git blame for line-by-line history - Step-by-Step Execution

Process Flow - git blame for line-by-line history

Start: Select file

↓

Run git blame

↓

Git finds last commit for each line

↓

Display line with commit info

↓

User reads who changed each line and when

↓

End

The flow shows how git blame takes a file, finds the last commit for each line, and displays that info line-by-line.

Execution Sample

Git

git blame example.txt

Shows who last changed each line in example.txt with commit info.

Process Table

Step	Action	Input/Line	Git Operation	Output/Result
1	Select file to blame	example.txt	Prepare file for blame	File example.txt ready
2	Run git blame	git blame example.txt	Git reads file lines	Starts processing lines
3	Process line 1	Line 1 content	Find last commit touching line 1	Commit abc123 by Alice, date
4	Process line 2	Line 2 content	Find last commit touching line 2	Commit def456 by Bob, date
5	Process line 3	Line 3 content	Find last commit touching line 3	Commit abc123 by Alice, date
6	Display results	All lines processed	Show line with commit info	Lines shown with commit, author, date
7	End	-	-	User sees full line-by-line history

💡 All lines processed and annotated with last commit info

Status Tracker

Variable	Start	After Line 1	After Line 2	After Line 3	Final
current_line	0	1	2	3	3
commit_info	{}	{line1: abc123, Alice}	{line1: abc123, Alice; line2: def456, Bob}	{line1: abc123, Alice; line2: def456, Bob; line3: abc123, Alice}	{line1: abc123, Alice; line2: def456, Bob; line3: abc123, Alice}

Key Moments - 3 Insights

Why does git blame show different commit IDs for different lines in the same file?

Because git blame finds the last commit that changed each specific line, so different lines can have different commit histories as shown in steps 3-5 of the execution table.

What if a line was never changed after the first commit?

Git blame will show the original commit that introduced that line, as seen in line 1 and 3 where commit abc123 appears, meaning those lines were last changed in that commit.

Does git blame modify the file or repository?

No, git blame only reads the history and shows info; it does not change the file or repository state, confirmed by the final step in the execution table.

Visual Quiz - 3 Questions

Look at the execution table, what commit does line 2 belong to?

A abc123 by Alice

B def456 by Bob

C xyz789 by Carol

D No commit found

💡 Hint

Check step 4 in the execution table where line 2 is processed.

At which step does git blame finish processing all lines?

A Step 6

B Step 5

C Step 7

D Step 3

💡 Hint

Look for the step where results are displayed after processing all lines.

If a new line is added and committed, how would the variable 'current_line' change?

A It would stay the same

B It would decrease

C It would increase by 1

D It would reset to 0

💡 Hint

Refer to the variable_tracker showing current_line increments after each line processed.

Concept Snapshot

git blame syntax: git blame <file>
Shows last commit info for each line in the file.
Each line is annotated with commit ID, author, and date.
Does not change file or repo, only reads history.
Useful to find who last changed a specific line.

Full Transcript

Git blame is a command that shows who last changed each line in a file. The process starts by selecting the file, then running git blame on it. Git reads each line and finds the last commit that modified that line. It then displays the file lines with commit ID, author, and date next to each line. This helps users understand the history of changes line-by-line without modifying the file or repository. The execution table shows each step from selecting the file to displaying the results. Variables like current_line and commit_info track progress and data collected. Key moments clarify why different lines have different commits and confirm that git blame is read-only. The visual quiz tests understanding of commit assignments, processing steps, and variable changes. The snapshot summarizes the command usage and behavior for quick reference.

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