How to Identify the First Good Commit in Git Without a Known Good Commit?

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In Git, finding the first good commit can be a challenging task, especially if you don't know an earlier commit that is stable. This situation can arise during development when a bug is introduced, and tracing it back becomes essential. In this article, we will discuss an effective script you can use to reverse loop through Git commits, testing each one until we identify the first good commit.

Understanding Git Bisect


Git bisect is a powerful tool that helps developers narrow down the commit that introduced a bug. You specify a bad commit (one that contains the bug) and a known good commit, and Git will guide you through testing each commit until the culprit is found. The command looks like this:

git bisect start <bad_commit> <good_commit>
git bisect run <test_command>


While this works well, what happens if you don’t know the good commit? In this case, utilizing a script to identify the first known good commit can be incredibly useful.

Why You Might Not Know a Good Commit


There are several reasons why you might not know which old commit is good:

• The last known state was not documented.
• The good state was from an external dependency that has been moved or deleted.
• Commits were made in rapid succession, making it hard to identify the stable point.

In these situations, you can write a Bash script that automates the process of checking each commit starting from the current HEAD, marking the first good one it encounters.

Bash Script to Find the First Good Commit


Let’s create a Bash script that will help in automatically testing each commit. This script will use git log to retrieve the commit hashes and will loop through them in reverse chronological order.

Step 1: Create the Script


Create a new Bash file with your preferred text editor, for instance, find_good_commit.sh and add the following content:

#!/bin/bash

# Function to test the commit
function test_commit {
if <test_condition>; then
echo "Good commit found: $1"
git checkout $1
exit 0 # Exit successfully
fi
}

# Start from HEAD and go backwards through commits
for commit in $(git rev-list HEAD --reverse); do
git checkout $commit
test_commit $commit
done

# If no good commit found
echo "No good commit found."
exit 1

Step 2: Modify to Include Your Testing Logic


The placeholder <test_condition> should be replaced with the logic that verifies whether the commit is good or bad. This could be a build command that needs to succeed, for instance:

if make; then


This modification would make the script check if the previous commit can be built successfully.

Step 3: Make the Script Executable


You need to grant execution permissions to the script. You can do that with:

chmod +x find_good_commit.sh

Step 4: Run the Script


To execute the script, just run:

./find_good_commit.sh


The script will iterate through each commit from the latest to the oldest one, checking each one as specified in your test_commit function. When it finds the first good commit, it will output the information and checkout that commit automatically.

Handling Edge Cases


Please note that this script assumes:

• The make command is your definitive test. Modify it according to your requirements.
• It’s ideal to run this script in a clean working directory to avoid conflicts.

Additional Considerations


Running this script could take a while depending on the number of commits in your repository. Ensure you have sufficient time and system resources for this.

Frequently Asked Questions

• What if my testing logic is complex? You can incorporate any logic you want in the test_commit function, just ensure it returns appropriate success status codes.
• Can I test multiple conditions? Absolutely! Just add more conditions in the test_commit function as needed.
• What if I run on a large repository? You might want to limit the number of commits you check by modifying the git rev-list command to include only a certain number of recent commits.

In conclusion, automating the process of identifying the first bad commit without knowing a good one can save you significant time and effort in troubleshooting. By leveraging Git commands and Bash scripting, you can create an effective solution tailored to your specific situation.

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