pre-commit hook in Git - Step-by-Step Execution

Process Flow - pre-commit hook

Make code changes

↓

Run 'git commit'

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Git triggers pre-commit hook script

↓

Hook script runs checks

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Commit

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Fix issues and retry

When you run 'git commit', Git runs the pre-commit hook script first. If the script passes, the commit happens. If it fails, the commit stops so you can fix problems.

Execution Sample

Git

#!/bin/sh
# pre-commit hook example

# Check for TODO comments
if git diff --cached | grep -q 'TODO'; then
  echo 'Error: TODO found in staged files.'
  exit 1
fi
exit 0

This pre-commit hook checks if any staged files contain 'TODO'. If yes, it stops the commit with an error.

Process Table

Step	Action	Command/Check	Result	Next Step
1	User runs 'git commit'	git commit	Triggers pre-commit hook	Run hook script
2	Hook script runs check	git diff --cached | grep 'TODO'	No TODO found	Allow commit
3	Commit proceeds	git commit completes	Commit saved	End
4	User runs 'git commit' again	git commit	Triggers pre-commit hook	Run hook script
5	Hook script runs check	git diff --cached | grep 'TODO'	TODO found	Abort commit
6	Commit aborted	exit 1	Commit stopped	User fixes TODO and retries

💡 Commit stops if TODO found; otherwise commit completes.

Status Tracker

Variable	Start	After Step 2	After Step 5	Final
TODO_found	undefined	false	true	true
Commit_status	not started	allowed	aborted	aborted or completed

Key Moments - 3 Insights

Why does the commit stop even though I typed 'git commit'?

The pre-commit hook script found a TODO in staged files (see execution_table step 5), so it exited with error to prevent committing unfinished code.

What happens if the pre-commit hook script passes all checks?

If no problems are found (execution_table step 2), the commit proceeds normally (step 3) and your changes are saved.

How can I fix a failed pre-commit hook?

Fix the issues reported by the hook (like removing TODO comments), stage the changes again, and run 'git commit' to retry (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the result of the hook check at step 2?

A TODO found, commit aborted

B No TODO found, commit allowed

C Hook script did not run

D Commit completed

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the commit get aborted due to a failed hook?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look for 'TODO found' and 'Abort commit' in the execution_table.

If the hook script always exits with 0, what will happen to the commit?

A Commit will always proceed

B Hook script will not run

C Commit will always be aborted

D Git will show an error

💡 Hint

Refer to the exit codes in the execution_table and how they affect commit status.

Concept Snapshot

pre-commit hook is a script Git runs before saving a commit.
It can check code or files and stop commit if problems found.
Place script in .git/hooks/pre-commit and make executable.
Exit 0 means pass; exit non-zero means fail and abort commit.
Useful to enforce code quality automatically.

Full Transcript

When you run 'git commit', Git first runs the pre-commit hook script if it exists. This script can check your staged files for issues like TODO comments. If the script finds a problem, it exits with an error code, stopping the commit so you can fix it. If no problems are found, the commit proceeds normally. This helps keep your code clean by preventing commits with unfinished or problematic code. You can create a pre-commit hook by adding an executable script named 'pre-commit' in the .git/hooks directory. The script should exit with 0 to allow commit or non-zero to stop it.