Why commit-msg hook for message validation in Git? - Purpose & Use Cases

The Big Idea

What if every commit message told a clear story without you lifting a finger?

The Scenario

Imagine you and your team are working on a project, and everyone writes commit messages in their own style. Some messages are clear, others are vague or missing important details. When you try to understand the history later, it feels like reading a messy diary with no order.

The Problem

Manually checking each commit message is slow and easy to forget. Mistakes slip through, causing confusion and wasted time when tracking changes or fixing bugs. It's like proofreading a long essay without any tools--tiring and error-prone.

The Solution

The commit-msg hook automatically checks every commit message before it's saved. It makes sure messages follow rules you set, like including a ticket number or a clear description. This keeps your project history neat and easy to understand without extra effort.

Before vs After

✗ Before

git commit -m "fix stuff"

✓ After

git commit -m "PROJ-123: Fix login button alignment"

What It Enables

It enables a clean, consistent project history that everyone can trust and understand instantly.

Real Life Example

A team working on a website uses a commit-msg hook to require ticket IDs in messages. When a bug appears, they quickly find the related commit and fix it, saving hours of searching.

Key Takeaways

Manual commit messages can be inconsistent and confusing.

Commit-msg hooks automatically enforce message rules.

This leads to clearer project history and faster teamwork.

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