Creating custom hook scripts in Git - Performance & Efficiency

Time Complexity: Creating custom hook scripts

O(n)

Understanding Time Complexity

When we create custom hook scripts in Git, we want to know how the time to run these scripts changes as the project grows.

We ask: How does running a hook script scale when the repository size or number of files increases?

Scenario Under Consideration

Analyze the time complexity of this simple pre-commit hook script.

#!/bin/sh
# pre-commit hook

# Check all staged files for TODO comments
files=$(git diff --cached --name-only)
for file in $files; do
  grep -q "TODO" "$file" && echo "TODO found in $file" && exit 1
 done
exit 0

This script checks each staged file for the word "TODO" and stops the commit if any are found.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping over each staged file and running a search inside it.
• How many times: Once for each staged file, so the number of files affects the count.

How Execution Grows With Input

As the number of staged files grows, the script checks more files one by one.

Input Size (n)	Approx. Operations
10 files	10 file checks
100 files	100 file checks
1000 files	1000 file checks

Pattern observation: The work grows directly with the number of files checked.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the hook grows linearly with the number of staged files.

Common Mistake

[X] Wrong: "The hook runs instantly no matter how many files are staged."

[OK] Correct: Each file adds work because the script checks them one by one, so more files mean more time.

Interview Connect

Understanding how hook scripts scale helps you write efficient checks that keep your workflow smooth as projects grow.

Self-Check

"What if the hook script also searched inside each file line by line? How would that affect the time complexity?"

Practice

1. What is the main purpose of creating a custom Git hook script?

easy

A. To delete branches remotely

B. To change the Git commit message manually

C. To create new Git repositories

D. To run scripts automatically at specific Git events

Solution

1. Step 1: Understand Git hooks

   Git hooks are scripts that run automatically at certain points in Git's workflow, like before a commit or after a push.

2. Step 2: Identify the purpose of custom hooks

   Custom hooks let you automate tasks or enforce rules during these Git events without manual intervention.

3. Final Answer:

   To run scripts automatically at specific Git events -> Option D

4. Quick Check:

   Git hooks = automatic scripts at Git events [OK]

Hint: Hooks run scripts automatically at Git events [OK]

Common Mistakes:

• Thinking hooks manually change commit messages
• Confusing hooks with branch deletion
• Assuming hooks create repositories

2. Which of the following is the correct way to make a Git hook script executable on Linux or macOS?

easy

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

B. git hook enable pre-commit

C. chmod 777 .git/hooks/pre-commit

D. git config hook.pre-commit true

Solution

1. Step 1: Understand file permissions for hooks

   Git hooks must be executable files to run automatically.

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

   The command chmod +x adds execute permission without changing other permissions.

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 that don't exist for hooks
• Setting overly permissive permissions like 777
• Trying to enable hooks via git config

3. Given this pre-commit hook script placed in .git/hooks/pre-commit and made executable:

#!/bin/sh
if git diff --cached --quiet; then
  echo "No changes staged for commit."
  exit 1
fi

What happens when you try to commit without staging any changes?

medium

A. The commit is blocked with message 'No changes staged for commit.'

B. The commit proceeds normally without any message

C. The commit is blocked with a syntax error

D. The commit proceeds but shows a warning

Solution

1. Step 1: Analyze the script logic

   The script checks if there are no staged changes using git diff --cached --quiet. If true, it prints a message and exits with code 1.

2. Step 2: Understand Git hook exit codes

   Exiting with code 1 in a pre-commit hook stops the commit process and shows the message.

3. Final Answer:

   The commit is blocked with message 'No changes staged for commit.' -> Option A

4. Quick Check:

   Exit 1 in pre-commit = block commit [OK]

Hint: Exit non-zero in pre-commit stops commit [OK]

Common Mistakes:

• Assuming commit proceeds despite exit 1
• Confusing staged vs unstaged changes
• Expecting syntax errors from correct shell script

4. You wrote this post-commit hook script but it does not run after commits:

#!/bin/sh
echo "Commit done!"

Which is the most likely reason?

medium

A. The script filename is incorrect

B. The script is not executable

C. Git hooks cannot echo messages

D. The script must be in the project root, not .git/hooks

Solution

1. Step 1: Check common hook issues

   Hooks must be executable files inside .git/hooks/ with correct names.

2. Step 2: Identify the most common cause

   Often, scripts don't run because they lack execute permission, even if named correctly.

3. Final Answer:

   The script is not executable -> Option B

4. Quick Check:

   Hooks must be executable to run [OK]

Hint: Make hook scripts executable with chmod +x [OK]

Common Mistakes:

• Placing scripts outside .git/hooks directory
• Assuming echo is not allowed in hooks
• Using wrong hook filenames

5. You want to create a custom Git hook that prevents commits if the commit message does not contain the word 'JIRA'. Which script placed in .git/hooks/commit-msg will achieve this?

hard

A. #!/bin/sh
if grep -q 'JIRA' "$1"; then
exit 1
fi

B. #!/bin/sh
grep 'JIRA' "$1" || echo "Commit message missing JIRA"

C. #!/bin/sh
if ! grep -q 'JIRA' "$1"; then
echo "Commit message must include 'JIRA'."
exit 1
fi

D. #!/bin/sh
echo "Checking commit message for JIRA"

Solution

1. Step 1: Understand commit-msg hook argument

   The commit message file path is passed as the first argument ($1) to the script.

2. Step 2: Check script logic for enforcing 'JIRA'

   #!/bin/sh if ! grep -q 'JIRA' "$1"; then echo "Commit message must include 'JIRA'." exit 1 fi uses grep -q to check if 'JIRA' is missing, then prints a message and exits with 1 to block the commit.

3. Step 3: Evaluate other options

   #!/bin/sh grep 'JIRA' "$1" || echo "Commit message missing JIRA" only echoes but does not block commit. #!/bin/sh if grep -q 'JIRA' "$1"; then exit 1 fi blocks if 'JIRA' is present (wrong logic). #!/bin/sh echo "Checking commit message for JIRA" just echoes without checks.

4. Final Answer:

   #!/bin/sh if ! grep -q 'JIRA' "$1"; then echo "Commit message must include 'JIRA'." exit 1 fi -> Option C

5. Quick Check:

   Exit 1 if 'JIRA' missing blocks commit [OK]

Hint: Use commit-msg hook with exit 1 to block bad messages [OK]

Common Mistakes:

• Not exiting with non-zero to block commit
• Checking for presence instead of absence
• Only printing message without blocking