commit-msg hook for message validation in Git - Step-by-Step Execution

Process Flow - commit-msg hook for message validation

Start commit-msg hook

↓

Read commit message file

↓

Validate message format

↓

Allow commit

↓

Show error message

↓

Exit hook

The commit-msg hook runs automatically during commit. It reads the commit message, checks if it matches rules, then allows or rejects the commit.

Execution Sample

Git

#!/bin/sh
MSG_FILE=$1
MSG=$(cat "$MSG_FILE")

if echo "$MSG" | grep -qE '^JIRA-[0-9]+: .{10,}'
then
  exit 0
else
  echo "Error: Commit message must start with JIRA-<number>: and be at least 10 chars"
  exit 1
fi

This hook script checks if the commit message starts with 'JIRA-' followed by a number and a colon, then has at least 10 characters.

Process Table

Step	Action	Commit Message Content	Validation Result	Hook Exit Code	Output
1	Read commit message file	JIRA-123: Fix bug in login	N/A	N/A
2	Check pattern '^JIRA-[0-9]+: .{10,}'	JIRA-123: Fix bug in login	Matches pattern	N/A
3	Exit hook	N/A	Valid message	0
4	Commit allowed	N/A	N/A	N/A	Commit proceeds
5	Read commit message file	Fix login bug	N/A	N/A
6	Check pattern '^JIRA-[0-9]+: .{10,}'	Fix login bug	Does not match	N/A
7	Print error message	N/A	Invalid message	N/A	Error: Commit message must start with JIRA-<number>: and be at least 10 chars
8	Exit hook	N/A	Invalid message	1
9	Commit rejected	N/A	N/A	N/A	Commit aborted

💡 Hook exits with 0 if message valid, 1 if invalid, controlling commit success

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 6	Final
MSG_FILE	unset	'.git/COMMIT_EDITMSG'	'.git/COMMIT_EDITMSG'	'.git/COMMIT_EDITMSG'	'.git/COMMIT_EDITMSG'
MSG	unset	'JIRA-123: Fix bug in login' or 'Fix login bug'	'JIRA-123: Fix bug in login' or 'Fix login bug'	'Fix login bug'	'Fix login bug' or 'JIRA-123: Fix bug in login'
Validation Result	unset	N/A	Matches pattern or Does not match	Does not match	Matches pattern or Does not match
Exit Code	unset	N/A	0 or 1	1	0 or 1

Key Moments - 3 Insights

Why does the commit get rejected even if the message looks descriptive?

The hook requires the message to start with 'JIRA-' followed by a number and a colon, then at least 10 characters. Messages without this prefix fail validation as shown in rows 6-9 of the execution table.

What does exit code 0 or 1 mean in the hook?

Exit code 0 means the commit message passed validation and the commit proceeds (rows 3-4). Exit code 1 means validation failed and the commit is aborted (rows 8-9).

How does the hook read the commit message to validate it?

The hook reads the commit message file passed as the first argument ($1) and stores its content in MSG variable (row 1). It then applies the pattern check on MSG.

Visual Quiz - 3 Questions

Look at the execution table, what is the exit code when the commit message is 'JIRA-123: Fix bug in login'?

A 2

B 1

C 0

D No exit code

💡 Hint

Check row 3 in the execution table where the message matches the pattern and exit code is set.

At which step does the hook print an error message for an invalid commit message?

A Step 7

B Step 4

C Step 2

D Step 1

💡 Hint

Look for the step where output shows the error message in the execution table.

If the commit message was 'JIRA-45: Add tests', how would the validation result change?

A It would match the pattern and pass

B It would fail because message is too short

C It would fail because no JIRA number

D It would pass without checks

💡 Hint

Refer to the regex pattern requiring at least 10 characters after the prefix, see execution table row 2.

Concept Snapshot

commit-msg hook syntax:
#!/bin/sh
MSG_FILE=$1
MSG=$(cat "$MSG_FILE")

Validation:
Use regex to check MSG format
Exit 0 to allow commit
Exit 1 to reject commit

Purpose: enforce commit message rules automatically

Full Transcript

The commit-msg hook runs automatically when you make a git commit. It reads the commit message from a file passed as an argument. Then it checks if the message matches a pattern: it must start with 'JIRA-' followed by a number and a colon, and have at least 10 characters. If the message matches, the hook exits with code 0, allowing the commit. If not, it prints an error message and exits with code 1, stopping the commit. This helps keep commit messages consistent and meaningful.