How to Master Git mailinfo in Punggol Tuition

When a learner opens a patch email, the visible message mixes several kinds of information: authorship headers, a proposed commit message, discussion text and the patch itself. Git mailinfo is the plumbing command that separates those channels.

git mailinfo reads one email message from standard input, writes the extracted commit log message to one file, writes the patch to another file, and prints author name, author email and subject information to standard output for git am. The Git manual notes that users normally call git am instead of mailinfo directly.

This guide teaches the command without applying untrusted patches: input and output routing, subject cleanup, encodings, Message-ID, scissors, quoted carriage returns, configurations, temporary fixtures, diagnostics, explained practice and parent decisions.

Punggol families can use this guide in short sessions around homework, CCAs and rest. The examples are proposed learning activities. They do not claim a physical tuition branch, timetable, fee, class size, school relationship or guaranteed result.

Use disposable data and repositories, preserve backups and check version-sensitive details against the linked primary source. Current documentation settles a technical contract; observation and explanation turn that contract into usable knowledge.

Find your next learning step

Choose the route that matches the present difficulty. A complete course can follow the index in order; a repair session can begin at the first unstable distinction.

Build the model

Chapters 1–4. Model one email input and three distinct outputs.

Read the contract

Chapters 5–8. Read subject cleanup, encoding and patch-preservation rules precisely.

Test the boundary

Chapters 9–12. Test Message-ID, scissors and quoted-CR options in disposable fixtures.

Diagnose and repair

Chapters 13–16. Diagnose MIME, file and pipeline failures without applying a patch.

Transfer with judgment

Chapters 17–20. Choose mailinfo, git am or ordinary email parsing by the real job.

Open the full chapter index · Jump to capstone practice · Use the How Studying Works hub · Read the primary technical source

Complete chapter index

Chapters 1–4 · Build the model

One email produces three output channels
mailinfo is normally plumbing beneath git am
The first positional path receives the commit message
The second positional path receives the patch

Chapters 5–8 · Read the contract

Standard output carries author and subject fields
Default subject cleanup removes email cruft
-k keeps the subject from being munged
-b limits bracket stripping to PATCH markers

Chapters 9–12 · Test the boundary

-u is the current default metadata recoding route
The patch is not charset-converted
–encoding chooses the target metadata encoding
-n disables metadata charset recoding

Chapters 13–16 · Diagnose and repair

–message-id appends discussion identity
–scissors discards discussion before the cut line
–quoted-cr controls decoded CRLF handling
Configuration can change default results

Chapters 17–20 · Transfer with judgment

Failures should preserve raw input and partial outputs safely
A format-patch round trip makes a strong fixture
Untrusted email and patches require review
Choose the command by the workflow

CHAPTER 1 OF 20 · Build the model

1. One email produces three output channels

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git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output. That is the invariant to preserve when names, data or surrounding project details change.

Confusing those channels can overwrite evidence or feed the wrong text to another command. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is redirecting standard output into the patch filename. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo msg.txt patch.diff < mail.eml

Explained result. The two positional files hold extracted bodies while stdout carries author and subject fields. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace input email to each of the three outputs. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “One email produces three output channels”, separate contract from coincidence. The contract is “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Name stdin, msg, patch and stdout destinations before running.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “One email produces three output channels” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” should remain invariant and what observation would expose redirecting standard output into the patch filename.

Reasoned route. Begin with “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” Apply this procedure: Name stdin, msg, patch and stdout destinations before running. For the homework evidence folder, introduce one controlled variation that exposes redirecting standard output into the patch filename. The two positional files hold extracted bodies while stdout carries author and subject fields. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “One email produces three output channels” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” should remain invariant and what observation would expose redirecting standard output into the patch filename.

Reasoned route. Begin with “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” Apply this procedure: Name stdin, msg, patch and stdout destinations before running. For the CCA media catalogue, introduce one controlled variation that exposes redirecting standard output into the patch filename. The two positional files hold extracted bodies while stdout carries author and subject fields. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “One email produces three output channels” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” should remain invariant and what observation would expose redirecting standard output into the patch filename.

Reasoned route. Begin with “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” Apply this procedure: Name stdin, msg, patch and stdout destinations before running. For the science investigation archive, introduce one controlled variation that exposes redirecting standard output into the patch filename. The two positional files hold extracted bodies while stdout carries author and subject fields. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “One email produces three output channels” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” should remain invariant and what observation would expose redirecting standard output into the patch filename.

Reasoned route. Begin with “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.” Apply this procedure: Name stdin, msg, patch and stdout destinations before running. For the revision-note builder, introduce one controlled variation that exposes redirecting standard output into the patch filename. The two positional files hold extracted bodies while stdout carries author and subject fields. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals redirecting standard output into the patch filename.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Name stdin, msg, patch and stdout destinations before running.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “One email produces three output channels”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.?”, and “Can you make a smaller example of redirecting standard output into the patch filename?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Draw a four-box data-flow diagram for one invocation. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The command separates commit text, patch data and metadata rather than returning one merged file. The final output alone earns only partial credit. A strong answer connects it to “git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output.”, shows the relevant intermediate evidence and states why redirecting standard output into the patch filename would lead to a different conclusion.

Decision and transfer

Keep output filenames distinct and use a disposable directory. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “One email produces three output channels” means the learner can predict the ordinary case, recognise redirecting standard output into the patch filename, diagnose the earliest mistaken assumption, recover with “Name stdin, msg, patch and stdout destinations before running.” and defend this decision: Keep output filenames distinct and use a disposable directory. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 2 OF 20 · Build the model

2. mailinfo is normally plumbing beneath git am

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The manual says direct use is usually unnecessary and points readers to git am. That is the invariant to preserve when names, data or surrounding project details change.

git am coordinates parsing and commit creation, while mailinfo exposes the extraction stage. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is teaching mailinfo as the everyday replacement for git am. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo msg patch < message.eml

Explained result. A direct mailinfo run stops after extraction and does not create a commit. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace extraction separately from any repository mutation. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “mailinfo is normally plumbing beneath git am”, separate contract from coincidence. The contract is “The manual says direct use is usually unnecessary and points readers to git am.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “mailinfo is normally plumbing beneath git am” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The manual says direct use is usually unnecessary and points readers to git am.” should remain invariant and what observation would expose teaching mailinfo as the everyday replacement for git am.

Reasoned route. Begin with “The manual says direct use is usually unnecessary and points readers to git am.” Apply this procedure: Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series. For the homework evidence folder, introduce one controlled variation that exposes teaching mailinfo as the everyday replacement for git am. A direct mailinfo run stops after extraction and does not create a commit. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “mailinfo is normally plumbing beneath git am” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The manual says direct use is usually unnecessary and points readers to git am.” should remain invariant and what observation would expose teaching mailinfo as the everyday replacement for git am.

Reasoned route. Begin with “The manual says direct use is usually unnecessary and points readers to git am.” Apply this procedure: Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series. For the CCA media catalogue, introduce one controlled variation that exposes teaching mailinfo as the everyday replacement for git am. A direct mailinfo run stops after extraction and does not create a commit. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “mailinfo is normally plumbing beneath git am” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The manual says direct use is usually unnecessary and points readers to git am.” should remain invariant and what observation would expose teaching mailinfo as the everyday replacement for git am.

Reasoned route. Begin with “The manual says direct use is usually unnecessary and points readers to git am.” Apply this procedure: Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series. For the science investigation archive, introduce one controlled variation that exposes teaching mailinfo as the everyday replacement for git am. A direct mailinfo run stops after extraction and does not create a commit. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “mailinfo is normally plumbing beneath git am” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The manual says direct use is usually unnecessary and points readers to git am.” should remain invariant and what observation would expose teaching mailinfo as the everyday replacement for git am.

Reasoned route. Begin with “The manual says direct use is usually unnecessary and points readers to git am.” Apply this procedure: Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series. For the revision-note builder, introduce one controlled variation that exposes teaching mailinfo as the everyday replacement for git am. A direct mailinfo run stops after extraction and does not create a commit. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals teaching mailinfo as the everyday replacement for git am.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “mailinfo is normally plumbing beneath git am”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The manual says direct use is usually unnecessary and points readers to git am.?”, and “Can you make a smaller example of teaching mailinfo as the everyday replacement for git am?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Explain what remains undone after a successful mailinfo command. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The patch has not been applied and no commit has been created. The final output alone earns only partial credit. A strong answer connects it to “The manual says direct use is usually unnecessary and points readers to git am.”, shows the relevant intermediate evidence and states why teaching mailinfo as the everyday replacement for git am would lead to a different conclusion.

Decision and transfer

Choose plumbing only when its narrower output is useful. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “mailinfo is normally plumbing beneath git am” means the learner can predict the ordinary case, recognise teaching mailinfo as the everyday replacement for git am, diagnose the earliest mistaken assumption, recover with “Use mailinfo for inspection, diagnostics or tooling and use git am when the intended workflow is applying an emailed patch series.” and defend this decision: Choose plumbing only when its narrower output is useful. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 3 OF 20 · Build the model

3. The first positional path receives the commit message

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The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject. That is the invariant to preserve when names, data or surrounding project details change.

Commit body and subject are intentionally separate pieces of metadata. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is expecting msg.txt to be an exact copy of the email body including headers. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo out.msg out.patch < sample.eml
cat out.msg

Explained result. The message file contains the extracted commit text rather than raw mail. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace headers, subject title and body destination. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “The first positional path receives the commit message”, separate contract from coincidence. The contract is “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Inspect stdout subject and msg file side by side.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “The first positional path receives the commit message” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” should remain invariant and what observation would expose expecting msg.txt to be an exact copy of the email body including headers.

Reasoned route. Begin with “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” Apply this procedure: Inspect stdout subject and msg file side by side. For the homework evidence folder, introduce one controlled variation that exposes expecting msg.txt to be an exact copy of the email body including headers. The message file contains the extracted commit text rather than raw mail. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “The first positional path receives the commit message” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” should remain invariant and what observation would expose expecting msg.txt to be an exact copy of the email body including headers.

Reasoned route. Begin with “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” Apply this procedure: Inspect stdout subject and msg file side by side. For the CCA media catalogue, introduce one controlled variation that exposes expecting msg.txt to be an exact copy of the email body including headers. The message file contains the extracted commit text rather than raw mail. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “The first positional path receives the commit message” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” should remain invariant and what observation would expose expecting msg.txt to be an exact copy of the email body including headers.

Reasoned route. Begin with “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” Apply this procedure: Inspect stdout subject and msg file side by side. For the science investigation archive, introduce one controlled variation that exposes expecting msg.txt to be an exact copy of the email body including headers. The message file contains the extracted commit text rather than raw mail. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “The first positional path receives the commit message” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” should remain invariant and what observation would expose expecting msg.txt to be an exact copy of the email body including headers.

Reasoned route. Begin with “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.” Apply this procedure: Inspect stdout subject and msg file side by side. For the revision-note builder, introduce one controlled variation that exposes expecting msg.txt to be an exact copy of the email body including headers. The message file contains the extracted commit text rather than raw mail. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals expecting msg.txt to be an exact copy of the email body including headers.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Inspect stdout subject and msg file side by side.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “The first positional path receives the commit message”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.?”, and “Can you make a smaller example of expecting msg.txt to be an exact copy of the email body including headers?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Create an email with a subject and two body paragraphs. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The subject is reported separately while the body becomes commit-message material. The final output alone earns only partial credit. A strong answer connects it to “The file receives the commit log message extracted from the email, usually excluding the title line derived from Subject.”, shows the relevant intermediate evidence and states why expecting msg.txt to be an exact copy of the email body including headers would lead to a different conclusion.

Decision and transfer

Preserve raw email separately when exact archival evidence matters. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “The first positional path receives the commit message” means the learner can predict the ordinary case, recognise expecting msg.txt to be an exact copy of the email body including headers, diagnose the earliest mistaken assumption, recover with “Inspect stdout subject and msg file side by side.” and defend this decision: Preserve raw email separately when exact archival evidence matters. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 4 OF 20 · Build the model

4. The second positional path receives the patch

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The file receives the patch extracted from the email. That is the invariant to preserve when names, data or surrounding project details change.

The patch is a different artifact from discussion and commit prose. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is opening the msg file and concluding that no patch was extracted. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo out.msg out.patch < sample.eml
sed -n '1,20p' out.patch

Explained result. Diff headers and hunks belong in the patch output. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace the boundary where patch content begins. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “The second positional path receives the patch”, separate contract from coincidence. The contract is “The file receives the patch extracted from the email.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Check both output files and their sizes after a successful run.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “The second positional path receives the patch” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the patch extracted from the email.” should remain invariant and what observation would expose opening the msg file and concluding that no patch was extracted.

Reasoned route. Begin with “The file receives the patch extracted from the email.” Apply this procedure: Check both output files and their sizes after a successful run. For the homework evidence folder, introduce one controlled variation that exposes opening the msg file and concluding that no patch was extracted. Diff headers and hunks belong in the patch output. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “The second positional path receives the patch” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the patch extracted from the email.” should remain invariant and what observation would expose opening the msg file and concluding that no patch was extracted.

Reasoned route. Begin with “The file receives the patch extracted from the email.” Apply this procedure: Check both output files and their sizes after a successful run. For the CCA media catalogue, introduce one controlled variation that exposes opening the msg file and concluding that no patch was extracted. Diff headers and hunks belong in the patch output. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “The second positional path receives the patch” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the patch extracted from the email.” should remain invariant and what observation would expose opening the msg file and concluding that no patch was extracted.

Reasoned route. Begin with “The file receives the patch extracted from the email.” Apply this procedure: Check both output files and their sizes after a successful run. For the science investigation archive, introduce one controlled variation that exposes opening the msg file and concluding that no patch was extracted. Diff headers and hunks belong in the patch output. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “The second positional path receives the patch” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The file receives the patch extracted from the email.” should remain invariant and what observation would expose opening the msg file and concluding that no patch was extracted.

Reasoned route. Begin with “The file receives the patch extracted from the email.” Apply this procedure: Check both output files and their sizes after a successful run. For the revision-note builder, introduce one controlled variation that exposes opening the msg file and concluding that no patch was extracted. Diff headers and hunks belong in the patch output. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals opening the msg file and concluding that no patch was extracted.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Check both output files and their sizes after a successful run.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “The second positional path receives the patch”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The file receives the patch extracted from the email.?”, and “Can you make a smaller example of opening the msg file and concluding that no patch was extracted?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Use a format-patch email and identify the first diff line. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The diff appears in the patch file, not the commit-message file. The final output alone earns only partial credit. A strong answer connects it to “The file receives the patch extracted from the email.”, shows the relevant intermediate evidence and states why opening the msg file and concluding that no patch was extracted would lead to a different conclusion.

Decision and transfer

Review the patch before any apply operation. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “The second positional path receives the patch” means the learner can predict the ordinary case, recognise opening the msg file and concluding that no patch was extracted, diagnose the earliest mistaken assumption, recover with “Check both output files and their sizes after a successful run.” and defend this decision: Review the patch before any apply operation. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 5 OF 20 · Read the contract

5. Standard output carries author and subject fields

Back to contents

Author name, author email and email subject information are written to stdout for git am. That is the invariant to preserve when names, data or surrounding project details change.

Programs can capture metadata without reading the two output files as one format. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is ignoring stdout and then scraping raw headers inconsistently. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo out.msg out.patch < sample.eml > meta.txt

Explained result. The metadata channel can be logged or parsed while files remain dedicated. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace stdout redirection without affecting msg or patch files. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Standard output carries author and subject fields”, separate contract from coincidence. The contract is “Author name, author email and email subject information are written to stdout for git am.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Capture stdout separately and treat its field format as command output, not human prose.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Standard output carries author and subject fields” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “Author name, author email and email subject information are written to stdout for git am.” should remain invariant and what observation would expose ignoring stdout and then scraping raw headers inconsistently.

Reasoned route. Begin with “Author name, author email and email subject information are written to stdout for git am.” Apply this procedure: Capture stdout separately and treat its field format as command output, not human prose. For the homework evidence folder, introduce one controlled variation that exposes ignoring stdout and then scraping raw headers inconsistently. The metadata channel can be logged or parsed while files remain dedicated. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Standard output carries author and subject fields” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “Author name, author email and email subject information are written to stdout for git am.” should remain invariant and what observation would expose ignoring stdout and then scraping raw headers inconsistently.

Reasoned route. Begin with “Author name, author email and email subject information are written to stdout for git am.” Apply this procedure: Capture stdout separately and treat its field format as command output, not human prose. For the CCA media catalogue, introduce one controlled variation that exposes ignoring stdout and then scraping raw headers inconsistently. The metadata channel can be logged or parsed while files remain dedicated. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Standard output carries author and subject fields” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “Author name, author email and email subject information are written to stdout for git am.” should remain invariant and what observation would expose ignoring stdout and then scraping raw headers inconsistently.

Reasoned route. Begin with “Author name, author email and email subject information are written to stdout for git am.” Apply this procedure: Capture stdout separately and treat its field format as command output, not human prose. For the science investigation archive, introduce one controlled variation that exposes ignoring stdout and then scraping raw headers inconsistently. The metadata channel can be logged or parsed while files remain dedicated. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Standard output carries author and subject fields” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “Author name, author email and email subject information are written to stdout for git am.” should remain invariant and what observation would expose ignoring stdout and then scraping raw headers inconsistently.

Reasoned route. Begin with “Author name, author email and email subject information are written to stdout for git am.” Apply this procedure: Capture stdout separately and treat its field format as command output, not human prose. For the revision-note builder, introduce one controlled variation that exposes ignoring stdout and then scraping raw headers inconsistently. The metadata channel can be logged or parsed while files remain dedicated. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals ignoring stdout and then scraping raw headers inconsistently.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Capture stdout separately and treat its field format as command output, not human prose.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Standard output carries author and subject fields”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests Author name, author email and email subject information are written to stdout for git am.?”, and “Can you make a smaller example of ignoring stdout and then scraping raw headers inconsistently?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Capture all three destinations and label them. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. meta.txt, out.msg and out.patch should serve different purposes. The final output alone earns only partial credit. A strong answer connects it to “Author name, author email and email subject information are written to stdout for git am.”, shows the relevant intermediate evidence and states why ignoring stdout and then scraping raw headers inconsistently would lead to a different conclusion.

Decision and transfer

Do not mix diagnostics on stderr with metadata on stdout. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Standard output carries author and subject fields” means the learner can predict the ordinary case, recognise ignoring stdout and then scraping raw headers inconsistently, diagnose the earliest mistaken assumption, recover with “Capture stdout separately and treat its field format as command output, not human prose.” and defend this decision: Do not mix diagnostics on stderr with metadata on stdout. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 6 OF 20 · Read the contract

6. Default subject cleanup removes email cruft

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Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace. That is the invariant to preserve when names, data or surrounding project details change.

Patch email subjects often contain transport or series markers not wanted in a commit title. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is assuming [PATCH v2 1/3] always remains in the title. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo msg patch < sample.eml

Explained result. The ordinary extracted title is cleaned for commit use. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace each removable prefix from the original Subject. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Default subject cleanup removes email cruft”, separate contract from coincidence. The contract is “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Compare default output with -k using the same saved email.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Default subject cleanup removes email cruft” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” should remain invariant and what observation would expose assuming [PATCH v2 1/3] always remains in the title.

Reasoned route. Begin with “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” Apply this procedure: Compare default output with -k using the same saved email. For the homework evidence folder, introduce one controlled variation that exposes assuming [PATCH v2 1/3] always remains in the title. The ordinary extracted title is cleaned for commit use. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Default subject cleanup removes email cruft” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” should remain invariant and what observation would expose assuming [PATCH v2 1/3] always remains in the title.

Reasoned route. Begin with “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” Apply this procedure: Compare default output with -k using the same saved email. For the CCA media catalogue, introduce one controlled variation that exposes assuming [PATCH v2 1/3] always remains in the title. The ordinary extracted title is cleaned for commit use. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Default subject cleanup removes email cruft” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” should remain invariant and what observation would expose assuming [PATCH v2 1/3] always remains in the title.

Reasoned route. Begin with “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” Apply this procedure: Compare default output with -k using the same saved email. For the science investigation archive, introduce one controlled variation that exposes assuming [PATCH v2 1/3] always remains in the title. The ordinary extracted title is cleaned for commit use. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Default subject cleanup removes email cruft” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” should remain invariant and what observation would expose assuming [PATCH v2 1/3] always remains in the title.

Reasoned route. Begin with “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.” Apply this procedure: Compare default output with -k using the same saved email. For the revision-note builder, introduce one controlled variation that exposes assuming [PATCH v2 1/3] always remains in the title. The ordinary extracted title is cleaned for commit use. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals assuming [PATCH v2 1/3] always remains in the title.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Compare default output with -k using the same saved email.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Default subject cleanup removes email cruft”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.?”, and “Can you make a smaller example of assuming [PATCH v2 1/3] always remains in the title?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Predict the cleaned title for Re: [PATCH v2] Fix parser. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The transport markers are removed under default cleanup, leaving the useful title. The final output alone earns only partial credit. A strong answer connects it to “Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace.”, shows the relevant intermediate evidence and states why assuming [PATCH v2 1/3] always remains in the title would lead to a different conclusion.

Decision and transfer

Keep raw subject in the original email for audit. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Default subject cleanup removes email cruft” means the learner can predict the ordinary case, recognise assuming [PATCH v2 1/3] always remains in the title, diagnose the earliest mistaken assumption, recover with “Compare default output with -k using the same saved email.” and defend this decision: Keep raw subject in the original email for audit. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 7 OF 20 · Read the contract

7. -k keeps the subject from being munged

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The -k option prevents the usual subject cleanup and is useful with git format-patch -k output. That is the invariant to preserve when names, data or surrounding project details change.

It preserves subject text that the default path would normalise or strip. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is using -k and still expecting [PATCH] removal. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo -k kept.msg kept.patch < sample.eml

Explained result. The -k metadata retains subject cruft that default mode removes. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace original subject directly to retained title. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “-k keeps the subject from being munged”, separate contract from coincidence. The contract is “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Run default and -k into separate temporary outputs.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “-k keeps the subject from being munged” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” should remain invariant and what observation would expose using -k and still expecting [PATCH] removal.

Reasoned route. Begin with “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” Apply this procedure: Run default and -k into separate temporary outputs. For the homework evidence folder, introduce one controlled variation that exposes using -k and still expecting [PATCH] removal. The -k metadata retains subject cruft that default mode removes. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “-k keeps the subject from being munged” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” should remain invariant and what observation would expose using -k and still expecting [PATCH] removal.

Reasoned route. Begin with “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” Apply this procedure: Run default and -k into separate temporary outputs. For the CCA media catalogue, introduce one controlled variation that exposes using -k and still expecting [PATCH] removal. The -k metadata retains subject cruft that default mode removes. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “-k keeps the subject from being munged” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” should remain invariant and what observation would expose using -k and still expecting [PATCH] removal.

Reasoned route. Begin with “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” Apply this procedure: Run default and -k into separate temporary outputs. For the science investigation archive, introduce one controlled variation that exposes using -k and still expecting [PATCH] removal. The -k metadata retains subject cruft that default mode removes. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “-k keeps the subject from being munged” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” should remain invariant and what observation would expose using -k and still expecting [PATCH] removal.

Reasoned route. Begin with “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.” Apply this procedure: Run default and -k into separate temporary outputs. For the revision-note builder, introduce one controlled variation that exposes using -k and still expecting [PATCH] removal. The -k metadata retains subject cruft that default mode removes. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals using -k and still expecting [PATCH] removal.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Run default and -k into separate temporary outputs.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “-k keeps the subject from being munged”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.?”, and “Can you make a smaller example of using -k and still expecting [PATCH] removal?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Compare [PATCH] Topic under default and -k. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The default cleans the marker; -k preserves it. The final output alone earns only partial credit. A strong answer connects it to “The -k option prevents the usual subject cleanup and is useful with git format-patch -k output.”, shows the relevant intermediate evidence and states why using -k and still expecting [PATCH] removal would lead to a different conclusion.

Decision and transfer

Use -k only when preservation is an explicit interoperability need. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “-k keeps the subject from being munged” means the learner can predict the ordinary case, recognise using -k and still expecting [PATCH] removal, diagnose the earliest mistaken assumption, recover with “Run default and -k into separate temporary outputs.” and defend this decision: Use -k only when preservation is an explicit interoperability need. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 8 OF 20 · Read the contract

8. -b limits bracket stripping to PATCH markers

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When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH. That is the invariant to preserve when names, data or surrounding project details change.

Other bracketed context can remain while patch markers are cleaned. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is assuming -b means keep every bracketed prefix. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo -b msg patch < sample.eml

Explained result. PATCH-labelled brackets are stripped while unrelated bracket context can remain. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace each bracketed group and whether it contains PATCH. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “-b limits bracket stripping to PATCH markers”, separate contract from coincidence. The contract is “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Create subjects with [PATCH], [project] and both, then compare.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “-b limits bracket stripping to PATCH markers” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” should remain invariant and what observation would expose assuming -b means keep every bracketed prefix.

Reasoned route. Begin with “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” Apply this procedure: Create subjects with [PATCH], [project] and both, then compare. For the homework evidence folder, introduce one controlled variation that exposes assuming -b means keep every bracketed prefix. PATCH-labelled brackets are stripped while unrelated bracket context can remain. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “-b limits bracket stripping to PATCH markers” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” should remain invariant and what observation would expose assuming -b means keep every bracketed prefix.

Reasoned route. Begin with “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” Apply this procedure: Create subjects with [PATCH], [project] and both, then compare. For the CCA media catalogue, introduce one controlled variation that exposes assuming -b means keep every bracketed prefix. PATCH-labelled brackets are stripped while unrelated bracket context can remain. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “-b limits bracket stripping to PATCH markers” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” should remain invariant and what observation would expose assuming -b means keep every bracketed prefix.

Reasoned route. Begin with “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” Apply this procedure: Create subjects with [PATCH], [project] and both, then compare. For the science investigation archive, introduce one controlled variation that exposes assuming -b means keep every bracketed prefix. PATCH-labelled brackets are stripped while unrelated bracket context can remain. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “-b limits bracket stripping to PATCH markers” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” should remain invariant and what observation would expose assuming -b means keep every bracketed prefix.

Reasoned route. Begin with “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.” Apply this procedure: Create subjects with [PATCH], [project] and both, then compare. For the revision-note builder, introduce one controlled variation that exposes assuming -b means keep every bracketed prefix. PATCH-labelled brackets are stripped while unrelated bracket context can remain. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals assuming -b means keep every bracketed prefix.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Create subjects with [PATCH], [project] and both, then compare.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “-b limits bracket stripping to PATCH markers”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.?”, and “Can you make a smaller example of assuming -b means keep every bracketed prefix?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Predict output for [project] [PATCH v3] Improve docs. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The patch marker is removed while project context can be preserved. The final output alone earns only partial credit. A strong answer connects it to “When -k is not active, -b restricts leading bracket removal to bracketed strings containing the word PATCH.”, shows the relevant intermediate evidence and states why assuming -b means keep every bracketed prefix would lead to a different conclusion.

Decision and transfer

Use -b when bracketed non-patch context carries meaning. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “-b limits bracket stripping to PATCH markers” means the learner can predict the ordinary case, recognise assuming -b means keep every bracketed prefix, diagnose the earliest mistaken assumption, recover with “Create subjects with [PATCH], [project] and both, then compare.” and defend this decision: Use -b when bracketed non-patch context carries meaning. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 9 OF 20 · Test the boundary

9. -u is the current default metadata recoding route

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The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8. That is the invariant to preserve when names, data or surrounding project details change.

Email metadata may arrive in encodings unsuitable for the repository’s commit text. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is assuming every byte is copied unchanged into the message file. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo -u msg patch < sample.eml

Explained result. Metadata is recoded under the default route, subject to documented conversion. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace transfer decoding, source charset and target encoding. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “-u is the current default metadata recoding route”, separate contract from coincidence. The contract is “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Record source charset and repository commit encoding in the fixture.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “-u is the current default metadata recoding route” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” should remain invariant and what observation would expose assuming every byte is copied unchanged into the message file.

Reasoned route. Begin with “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” Apply this procedure: Record source charset and repository commit encoding in the fixture. For the homework evidence folder, introduce one controlled variation that exposes assuming every byte is copied unchanged into the message file. Metadata is recoded under the default route, subject to documented conversion. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “-u is the current default metadata recoding route” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” should remain invariant and what observation would expose assuming every byte is copied unchanged into the message file.

Reasoned route. Begin with “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” Apply this procedure: Record source charset and repository commit encoding in the fixture. For the CCA media catalogue, introduce one controlled variation that exposes assuming every byte is copied unchanged into the message file. Metadata is recoded under the default route, subject to documented conversion. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “-u is the current default metadata recoding route” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” should remain invariant and what observation would expose assuming every byte is copied unchanged into the message file.

Reasoned route. Begin with “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” Apply this procedure: Record source charset and repository commit encoding in the fixture. For the science investigation archive, introduce one controlled variation that exposes assuming every byte is copied unchanged into the message file. Metadata is recoded under the default route, subject to documented conversion. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “-u is the current default metadata recoding route” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” should remain invariant and what observation would expose assuming every byte is copied unchanged into the message file.

Reasoned route. Begin with “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.” Apply this procedure: Record source charset and repository commit encoding in the fixture. For the revision-note builder, introduce one controlled variation that exposes assuming every byte is copied unchanged into the message file. Metadata is recoded under the default route, subject to documented conversion. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals assuming every byte is copied unchanged into the message file.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Record source charset and repository commit encoding in the fixture.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “-u is the current default metadata recoding route”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.?”, and “Can you make a smaller example of assuming every byte is copied unchanged into the message file?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Explain why accented author names need an encoding-aware test. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Visible equality in one terminal does not prove byte-level handling. The final output alone earns only partial credit. A strong answer connects it to “The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8.”, shows the relevant intermediate evidence and states why assuming every byte is copied unchanged into the message file would lead to a different conclusion.

Decision and transfer

Use UTF-8 fixtures plus one explicit non-ASCII case. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “-u is the current default metadata recoding route” means the learner can predict the ordinary case, recognise assuming every byte is copied unchanged into the message file, diagnose the earliest mistaken assumption, recover with “Record source charset and repository commit encoding in the fixture.” and defend this decision: Use UTF-8 fixtures plus one explicit non-ASCII case. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 10 OF 20 · Test the boundary

10. The patch is not charset-converted

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The manual states that the patch is always used as-is without charset conversion, even with -u. That is the invariant to preserve when names, data or surrounding project details change.

Changing patch bytes could corrupt context lines or file data. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is assuming -u recodes every output channel. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

sha256sum out.patch

Explained result. Metadata may be recoded while the patch bytes remain under the patch-preservation rule. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace message recoding and patch preservation as parallel policies. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “The patch is not charset-converted”, separate contract from coincidence. The contract is “The manual states that the patch is always used as-is without charset conversion, even with -u.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Hash or byte-compare the extracted patch separately from message metadata.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “The patch is not charset-converted” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The manual states that the patch is always used as-is without charset conversion, even with -u.” should remain invariant and what observation would expose assuming -u recodes every output channel.

Reasoned route. Begin with “The manual states that the patch is always used as-is without charset conversion, even with -u.” Apply this procedure: Hash or byte-compare the extracted patch separately from message metadata. For the homework evidence folder, introduce one controlled variation that exposes assuming -u recodes every output channel. Metadata may be recoded while the patch bytes remain under the patch-preservation rule. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “The patch is not charset-converted” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The manual states that the patch is always used as-is without charset conversion, even with -u.” should remain invariant and what observation would expose assuming -u recodes every output channel.

Reasoned route. Begin with “The manual states that the patch is always used as-is without charset conversion, even with -u.” Apply this procedure: Hash or byte-compare the extracted patch separately from message metadata. For the CCA media catalogue, introduce one controlled variation that exposes assuming -u recodes every output channel. Metadata may be recoded while the patch bytes remain under the patch-preservation rule. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “The patch is not charset-converted” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The manual states that the patch is always used as-is without charset conversion, even with -u.” should remain invariant and what observation would expose assuming -u recodes every output channel.

Reasoned route. Begin with “The manual states that the patch is always used as-is without charset conversion, even with -u.” Apply this procedure: Hash or byte-compare the extracted patch separately from message metadata. For the science investigation archive, introduce one controlled variation that exposes assuming -u recodes every output channel. Metadata may be recoded while the patch bytes remain under the patch-preservation rule. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “The patch is not charset-converted” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The manual states that the patch is always used as-is without charset conversion, even with -u.” should remain invariant and what observation would expose assuming -u recodes every output channel.

Reasoned route. Begin with “The manual states that the patch is always used as-is without charset conversion, even with -u.” Apply this procedure: Hash or byte-compare the extracted patch separately from message metadata. For the revision-note builder, introduce one controlled variation that exposes assuming -u recodes every output channel. Metadata may be recoded while the patch bytes remain under the patch-preservation rule. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals assuming -u recodes every output channel.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Hash or byte-compare the extracted patch separately from message metadata.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “The patch is not charset-converted”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The manual states that the patch is always used as-is without charset conversion, even with -u.?”, and “Can you make a smaller example of assuming -u recodes every output channel?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Create a fixture with non-ASCII metadata and a controlled patch. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. The explanation should distinguish metadata conversion from patch handling. The final output alone earns only partial credit. A strong answer connects it to “The manual states that the patch is always used as-is without charset conversion, even with -u.”, shows the relevant intermediate evidence and states why assuming -u recodes every output channel would lead to a different conclusion.

Decision and transfer

Verify patch integrity independently. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “The patch is not charset-converted” means the learner can predict the ordinary case, recognise assuming -u recodes every output channel, diagnose the earliest mistaken assumption, recover with “Hash or byte-compare the extracted patch separately from message metadata.” and defend this decision: Verify patch integrity independently. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 11 OF 20 · Test the boundary

11. –encoding chooses the target metadata encoding

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–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8. That is the invariant to preserve when names, data or surrounding project details change.

Tooling sometimes needs a declared output charset. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is setting an encoding name without verifying downstream support. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo --encoding=UTF-8 msg patch < sample.eml

Explained result. The commit message and author metadata target the specified encoding while the patch remains as-is. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace declared target encoding and affected outputs. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “–encoding chooses the target metadata encoding”, separate contract from coincidence. The contract is “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Use a small non-ASCII fixture and inspect bytes with a trusted tool.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “–encoding chooses the target metadata encoding” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” should remain invariant and what observation would expose setting an encoding name without verifying downstream support.

Reasoned route. Begin with “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” Apply this procedure: Use a small non-ASCII fixture and inspect bytes with a trusted tool. For the homework evidence folder, introduce one controlled variation that exposes setting an encoding name without verifying downstream support. The commit message and author metadata target the specified encoding while the patch remains as-is. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “–encoding chooses the target metadata encoding” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” should remain invariant and what observation would expose setting an encoding name without verifying downstream support.

Reasoned route. Begin with “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” Apply this procedure: Use a small non-ASCII fixture and inspect bytes with a trusted tool. For the CCA media catalogue, introduce one controlled variation that exposes setting an encoding name without verifying downstream support. The commit message and author metadata target the specified encoding while the patch remains as-is. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “–encoding chooses the target metadata encoding” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” should remain invariant and what observation would expose setting an encoding name without verifying downstream support.

Reasoned route. Begin with “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” Apply this procedure: Use a small non-ASCII fixture and inspect bytes with a trusted tool. For the science investigation archive, introduce one controlled variation that exposes setting an encoding name without verifying downstream support. The commit message and author metadata target the specified encoding while the patch remains as-is. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “–encoding chooses the target metadata encoding” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” should remain invariant and what observation would expose setting an encoding name without verifying downstream support.

Reasoned route. Begin with “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.” Apply this procedure: Use a small non-ASCII fixture and inspect bytes with a trusted tool. For the revision-note builder, introduce one controlled variation that exposes setting an encoding name without verifying downstream support. The commit message and author metadata target the specified encoding while the patch remains as-is. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals setting an encoding name without verifying downstream support.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Use a small non-ASCII fixture and inspect bytes with a trusted tool.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “–encoding chooses the target metadata encoding”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests –encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.?”, and “Can you make a smaller example of setting an encoding name without verifying downstream support?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Compare default UTF-8 with one explicit encoding in a safe fixture. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Only metadata channels are candidates for recoding. The final output alone earns only partial credit. A strong answer connects it to “–encoding= behaves like -u but uses the named encoding instead of i18n.commitEncoding or UTF-8.”, shows the relevant intermediate evidence and states why setting an encoding name without verifying downstream support would lead to a different conclusion.

Decision and transfer

Prefer UTF-8 unless a real repository contract requires otherwise. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “–encoding chooses the target metadata encoding” means the learner can predict the ordinary case, recognise setting an encoding name without verifying downstream support, diagnose the earliest mistaken assumption, recover with “Use a small non-ASCII fixture and inspect bytes with a trusted tool.” and defend this decision: Prefer UTF-8 unless a real repository contract requires otherwise. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 12 OF 20 · Test the boundary

12. -n disables metadata charset recoding

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The -n option disables all charset recoding of the metadata. That is the invariant to preserve when names, data or surrounding project details change.

Rawer metadata can be useful for diagnosis but may not be ready for commit use. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is using -n as a universal fix for garbled text. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo -n msg patch < sample.eml

Explained result. The command avoids recoding but still performs its extraction work. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace transfer decoding versus charset recoding carefully. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “-n disables metadata charset recoding”, separate contract from coincidence. The contract is “The -n option disables all charset recoding of the metadata.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “-n disables metadata charset recoding” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The -n option disables all charset recoding of the metadata.” should remain invariant and what observation would expose using -n as a universal fix for garbled text.

Reasoned route. Begin with “The -n option disables all charset recoding of the metadata.” Apply this procedure: Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed. For the homework evidence folder, introduce one controlled variation that exposes using -n as a universal fix for garbled text. The command avoids recoding but still performs its extraction work. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “-n disables metadata charset recoding” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The -n option disables all charset recoding of the metadata.” should remain invariant and what observation would expose using -n as a universal fix for garbled text.

Reasoned route. Begin with “The -n option disables all charset recoding of the metadata.” Apply this procedure: Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed. For the CCA media catalogue, introduce one controlled variation that exposes using -n as a universal fix for garbled text. The command avoids recoding but still performs its extraction work. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “-n disables metadata charset recoding” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The -n option disables all charset recoding of the metadata.” should remain invariant and what observation would expose using -n as a universal fix for garbled text.

Reasoned route. Begin with “The -n option disables all charset recoding of the metadata.” Apply this procedure: Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed. For the science investigation archive, introduce one controlled variation that exposes using -n as a universal fix for garbled text. The command avoids recoding but still performs its extraction work. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “-n disables metadata charset recoding” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The -n option disables all charset recoding of the metadata.” should remain invariant and what observation would expose using -n as a universal fix for garbled text.

Reasoned route. Begin with “The -n option disables all charset recoding of the metadata.” Apply this procedure: Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed. For the revision-note builder, introduce one controlled variation that exposes using -n as a universal fix for garbled text. The command avoids recoding but still performs its extraction work. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals using -n as a universal fix for garbled text.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “-n disables metadata charset recoding”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The -n option disables all charset recoding of the metadata.?”, and “Can you make a smaller example of using -n as a universal fix for garbled text?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Diagnose a fixture whose declared charset is wrong. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Disabling recoding may preserve bytes but does not make mislabeled data correct. The final output alone earns only partial credit. A strong answer connects it to “The -n option disables all charset recoding of the metadata.”, shows the relevant intermediate evidence and states why using -n as a universal fix for garbled text would lead to a different conclusion.

Decision and transfer

Use -n for controlled inspection, not to hide encoding errors. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “-n disables metadata charset recoding” means the learner can predict the ordinary case, recognise using -n as a universal fix for garbled text, diagnose the earliest mistaken assumption, recover with “Compare bytes and declared MIME charset before deciding whether recoding or source repair is needed.” and defend this decision: Use -n for controlled inspection, not to hide encoding errors. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 13 OF 20 · Diagnose and repair

13. –message-id appends discussion identity

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The -m or –message-id option copies the Message-ID header to the end of the commit message. That is the invariant to preserve when names, data or surrounding project details change.

This can associate a commit with its mailing-list discussion. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is assuming Message-ID authenticates the author or patch. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo --message-id msg patch < sample.eml

Explained result. The message file gains the email’s identifier when present. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace header value to appended commit-message text. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “–message-id appends discussion identity”, separate contract from coincidence. The contract is “The -m or –message-id option copies the Message-ID header to the end of the commit message.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Inspect the appended line and treat it as a reference, not a trust proof.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “–message-id appends discussion identity” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “The -m or –message-id option copies the Message-ID header to the end of the commit message.” should remain invariant and what observation would expose assuming Message-ID authenticates the author or patch.

Reasoned route. Begin with “The -m or –message-id option copies the Message-ID header to the end of the commit message.” Apply this procedure: Inspect the appended line and treat it as a reference, not a trust proof. For the homework evidence folder, introduce one controlled variation that exposes assuming Message-ID authenticates the author or patch. The message file gains the email’s identifier when present. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “–message-id appends discussion identity” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “The -m or –message-id option copies the Message-ID header to the end of the commit message.” should remain invariant and what observation would expose assuming Message-ID authenticates the author or patch.

Reasoned route. Begin with “The -m or –message-id option copies the Message-ID header to the end of the commit message.” Apply this procedure: Inspect the appended line and treat it as a reference, not a trust proof. For the CCA media catalogue, introduce one controlled variation that exposes assuming Message-ID authenticates the author or patch. The message file gains the email’s identifier when present. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “–message-id appends discussion identity” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “The -m or –message-id option copies the Message-ID header to the end of the commit message.” should remain invariant and what observation would expose assuming Message-ID authenticates the author or patch.

Reasoned route. Begin with “The -m or –message-id option copies the Message-ID header to the end of the commit message.” Apply this procedure: Inspect the appended line and treat it as a reference, not a trust proof. For the science investigation archive, introduce one controlled variation that exposes assuming Message-ID authenticates the author or patch. The message file gains the email’s identifier when present. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “–message-id appends discussion identity” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “The -m or –message-id option copies the Message-ID header to the end of the commit message.” should remain invariant and what observation would expose assuming Message-ID authenticates the author or patch.

Reasoned route. Begin with “The -m or –message-id option copies the Message-ID header to the end of the commit message.” Apply this procedure: Inspect the appended line and treat it as a reference, not a trust proof. For the revision-note builder, introduce one controlled variation that exposes assuming Message-ID authenticates the author or patch. The message file gains the email’s identifier when present. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals assuming Message-ID authenticates the author or patch.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Inspect the appended line and treat it as a reference, not a trust proof.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “–message-id appends discussion identity”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests The -m or –message-id option copies the Message-ID header to the end of the commit message.?”, and “Can you make a smaller example of assuming Message-ID authenticates the author or patch?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Compare fixtures with and without Message-ID. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Only the header-bearing fixture can supply the reference. The final output alone earns only partial credit. A strong answer connects it to “The -m or –message-id option copies the Message-ID header to the end of the commit message.”, shows the relevant intermediate evidence and states why assuming Message-ID authenticates the author or patch would lead to a different conclusion.

Decision and transfer

Use it when traceability is valuable and repository policy permits. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “–message-id appends discussion identity” means the learner can predict the ordinary case, recognise assuming Message-ID authenticates the author or patch, diagnose the earliest mistaken assumption, recover with “Inspect the appended line and treat it as a reference, not a trust proof.” and defend this decision: Use it when traceability is valuable and repository policy permits. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 14 OF 20 · Diagnose and repair

14. –scissors discards discussion before the cut line

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With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch. That is the invariant to preserve when names, data or surrounding project details change.

A reply can contain discussion before the intended commit message and patch. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is manually deleting lines after extraction and losing reproducibility. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo --scissors msg patch < reply.eml

Explained result. The extracted message begins after the cut boundary while the patch remains available. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace discussion, scissors line, commit text and patch in order. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “–scissors discards discussion before the cut line”, separate contract from coincidence. The contract is “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Put a documented scissors marker in a disposable email and compare with –no-scissors.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “–scissors discards discussion before the cut line” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” should remain invariant and what observation would expose manually deleting lines after extraction and losing reproducibility.

Reasoned route. Begin with “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” Apply this procedure: Put a documented scissors marker in a disposable email and compare with –no-scissors. For the homework evidence folder, introduce one controlled variation that exposes manually deleting lines after extraction and losing reproducibility. The extracted message begins after the cut boundary while the patch remains available. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “–scissors discards discussion before the cut line” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” should remain invariant and what observation would expose manually deleting lines after extraction and losing reproducibility.

Reasoned route. Begin with “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” Apply this procedure: Put a documented scissors marker in a disposable email and compare with –no-scissors. For the CCA media catalogue, introduce one controlled variation that exposes manually deleting lines after extraction and losing reproducibility. The extracted message begins after the cut boundary while the patch remains available. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “–scissors discards discussion before the cut line” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” should remain invariant and what observation would expose manually deleting lines after extraction and losing reproducibility.

Reasoned route. Begin with “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” Apply this procedure: Put a documented scissors marker in a disposable email and compare with –no-scissors. For the science investigation archive, introduce one controlled variation that exposes manually deleting lines after extraction and losing reproducibility. The extracted message begins after the cut boundary while the patch remains available. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “–scissors discards discussion before the cut line” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” should remain invariant and what observation would expose manually deleting lines after extraction and losing reproducibility.

Reasoned route. Begin with “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.” Apply this procedure: Put a documented scissors marker in a disposable email and compare with –no-scissors. For the revision-note builder, introduce one controlled variation that exposes manually deleting lines after extraction and losing reproducibility. The extracted message begins after the cut boundary while the patch remains available. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals manually deleting lines after extraction and losing reproducibility.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Put a documented scissors marker in a disposable email and compare with –no-scissors.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “–scissors discards discussion before the cut line”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.?”, and “Can you make a smaller example of manually deleting lines after extraction and losing reproducibility?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Create a reply with two comments before — >8 –. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Those comments and the marker are ignored under scissors mode. The final output alone earns only partial credit. A strong answer connects it to “With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch.”, shows the relevant intermediate evidence and states why manually deleting lines after extraction and losing reproducibility would lead to a different conclusion.

Decision and transfer

Use only when the sender’s boundary is intentional. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “–scissors discards discussion before the cut line” means the learner can predict the ordinary case, recognise manually deleting lines after extraction and losing reproducibility, diagnose the earliest mistaken assumption, recover with “Put a documented scissors marker in a disposable email and compare with –no-scissors.” and defend this decision: Use only when the sender’s boundary is intentional. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 15 OF 20 · Diagnose and repair

15. –quoted-cr controls decoded CRLF handling

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–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF. That is the invariant to preserve when names, data or surrounding project details change.

Line endings can create warnings or unwanted carriage returns after transfer decoding. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is using strip without recording that bytes were normalised. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo --quoted-cr=warn msg patch < sample.eml

Explained result. nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace transfer decoding, detected CRLF and selected action. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “–quoted-cr controls decoded CRLF handling”, separate contract from coincidence. The contract is “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Build a controlled quoted-printable fixture and choose an action based on downstream needs.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “–quoted-cr controls decoded CRLF handling” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” should remain invariant and what observation would expose using strip without recording that bytes were normalised.

Reasoned route. Begin with “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” Apply this procedure: Build a controlled quoted-printable fixture and choose an action based on downstream needs. For the homework evidence folder, introduce one controlled variation that exposes using strip without recording that bytes were normalised. nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “–quoted-cr controls decoded CRLF handling” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” should remain invariant and what observation would expose using strip without recording that bytes were normalised.

Reasoned route. Begin with “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” Apply this procedure: Build a controlled quoted-printable fixture and choose an action based on downstream needs. For the CCA media catalogue, introduce one controlled variation that exposes using strip without recording that bytes were normalised. nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “–quoted-cr controls decoded CRLF handling” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” should remain invariant and what observation would expose using strip without recording that bytes were normalised.

Reasoned route. Begin with “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” Apply this procedure: Build a controlled quoted-printable fixture and choose an action based on downstream needs. For the science investigation archive, introduce one controlled variation that exposes using strip without recording that bytes were normalised. nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “–quoted-cr controls decoded CRLF handling” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” should remain invariant and what observation would expose using strip without recording that bytes were normalised.

Reasoned route. Begin with “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.” Apply this procedure: Build a controlled quoted-printable fixture and choose an action based on downstream needs. For the revision-note builder, introduce one controlled variation that exposes using strip without recording that bytes were normalised. nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals using strip without recording that bytes were normalised.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Build a controlled quoted-printable fixture and choose an action based on downstream needs.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “–quoted-cr controls decoded CRLF handling”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests –quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.?”, and “Can you make a smaller example of using strip without recording that bytes were normalised?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Compare warn and strip without applying the patch. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. One preserves with warning; the other normalises decoded line endings. The final output alone earns only partial credit. A strong answer connects it to “–quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF.”, shows the relevant intermediate evidence and states why using strip without recording that bytes were normalised would lead to a different conclusion.

Decision and transfer

Treat line-ending normalisation as a documented policy. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “–quoted-cr controls decoded CRLF handling” means the learner can predict the ordinary case, recognise using strip without recording that bytes were normalised, diagnose the earliest mistaken assumption, recover with “Build a controlled quoted-printable fixture and choose an action based on downstream needs.” and defend this decision: Treat line-ending normalisation as a documented policy. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 16 OF 20 · Diagnose and repair

16. Configuration can change default results

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mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them. That is the invariant to preserve when names, data or surrounding project details change.

Two machines can parse the same email differently because their Git configuration differs. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is blaming the email while ignoring local configuration. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git config --get mailinfo.scissors
git config --get mailinfo.quotedCR

Explained result. The effective behaviour follows command flags plus configured defaults. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace system, global and repository configuration sources. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Configuration can change default results”, separate contract from coincidence. The contract is “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Query the relevant configuration and pin flags in reproducible tooling.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Configuration can change default results” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” should remain invariant and what observation would expose blaming the email while ignoring local configuration.

Reasoned route. Begin with “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” Apply this procedure: Query the relevant configuration and pin flags in reproducible tooling. For the homework evidence folder, introduce one controlled variation that exposes blaming the email while ignoring local configuration. The effective behaviour follows command flags plus configured defaults. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Configuration can change default results” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” should remain invariant and what observation would expose blaming the email while ignoring local configuration.

Reasoned route. Begin with “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” Apply this procedure: Query the relevant configuration and pin flags in reproducible tooling. For the CCA media catalogue, introduce one controlled variation that exposes blaming the email while ignoring local configuration. The effective behaviour follows command flags plus configured defaults. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Configuration can change default results” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” should remain invariant and what observation would expose blaming the email while ignoring local configuration.

Reasoned route. Begin with “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” Apply this procedure: Query the relevant configuration and pin flags in reproducible tooling. For the science investigation archive, introduce one controlled variation that exposes blaming the email while ignoring local configuration. The effective behaviour follows command flags plus configured defaults. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Configuration can change default results” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” should remain invariant and what observation would expose blaming the email while ignoring local configuration.

Reasoned route. Begin with “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.” Apply this procedure: Query the relevant configuration and pin flags in reproducible tooling. For the revision-note builder, introduce one controlled variation that exposes blaming the email while ignoring local configuration. The effective behaviour follows command flags plus configured defaults. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals blaming the email while ignoring local configuration.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Query the relevant configuration and pin flags in reproducible tooling.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Configuration can change default results”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.?”, and “Can you make a smaller example of blaming the email while ignoring local configuration?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Explain how to reproduce a surprising scissors result. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Record the config and rerun with an explicit opposing flag. The final output alone earns only partial credit. A strong answer connects it to “mailinfo.scissors and mailinfo.quotedCR can supply defaults when command-line options do not override them.”, shows the relevant intermediate evidence and states why blaming the email while ignoring local configuration would lead to a different conclusion.

Decision and transfer

Make hidden defaults visible in diagnostics. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Configuration can change default results” means the learner can predict the ordinary case, recognise blaming the email while ignoring local configuration, diagnose the earliest mistaken assumption, recover with “Query the relevant configuration and pin flags in reproducible tooling.” and defend this decision: Make hidden defaults visible in diagnostics. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 17 OF 20 · Transfer with judgment

17. Failures should preserve raw input and partial outputs safely

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Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers. That is the invariant to preserve when names, data or surrounding project details change.

A generic parse failed message gives no repair route. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is overwriting the only raw email during experimentation. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

tmp=$(mktemp -d)
git mailinfo "$tmp/msg" "$tmp/patch" < sample.eml

Explained result. The earliest failed layer remains diagnosable without applying anything. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace input read, output opens, decoding and extraction. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Failures should preserve raw input and partial outputs safely”, separate contract from coincidence. The contract is “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Failures should preserve raw input and partial outputs safely” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” should remain invariant and what observation would expose overwriting the only raw email during experimentation.

Reasoned route. Begin with “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” Apply this procedure: Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts. For the homework evidence folder, introduce one controlled variation that exposes overwriting the only raw email during experimentation. The earliest failed layer remains diagnosable without applying anything. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Failures should preserve raw input and partial outputs safely” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” should remain invariant and what observation would expose overwriting the only raw email during experimentation.

Reasoned route. Begin with “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” Apply this procedure: Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts. For the CCA media catalogue, introduce one controlled variation that exposes overwriting the only raw email during experimentation. The earliest failed layer remains diagnosable without applying anything. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Failures should preserve raw input and partial outputs safely” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” should remain invariant and what observation would expose overwriting the only raw email during experimentation.

Reasoned route. Begin with “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” Apply this procedure: Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts. For the science investigation archive, introduce one controlled variation that exposes overwriting the only raw email during experimentation. The earliest failed layer remains diagnosable without applying anything. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Failures should preserve raw input and partial outputs safely” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” should remain invariant and what observation would expose overwriting the only raw email during experimentation.

Reasoned route. Begin with “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.” Apply this procedure: Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts. For the revision-note builder, introduce one controlled variation that exposes overwriting the only raw email during experimentation. The earliest failed layer remains diagnosable without applying anything. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals overwriting the only raw email during experimentation.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Failures should preserve raw input and partial outputs safely”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.?”, and “Can you make a smaller example of overwriting the only raw email during experimentation?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Design wrong-path, malformed-message and empty-patch fixtures. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Each case should retain command status and identify which expected artifact is absent. The final output alone earns only partial credit. A strong answer connects it to “Malformed MIME, unwritable destinations and unsuitable line endings are different failure layers.”, shows the relevant intermediate evidence and states why overwriting the only raw email during experimentation would lead to a different conclusion.

Decision and transfer

Never retry by loosening every option at once. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Failures should preserve raw input and partial outputs safely” means the learner can predict the ordinary case, recognise overwriting the only raw email during experimentation, diagnose the earliest mistaken assumption, recover with “Keep the source immutable, use a fresh temporary directory, capture status and inspect only safe partial artifacts.” and defend this decision: Never retry by loosening every option at once. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 18 OF 20 · Transfer with judgment

18. A format-patch round trip makes a strong fixture

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git format-patch can produce a controlled patch email that mailinfo then separates for inspection. That is the invariant to preserve when names, data or surrounding project details change.

A known commit supplies predictable author, title, message and diff data. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is testing with an unknown downloaded email and guessing the expected outputs. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git format-patch -1 --stdout HEAD > one.eml
git mailinfo msg patch < one.eml

Explained result. The extracted outputs can be compared with the known commit and patch. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace commit to email to separated artifacts. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “A format-patch round trip makes a strong fixture”, separate contract from coincidence. The contract is “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Create a disposable repository and one harmless commit, then generate one email without applying it.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “A format-patch round trip makes a strong fixture” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” should remain invariant and what observation would expose testing with an unknown downloaded email and guessing the expected outputs.

Reasoned route. Begin with “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” Apply this procedure: Create a disposable repository and one harmless commit, then generate one email without applying it. For the homework evidence folder, introduce one controlled variation that exposes testing with an unknown downloaded email and guessing the expected outputs. The extracted outputs can be compared with the known commit and patch. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “A format-patch round trip makes a strong fixture” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” should remain invariant and what observation would expose testing with an unknown downloaded email and guessing the expected outputs.

Reasoned route. Begin with “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” Apply this procedure: Create a disposable repository and one harmless commit, then generate one email without applying it. For the CCA media catalogue, introduce one controlled variation that exposes testing with an unknown downloaded email and guessing the expected outputs. The extracted outputs can be compared with the known commit and patch. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “A format-patch round trip makes a strong fixture” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” should remain invariant and what observation would expose testing with an unknown downloaded email and guessing the expected outputs.

Reasoned route. Begin with “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” Apply this procedure: Create a disposable repository and one harmless commit, then generate one email without applying it. For the science investigation archive, introduce one controlled variation that exposes testing with an unknown downloaded email and guessing the expected outputs. The extracted outputs can be compared with the known commit and patch. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “A format-patch round trip makes a strong fixture” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” should remain invariant and what observation would expose testing with an unknown downloaded email and guessing the expected outputs.

Reasoned route. Begin with “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.” Apply this procedure: Create a disposable repository and one harmless commit, then generate one email without applying it. For the revision-note builder, introduce one controlled variation that exposes testing with an unknown downloaded email and guessing the expected outputs. The extracted outputs can be compared with the known commit and patch. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals testing with an unknown downloaded email and guessing the expected outputs.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Create a disposable repository and one harmless commit, then generate one email without applying it.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “A format-patch round trip makes a strong fixture”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests git format-patch can produce a controlled patch email that mailinfo then separates for inspection.?”, and “Can you make a smaller example of testing with an unknown downloaded email and guessing the expected outputs?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Verify subject, body and first diff headers. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Each output should map back to the controlled commit source. The final output alone earns only partial credit. A strong answer connects it to “git format-patch can produce a controlled patch email that mailinfo then separates for inspection.”, shows the relevant intermediate evidence and states why testing with an unknown downloaded email and guessing the expected outputs would lead to a different conclusion.

Decision and transfer

Use temporary repositories for repeatable learning. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “A format-patch round trip makes a strong fixture” means the learner can predict the ordinary case, recognise testing with an unknown downloaded email and guessing the expected outputs, diagnose the earliest mistaken assumption, recover with “Create a disposable repository and one harmless commit, then generate one email without applying it.” and defend this decision: Use temporary repositories for repeatable learning. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 19 OF 20 · Transfer with judgment

19. Untrusted email and patches require review

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mailinfo parses content; it does not prove identity, intent, safety or correctness. That is the invariant to preserve when names, data or surrounding project details change.

A syntactically valid patch can still be malicious or inappropriate. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is treating successful extraction as permission to apply or run code. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git diff --no-index /dev/null out.patch

Explained result. Extraction supplies evidence, not trust. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace source provenance, extracted scope, review and only then any authorised apply step. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Untrusted email and patches require review”, separate contract from coincidence. The contract is “mailinfo parses content; it does not prove identity, intent, safety or correctness.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “Review metadata, patch scope and repository policy before any git am or execution step.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Untrusted email and patches require review” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo parses content; it does not prove identity, intent, safety or correctness.” should remain invariant and what observation would expose treating successful extraction as permission to apply or run code.

Reasoned route. Begin with “mailinfo parses content; it does not prove identity, intent, safety or correctness.” Apply this procedure: Review metadata, patch scope and repository policy before any git am or execution step. For the homework evidence folder, introduce one controlled variation that exposes treating successful extraction as permission to apply or run code. Extraction supplies evidence, not trust. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Untrusted email and patches require review” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo parses content; it does not prove identity, intent, safety or correctness.” should remain invariant and what observation would expose treating successful extraction as permission to apply or run code.

Reasoned route. Begin with “mailinfo parses content; it does not prove identity, intent, safety or correctness.” Apply this procedure: Review metadata, patch scope and repository policy before any git am or execution step. For the CCA media catalogue, introduce one controlled variation that exposes treating successful extraction as permission to apply or run code. Extraction supplies evidence, not trust. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Untrusted email and patches require review” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo parses content; it does not prove identity, intent, safety or correctness.” should remain invariant and what observation would expose treating successful extraction as permission to apply or run code.

Reasoned route. Begin with “mailinfo parses content; it does not prove identity, intent, safety or correctness.” Apply this procedure: Review metadata, patch scope and repository policy before any git am or execution step. For the science investigation archive, introduce one controlled variation that exposes treating successful extraction as permission to apply or run code. Extraction supplies evidence, not trust. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Untrusted email and patches require review” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo parses content; it does not prove identity, intent, safety or correctness.” should remain invariant and what observation would expose treating successful extraction as permission to apply or run code.

Reasoned route. Begin with “mailinfo parses content; it does not prove identity, intent, safety or correctness.” Apply this procedure: Review metadata, patch scope and repository policy before any git am or execution step. For the revision-note builder, introduce one controlled variation that exposes treating successful extraction as permission to apply or run code. Extraction supplies evidence, not trust. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals treating successful extraction as permission to apply or run code.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: Review metadata, patch scope and repository policy before any git am or execution step.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Untrusted email and patches require review”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests mailinfo parses content; it does not prove identity, intent, safety or correctness.?”, and “Can you make a smaller example of treating successful extraction as permission to apply or run code?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. List review questions for a patch touching scripts and configuration. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Check sender verification, changed paths, commands, tests and policy. The final output alone earns only partial credit. A strong answer connects it to “mailinfo parses content; it does not prove identity, intent, safety or correctness.”, shows the relevant intermediate evidence and states why treating successful extraction as permission to apply or run code would lead to a different conclusion.

Decision and transfer

Keep parsing and execution separated by human or policy review. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Untrusted email and patches require review” means the learner can predict the ordinary case, recognise treating successful extraction as permission to apply or run code, diagnose the earliest mistaken assumption, recover with “Review metadata, patch scope and repository policy before any git am or execution step.” and defend this decision: Keep parsing and execution separated by human or policy review. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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CHAPTER 20 OF 20 · Transfer with judgment

20. Choose the command by the workflow

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mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing. That is the invariant to preserve when names, data or surrounding project details change.

The right boundary reduces accidental repository mutation and custom parsing. The useful question is not “Which line should I copy?” but “Which state, value or rendering decision does this operation control?” Answering that question prevents a correct-looking result from hiding a wrong mental model.

The most revealing near-miss is using mailinfo for every email merely because it understands MIME. A learner should predict the result first, run the smallest safe case, label the observation and explain the earliest point where the prediction diverges. This turns an error into evidence rather than a verdict on ability.

Core worked example

git mailinfo msg patch < one.eml

Explained result. The selected tool performs no more authority or mutation than necessary. The explanation should identify the input, the governing rule, the observable outcome and one boundary that would change the conclusion. If any of those four parts is missing, reduce the example and trace it again.

Read the trace, not just the last line

Trace input kind, desired artifacts and allowed side effects. Write the trace as a sequence of labelled states. For Git mailinfo, the decisive evidence may be an iterator close, a cached classification, a glyph-spacing decision or a repository object path; printing only the final Boolean or filename can hide the mechanism that produced it.

For “Choose the command by the workflow”, separate contract from coincidence. The contract is “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” and comes from the primary specification or manual. Coincidence is what happened on one machine, font, filesystem or repository. Record both, then use “State whether the job is inspection, patch application or general email analysis.” to decide whether another observation challenges the model or merely reflects the environment.

Four purposeful transfer cases

Case 1: homework evidence folder. Model “Choose the command by the workflow” with subject folders, draft answers and a reviewed copy. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” should remain invariant and what observation would expose using mailinfo for every email merely because it understands MIME.

Reasoned route. Begin with “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” Apply this procedure: State whether the job is inspection, patch application or general email analysis. For the homework evidence folder, introduce one controlled variation that exposes using mailinfo for every email merely because it understands MIME. The selected tool performs no more authority or mutation than necessary. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 2: CCA media catalogue. Contrast “Choose the command by the workflow” with event folders, selected images and caption notes. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” should remain invariant and what observation would expose using mailinfo for every email merely because it understands MIME.

Reasoned route. Begin with “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” Apply this procedure: State whether the job is inspection, patch application or general email analysis. For the CCA media catalogue, introduce one controlled variation that exposes using mailinfo for every email merely because it understands MIME. The selected tool performs no more authority or mutation than necessary. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 3: science investigation archive. Stress-test “Choose the command by the workflow” with raw readings, cleaned tables and chart exports. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” should remain invariant and what observation would expose using mailinfo for every email merely because it understands MIME.

Reasoned route. Begin with “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” Apply this procedure: State whether the job is inspection, patch application or general email analysis. For the science investigation archive, introduce one controlled variation that exposes using mailinfo for every email merely because it understands MIME. The selected tool performs no more authority or mutation than necessary. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Case 4: revision-note builder. Explain “Choose the command by the workflow” with topic notes, worked examples and a generated index. Before touching a tool, state which part of the case corresponds to the input, why “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” should remain invariant and what observation would expose using mailinfo for every email merely because it understands MIME.

Reasoned route. Begin with “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.” Apply this procedure: State whether the job is inspection, patch application or general email analysis. For the revision-note builder, introduce one controlled variation that exposes using mailinfo for every email merely because it understands MIME. The selected tool performs no more authority or mutation than necessary. A complete answer explains why the near-miss fails, names the evidence that distinguishes it from the corrected case and transfers the rule without relying on the original variable names.

Diagnostic route

Model check: draw or list the exact values, objects, paths, characters or repository states before the operation.
Boundary check: construct the smallest safe input that reveals using mailinfo for every email merely because it understands MIME.
Evidence check: distinguish a displayed result from ownership, caching, consumption, font behaviour or object type.
Repair check: State whether the job is inspection, patch application or general email analysis.
Transfer check: repeat the rule in a second context and name what remains unchanged.

A parent does not need to memorise Git mailinfo to support “Choose the command by the workflow”. Useful prompts are: “What did you expect?”, “Which state changed first?”, “What evidence tests mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.?”, and “Can you make a smaller example of using mailinfo for every email merely because it understands MIME?” These questions keep ownership with the learner while making the reasoning visible.

Practice with an explained answer

Task. Choose tools for inspecting one patch email, applying a reviewed series and archiving newsletters. Write the prediction before running anything. Include one ordinary case and one boundary case, then explain the difference in two causal sentences.

Answer route. Use mailinfo, git am and a mail parser respectively. The final output alone earns only partial credit. A strong answer connects it to “mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing.”, shows the relevant intermediate evidence and states why using mailinfo for every email merely because it understands MIME would lead to a different conclusion.

Decision and transfer

Prefer the narrowest tool that matches the authorised job. Prefer the choice whose semantics and failure modes are easiest to explain for the actual job. Shorter syntax is useful only when it preserves that clarity.

Mastery of “Choose the command by the workflow” means the learner can predict the ordinary case, recognise using mailinfo for every email merely because it understands MIME, diagnose the earliest mistaken assumption, recover with “State whether the job is inspection, patch application or general email analysis.” and defend this decision: Prefer the narrowest tool that matches the authorised job. Once that causal chain is stable, continue to the next chapter rather than adding repetitions of the same surface exercise.

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Parent guide: choose the next useful step

For Git mailinfo, start with evidence rather than a label such as careless. Ask for one prediction and one trace. If the first transition is wrong, rebuild the model. If the model is sound but reference use is weak, practise finding the governing contract. If routine cases work but boundaries fail, vary one condition. If explanations transfer, move to a small project.

Punggol family routines are often busy, so use one complete Git mailinfo prediction–observation–explanation cycle while attention is good. Ten to twenty focused minutes can be enough. Keep a four-line record: concept, prediction, observed difference and next test. Stop when fatigue replaces reasoning.

Use disposable fixtures, temporary directories and test repositories. Backups remain necessary. The activities in this guide are proposed home-learning exercises, not advertised lesson times, branch facilities, school affiliations or outcome guarantees.

Capstone practice with explained routes

1. Separate all outputs

Run mailinfo on a controlled email and label stdin, stdout, msg and patch. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: git mailinfo reads one email from standard input, writes a message file and patch file, and emits selected metadata on standard output. Apply: Name stdin, msg, patch and stdout destinations before running. Verify: The two positional files hold extracted bodies while stdout carries author and subject fields. Then connect the result to the message and patch distinctions in Chapters 3–5. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

2. Compare subject policies

Test default, -k and -b on subjects with PATCH and project markers. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: Without -k, mailinfo trims whitespace, removes leading Re-like markers and leading bracketed strings, then normalises whitespace. Apply: Compare default output with -k using the same saved email. Verify: The ordinary extracted title is cleaned for commit use. Then connect the result to the plumbing role in Chapter 2. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

3. Trace encoding boundaries

Explain which outputs can be recoded and which patch bytes remain as-is. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: The -u behaviour minimally decodes MIME transfer encoding and recodes commit message and author metadata to i18n.commitEncoding, defaulting to UTF-8. Apply: Record source charset and repository commit encoding in the fixture. Verify: Metadata is recoded under the default route, subject to documented conversion. Then connect the result to the explicit encoding choices in Chapters 11–12. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

4. Preserve discussion identity

Use a fixture with Message-ID and inspect the appended commit text. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: The -m or –message-id option copies the Message-ID header to the end of the commit message. Apply: Inspect the appended line and treat it as a reference, not a trust proof. Verify: The message file gains the email’s identifier when present. Then connect the result to the trust warning in Chapter 20. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

5. Test scissors safely

Compare scissors and no-scissors without applying the patch. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: With –scissors, body content before a scissors line is removed, including the line itself, when it precedes the patch. Apply: Put a documented scissors marker in a disposable email and compare with –no-scissors. Verify: The extracted message begins after the cut boundary while the patch remains available. Then connect the result to the configuration override in Chapter 16. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

6. Handle quoted CR

Predict nowarn, warn and strip on one controlled MIME fixture. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: –quoted-cr accepts nowarn, warn or strip when decoded base64 or quoted-printable lines end in CRLF instead of LF. Apply: Build a controlled quoted-printable fixture and choose an action based on downstream needs. Verify: nowarn leaves the condition alone, warn reports it, and strip converts CRLF to LF. Then connect the result to the encoding route in Chapter 9. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

7. Build a round trip

Generate a one-commit format-patch email and inspect extracted outputs. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: git format-patch can produce a controlled patch email that mailinfo then separates for inspection. Apply: Create a disposable repository and one harmless commit, then generate one email without applying it. Verify: The extracted outputs can be compared with the known commit and patch. Then connect the result to the diagnostic discipline in Chapter 17. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

8. Defend the tool choice

Choose mailinfo, git am or a mail parser for three jobs. Include an ordinary case, a boundary, a deliberate failure and a recovery. Write the expected state before each operation.

Explained route. Start with: mailinfo is for extracting one email; git am manages applying emailed patches; general mail tools are better for unrelated MIME processing. Apply: State whether the job is inspection, patch application or general email analysis. Verify: The selected tool performs no more authority or mutation than necessary. Then connect the result to the no-apply boundary in Chapter 2. A complete solution contains the input model, a trace, observed evidence, a correction and one transfer statement.

Frequently asked questions

How long should a practice session be?

Use one complete Git mailinfo prediction–observation–explanation cycle while attention remains good. Ten to twenty focused minutes can be enough.

Should every option or method be memorised?

No. For Git mailinfo, memorise governing distinctions and practise retrieving the primary reference. Understanding means predicting and explaining, not reciting a parameter list.

What if the result is correct but the explanation is weak?

Treat it as partial success. Ask for a trace and change one boundary. A reliable model survives controlled variation.

Is the shortest solution the best?

Not automatically. Prefer the solution whose semantics, failure modes and maintenance cost are easiest to justify for the actual project.

When should the official reference be used?

Use it whenever Git mailinfo syntax, supported values, version behaviour or implementation boundaries matter. The primary source settles the current contract.

How can a parent help without technical expertise?

Ask what was predicted, where the first difference appeared, what evidence matters and which smaller example could isolate it.

How do we test transfer?

Change the context, vocabulary and one boundary. Require the learner to identify the invariant before using a tool.

What should be saved after practice?

Keep the corrected rule, one trace, one boundary case and the next question. Avoid storing pages of unexplained output.

Can these exercises replace backups?

No. Use disposable examples and proper backups. Learning should not endanger schoolwork, repositories or personal data.

What counts as mastery?

The learner can predict, verify, diagnose, recover and justify a choice across more than one context, while knowing when to consult the current reference.

Primary and supporting references

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Transfer with judgment

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Build the model

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Transfer with judgment

Complete chapter index

1. One email produces three output channels

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

2. mailinfo is normally plumbing beneath git am

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

3. The first positional path receives the commit message

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

4. The second positional path receives the patch

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

5. Standard output carries author and subject fields

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

6. Default subject cleanup removes email cruft

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

7. -k keeps the subject from being munged

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

8. -b limits bracket stripping to PATCH markers

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

9. -u is the current default metadata recoding route

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

10. The patch is not charset-converted

Core worked example

Read the trace, not just the last line

Four purposeful transfer cases

Diagnostic route

Practice with an explained answer

Decision and transfer

11. –encoding chooses the target metadata encoding