commit-msg hook for message validation in Git - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a commit-msg hook in Git?

A commit-msg hook is a script that runs automatically after a commit message is entered but before the commit is finalized. It checks the commit message to ensure it meets certain rules or formats.

beginner

Where do you place a commit-msg hook script in a Git repository?

You place the commit-msg hook script inside the .git/hooks/ directory of your repository, and it must be executable.

intermediate

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

If the commit-msg hook script exits with a non-zero status, Git stops the commit process and shows an error message. This prevents the commit from being saved.

beginner

Give an example of a simple rule a commit-msg hook might enforce.

A simple rule could be that the commit message must start with a capital letter and be at least 10 characters long.

beginner

How can you test a commit-msg hook script?

You can test it by trying to make a commit with different messages and seeing if the hook allows or blocks the commit based on your rules.

Where is the commit-msg hook script located in a Git repository?

A .git/hooks/

B .git/commit-msg/

C root directory

D any directory

The commit-msg hook script must be placed inside the .git/hooks/ directory.

What does a non-zero exit code from a commit-msg hook script do?

A Automatically fixes the commit message

B Allows the commit to proceed

C Deletes the commit message

D Stops the commit and shows an error

A non-zero exit code stops the commit process and shows an error message.

Which of these is a valid use of a commit-msg hook?

A Run unit tests

B Check commit message format

C Push to remote repository

D Merge branches

The commit-msg hook is used to check the commit message format before committing.

How do you make a commit-msg hook script executable?

A No action needed

B Rename it to <code>commit-msg.sh</code>

C Run <code>chmod +x .git/hooks/commit-msg</code>

D Place it in the root directory

You must make the script executable with chmod +x.

What file does the commit-msg hook receive as input?

A The file containing the commit message

B The staged files

C The repository config file

D The last commit hash

The hook receives the path to the file containing the commit message as its first argument.

Explain how a commit-msg hook can help maintain commit message quality.

Think about how automatic checks can stop bad messages.

Describe the steps to create and enable a commit-msg hook for message validation.

Focus on script location, permissions, and testing.

Practice

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

easy

A. To check and validate the commit message before the commit is saved

B. To automatically push commits to the remote repository

C. To merge branches after a commit

D. To delete old commits from history

Solution

1. Step 1: Understand the role of Git hooks

   Git hooks are scripts that run at specific points in Git's workflow to automate tasks or checks.

2. Step 2: Identify the function of the commit-msg hook

   The commit-msg hook runs after a commit message is entered but before the commit is finalized, allowing validation of the message.

3. Final Answer:

   To check and validate the commit message before the commit is saved -> Option A

4. Quick Check:

   commit-msg hook = validate commit message [OK]

Hint: commit-msg hook validates commit messages before saving [OK]

Common Mistakes:

• Confusing commit-msg hook with pre-push or post-commit hooks
• Thinking it automatically pushes commits
• Assuming it modifies commit history

2. Which of the following is the correct way to make a commit-msg hook executable on a Unix-like system?

easy

A. chmod 000 .git/hooks/commit-msg

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

C. git commit --hook commit-msg

D. git hook enable commit-msg

Solution

1. Step 1: Understand file permissions for hooks

   Git hooks are scripts stored in the .git/hooks directory and must be executable to run.

2. Step 2: Identify the correct command to make a file executable

   The command chmod +x adds execute permission to the file, enabling the hook to run.

3. Final Answer:

   chmod +x .git/hooks/commit-msg -> Option B

4. Quick Check:

   chmod +x = make executable [OK]

Hint: Use chmod +x to enable hook execution [OK]

Common Mistakes:

• Using chmod 000 which removes all permissions
• Trying git commands that don't exist for hooks
• Not setting executable permission, so hook won't run

3. Given this commit-msg hook script snippet, what will happen if the commit message is empty?

#!/bin/sh
if [ -z "$1" ]; then
  echo "Error: Commit message is empty." >&2
  exit 1
fi

medium

A. The commit will be blocked with an error message

B. The commit message will be replaced with a default message

C. The commit will proceed without any message

D. The script will ignore the message and exit with 0

Solution

1. Step 1: Analyze the script condition

   The script checks if the first argument $1 is empty using [ -z "$1" ]. In the commit-msg hook, $1 is the path to a temporary file containing the proposed commit message.

2. Step 2: Understand Git's behavior

   Since $1 is the file path, it is never empty, so the condition [ -z "$1" ] is false. However, the script as written incorrectly checks the variable $1 instead of the content of the file. To check if the commit message is empty, the script should check the content of the file, e.g., if [ ! -s "$1" ] or if [ -z "$(cat $1)" ]. As is, the script will not block empty commit messages.

3. Final Answer:

   The commit will proceed without any message -> Option C

4. Quick Check:

   $1 is file path, not content; checking it for emptiness is incorrect [OK]

Hint: $1 is file path, not content; check with `cat $1` [OK]

Common Mistakes:

• Thinking exit 1 allows commit to proceed
• Assuming the message is replaced automatically
• Confusing $1 with the commit message text instead of file path

4. You wrote this commit-msg hook to reject messages shorter than 10 characters:

#!/bin/sh
MSG=$(cat "$1")
if [ ${#MSG} -lt 10 ]; then
  echo "Commit message too short." >&2
  exit 1
fi

But it does not block short messages. What is the likely problem?

medium

A. The hook script is not executable

B. The commit message file path is incorrect

C. The shell does not support ${#MSG} syntax for string length

D. The script should use -z instead of -lt

Solution

1. Step 1: Check the script logic

   The script correctly reads the commit message and checks its length using ${#MSG} which is valid in sh/bash.

2. Step 2: Consider common hook issues

   If the hook is not executable, Git will ignore it and allow commits without running the script.

3. Final Answer:

   The hook script is not executable -> Option A

4. Quick Check:

   Non-executable hook = script not run [OK]

Hint: Make sure hook script is executable with chmod +x [OK]

Common Mistakes:

• Assuming ${#MSG} is invalid in shell
• Confusing file path with message content
• Using wrong test operators like -z for length comparison

5. You want to enforce commit messages to start with a ticket number like PROJ-123: followed by a space and description. Which commit-msg hook snippet correctly validates this pattern?

hard

A. #!/bin/sh
MSG=$(cat "$1")
if echo "$MSG" | grep -qE '^PROJ-[a-z]+: .+'; then
echo "Invalid ticket format" >&2
exit 1
fi

B. #!/bin/sh
MSG=$(cat "$1")
if echo "$MSG" | grep -q '^PROJ-[0-9]+: .+'; then
exit 1
fi

C. #!/bin/sh
if [ -z "$1" ]; then
echo "Empty message" >&2
exit 0
fi

D. #!/bin/sh
MSG=$(cat "$1")
if ! echo "$MSG" | grep -qE '^PROJ-[0-9]+: .+'; then
echo "Commit message must start with 'PROJ-<number>: '" >&2
exit 1
fi

Solution

1. Step 1: Understand the required pattern

   The commit message must start with 'PROJ-' followed by digits, a colon, a space, then some text.

2. Step 2: Analyze each option's regex and logic

   #!/bin/sh MSG=$(cat "$1") if ! echo "$MSG" | grep -qE '^PROJ-[0-9]+: .+'; then echo "Commit message must start with 'PROJ-<number>: '" >&2 exit 1 fi uses grep -qE with '^PROJ-[0-9]+: .+' and exits with error if pattern not matched, which is correct. #!/bin/sh MSG=$(cat "$1") if echo "$MSG" | grep -q '^PROJ-[0-9]+: .+'; then exit 1 fi exits on match (wrong logic). #!/bin/sh if [ -z "$1" ]; then echo "Empty message" >&2 exit 0 fi exits 0 on empty message (wrong). #!/bin/sh MSG=$(cat "$1") if echo "$MSG" | grep -qE '^PROJ-[a-z]+: .+'; then echo "Invalid ticket format" >&2 exit 1 fi checks for lowercase letters after PROJ- which is incorrect.

3. Final Answer:

   #!/bin/sh MSG=$(cat "$1") if ! echo "$MSG" | grep -qE '^PROJ-[0-9]+: .+'; then echo "Commit message must start with 'PROJ-<number>: '" >&2 exit 1 fi -> Option D

4. Quick Check:

   grep -qE '^PROJ-[0-9]+: .+' = valid pattern check [OK]

Hint: Use grep -qE with ^PROJ-[0-9]+: .+ to validate pattern [OK]

Common Mistakes:

• Reversing logic: exiting on match instead of no match
• Using wrong regex for digits or colon
• Exiting with 0 on error conditions