pre-commit hook in Git - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pre-commit hook in Git?

A pre-commit hook is a script that runs automatically before a commit is saved. It helps check or change files to keep code quality before saving changes.

beginner

Where do you place a pre-commit hook script in a Git project?

You place the pre-commit hook script inside the .git/hooks/ folder of your project, naming it pre-commit and making it executable.

intermediate

What happens if a pre-commit hook script exits with a non-zero status?

Git stops the commit process and shows the error. This prevents bad or unwanted changes from being saved.

beginner

Give an example of a simple pre-commit hook use case.

A pre-commit hook can check if code files have no syntax errors or if commit messages follow a style guide before allowing the commit.

beginner

How can you make a pre-commit hook script executable on Unix systems?

Run the command chmod +x .git/hooks/pre-commit to give execute permission to the script.

What is the main purpose of a pre-commit hook in Git?

A To delete old commits

B To merge branches automatically

C To push changes to remote

D To run checks before saving a commit

A pre-commit hook runs scripts before a commit is saved to check or modify files.

Where is the pre-commit hook script located in a Git project?

A config/pre-commit

B root/pre-commit

C .git/hooks/pre-commit

D src/hooks/pre-commit

Git looks for hook scripts inside the .git/hooks directory.

What happens if a pre-commit hook script fails (exits with error)?

A Commit is stopped

B Commit proceeds anyway

C Git deletes the commit

D Git pushes the commit automatically

A failing pre-commit hook stops the commit to prevent bad changes.

Which command makes a pre-commit hook script executable on Unix?

A git enable pre-commit

B chmod +x .git/hooks/pre-commit

C chmod 777 pre-commit

D git hook activate pre-commit

chmod +x sets execute permission needed for the hook script to run.

Which of these is a common use for a pre-commit hook?

A Check code style before commit

B Automatically merge branches

C Create new branches

D Delete remote branches

Pre-commit hooks often check code style or run tests before committing.

Explain what a pre-commit hook is and why it is useful in Git.

Think about what happens before you save your work in Git.

Describe the steps to create and enable a simple pre-commit hook script in a Git project.

Focus on file location, permissions, and testing.

Practice

1. What is the main purpose of a pre-commit hook in Git?

easy

A. To push commits to the remote repository automatically

B. To run scripts before a commit is saved to catch errors early

C. To delete files from the repository before committing

D. To automatically merge branches after a commit

Solution

1. Step 1: Understand what pre-commit hooks do

   Pre-commit hooks run scripts before a commit is finalized to check code or enforce rules.

2. Step 2: Identify the correct purpose

   They help catch errors early by running checks before saving changes.

3. Final Answer:

   To run scripts before a commit is saved to catch errors early -> Option B

4. Quick Check:

   Pre-commit hook purpose = run scripts before commit [OK]

Hint: Pre-commit hooks run before saving commits to catch issues [OK]

Common Mistakes:

• Confusing pre-commit with post-commit hooks
• Thinking hooks automatically push commits
• Believing hooks delete files automatically

2. Which of the following is the correct way to make a pre-commit hook script executable on Linux?

easy

A. chmod +x .git/hooks/pre-commit

B. git enable pre-commit

C. chmod 000 .git/hooks/pre-commit

D. git hook run pre-commit

Solution

1. Step 1: Identify how to make scripts executable

   On Linux, chmod +x sets execute permission on a file.

2. Step 2: Apply to pre-commit hook script

   Running chmod +x .git/hooks/pre-commit makes the hook script executable so Git can run it.

3. Final Answer:

   chmod +x .git/hooks/pre-commit -> Option A

4. Quick Check:

   Make hook executable = chmod +x [OK]

Hint: Use chmod +x to make hook scripts executable [OK]

Common Mistakes:

• Using git commands instead of chmod for permissions
• Setting permissions to 000 disables execution
• Assuming hooks run without executable permission

3. Given this pre-commit hook script content:

#!/bin/sh
if git diff --cached --name-only | grep -q '\.py$'; then
  echo 'Python files changed'
else
  echo 'No Python files changed'
fi

What will be the output if you commit a changed file named app.js?

medium

A. No Python files changed

B. Python files changed

C. Syntax error in script

D. Hook will not run

Solution

1. Step 1: Understand the script logic

   The script checks staged files for names ending with '.py'. If found, it prints 'Python files changed'. Otherwise, it prints 'No Python files changed'.

2. Step 2: Apply to the committed file

   The committed file is 'app.js', which does not end with '.py', so the else branch runs.

3. Final Answer:

   No Python files changed -> Option A

4. Quick Check:

   File extension check = no match = else output [OK]

Hint: Check file extensions in staged files to predict output [OK]

Common Mistakes:

• Assuming any changed file triggers 'Python files changed'
• Confusing staged vs unstaged files
• Ignoring the file extension check

4. You wrote a pre-commit hook script but it does not run when committing. Which of these is the most likely cause?

medium

A. The script is placed outside the .git/hooks directory

B. The script is named pre-commit.sh instead of pre-commit

C. All of the above

D. The script is not executable

Solution

1. Step 1: Check script permissions

   If the script is not executable, Git cannot run it.

2. Step 2: Check script name and location

   The script must be named exactly 'pre-commit' and placed inside the '.git/hooks' folder to be recognized.

3. Step 3: Combine all issues

   Any of these problems will prevent the hook from running, so all are valid causes.

4. Final Answer:

   All of the above -> Option C

5. Quick Check:

   Hook runs only if name, location, and permissions are correct [OK]

Hint: Check name, location, and execute permission for hooks [OK]

Common Mistakes:

• Ignoring file permissions
• Naming script with extensions like .sh
• Placing script outside .git/hooks folder

5. You want a pre-commit hook that prevents commits if any staged file contains the word 'TODO'. Which script snippet correctly implements this?

hard

A. #!/bin/sh
git commit -m 'Check TODOs' || exit 1

B. #!/bin/sh
grep -r 'TODO' . && echo 'TODO found' && exit 1

C. #!/bin/sh
git status | grep 'TODO' && exit 1

D. #!/bin/sh
git diff --cached | grep -q 'TODO' && echo 'Remove TODOs before commit' && exit 1

Solution

1. Step 1: Understand the goal

   The hook must check staged changes for the word 'TODO' and stop commit if found.

2. Step 2: Analyze each option

   #!/bin/sh git diff --cached | grep -q 'TODO' && echo 'Remove TODOs before commit' && exit 1 uses git diff --cached to check staged changes and exits with error if 'TODO' is found, which is correct.
   #!/bin/sh grep -r 'TODO' . && echo 'TODO found' && exit 1 searches all files recursively, not just staged changes.
   #!/bin/sh git commit -m 'Check TODOs' || exit 1 tries to run a commit inside the hook, which is incorrect.
   #!/bin/sh git status | grep 'TODO' && exit 1 checks git status output, which does not show file contents.

3. Final Answer:

   #!/bin/sh git diff --cached | grep -q 'TODO' && echo 'Remove TODOs before commit' && exit 1 -> Option D

4. Quick Check:

   Check staged diff for TODO and exit if found [OK]

Hint: Use git diff --cached to scan staged changes for keywords [OK]

Common Mistakes:

• Searching unstaged files instead of staged
• Running git commit inside hook
• Checking git status output for content