git blame for line-by-line history - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify the commit hash from git blame output

You run git blame -L 10,10 README.md and see this output:

e3a1b2c4 (Alice 2024-05-01 10:15:30 +0000 10) Fix typo in introduction

What is the commit hash for the change on line 10?

A Alice

B e3a1b2c4

C 2024-05-01

D 10

💡 Hint

The commit hash is the first part before the author name.

The commit hash is the unique identifier for the commit and appears at the start of the git blame line.

🧠 Conceptual

intermediate

Purpose of git blame

What is the main purpose of the git blame command?

A To show who last modified each line of a file

B To undo the last commit

C To merge two branches

D To list all branches

💡 Hint

Think about tracking changes line by line.

git blame shows the author and commit info for each line in a file.

❓ Troubleshoot

advanced

Why does git blame show wrong author for a line?

You notice git blame shows an unexpected author for a line you recently changed. What could cause this?

A You forgot to stage the file before committing

B You ran <code>git blame</code> on a different branch

C The line was copied from another file without changes

D Your local git config has wrong user.email

💡 Hint

Think about how git tracks line history when lines are copied.

If a line is copied from another file or commit without changes, git blame attributes it to the original author.

🔀 Workflow

advanced

Using git blame to find when a bug was introduced

You want to find when a bug was introduced in a file. Which git command sequence helps you use git blame effectively?

A Run <code>git blame</code> on the buggy line, then use <code>git show</code> on the commit hash

B Run <code>git log</code> to list commits, then <code>git merge</code> to combine fixes

C Run <code>git diff</code> between branches, then <code>git blame</code> on the whole file

D Run <code>git reset</code> to undo changes, then <code>git blame</code> to check authors

💡 Hint

First find the commit that last changed the line, then inspect that commit.

Using git blame on the line shows the commit hash that last changed it. Then git show reveals details of that commit.

✅ Best Practice

expert

Optimizing git blame output for large files

For a large file, running git blame is slow. Which option improves performance without losing line history?

A Use <code>git blame --show-name</code> to display file names

B Use <code>git blame --incremental</code> to get detailed output efficiently

C Use <code>git blame --reverse</code> to blame lines backwards

D Use <code>git blame --ignore-rev</code> with a commit hash to skip known refactors

💡 Hint

Skipping commits that only refactor code can speed up blame.

Using --ignore-rev skips commits that do not affect logic, improving blame speed and clarity.

Practice

1. What does the git blame command primarily show?

easy

A. Who last changed each line in a file

B. The current status of the repository

C. The list of branches in the repository

D. The commit history summary

Solution

1. Step 1: Understand the purpose of git blame

   git blame is used to show the author information for each line in a file, indicating who last modified that line.

2. Step 2: Compare with other git commands

   Other commands like git status, git branch, and git log serve different purposes, not line-by-line author tracking.

3. Final Answer:

   Who last changed each line in a file -> Option A

4. Quick Check:

   git blame = line author info [OK]

Hint: Remember: blame = who changed each line last [OK]

Common Mistakes:

• Confusing git blame with git log
• Thinking git blame shows branch info
• Assuming git blame shows file status

2. Which of the following is the correct syntax to show blame for a file named app.js?

easy

A. git blame --file app.js

B. git blame app.js

C. git blame --show app.js

D. git blame -f app.js

Solution

1. Step 1: Recall basic git blame syntax

   The basic syntax to run blame on a file is simply git blame <filename>. No extra flags are needed for a simple blame.

2. Step 2: Check the options given

   Options like -f, --show, or --file are not valid or required for basic blame usage.

3. Final Answer:

   git blame app.js -> Option B

4. Quick Check:

   Basic blame = git blame filename [OK]

Hint: Use just 'git blame filename' for simple blame [OK]

Common Mistakes:

• Adding unnecessary flags
• Using incorrect or unsupported options
• Confusing blame syntax with other git commands

3. Given the command git blame -L 10,15 README.md, what will it show?

medium

A. Blame info for lines 1 to 15 of README.md

B. Blame info for the whole README.md file

C. Blame info for lines 10 to 15 of README.md

D. An error because -L requires a commit hash

Solution

1. Step 1: Understand the -L option in git blame

   The -L option limits the blame output to a specific line range. Here, -L 10,15 means lines 10 through 15.

2. Step 2: Apply to the file README.md

   The command will show blame info only for lines 10 to 15 of the file README.md, not the entire file or any other range.

3. Final Answer:

   Blame info for lines 10 to 15 of README.md -> Option C

4. Quick Check:

   -L limits lines = lines 10-15 [OK]

Hint: -L start,end shows blame only for those lines [OK]

Common Mistakes:

• Assuming -L shows whole file blame
• Thinking -L needs a commit hash
• Confusing line numbers with byte offsets

4. You run git blame --since=2.weeks README.md but get an error. What is the likely cause?

medium

A. You need to specify a commit hash with --since

B. README.md file does not exist

C. Incorrect date format for --since option

D. git blame does not support --since option

Solution

1. Step 1: Check if git blame supports --since

   The git blame command does not have a --since option. This option is valid for git log but not for blame.

2. Step 2: Understand the error cause

   Using an unsupported option causes git blame to error out. The file existence or commit hash is unrelated here.

3. Final Answer:

   git blame does not support --since option -> Option D

4. Quick Check:

   git blame lacks --since option [OK]

Hint: git blame has no --since; use git log for date filters [OK]

Common Mistakes:

• Assuming git blame supports --since
• Blaming file existence without checking
• Thinking commit hash is mandatory with --since

5. You want to find who last changed line 42 in server.py but only for commits on the feature branch. Which command correctly achieves this?

hard

A. git blame feature -- server.py -L 42,42

B. git blame -L 42,42 server.py feature

C. git blame -L 42,42 -- server.py feature

D. git blame -L 42,42 --first-parent feature -- server.py

Solution

1. Step 1: Understand branch limitation in git blame

   To blame a file as it appears on a specific branch, specify the branch name before the -- separator, then the file name.

2. Step 2: Apply line range and branch correctly

   The correct syntax is git blame <branch> -- <file> -L <start,end>. So git blame feature -- server.py -L 42,42 limits blame to line 42 on the feature branch.

3. Final Answer:

   git blame feature -- server.py -L 42,42 -> Option A

4. Quick Check:

   Branch before --, file after, -L for lines [OK]

Hint: Put branch before --, file after, use -L for lines [OK]

Common Mistakes:

• Placing branch after file name
• Misordering -L option
• Using unsupported options like --first-parent with blame