Validating commit messages with dart and Lefthook

Validating commit messages with dart and Lefthook
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Something I had been trying to work out for my Flutter project for some time was validating commit messages, initially, I used commit-msg-validate but having a node dependency in a Flutter/Dart project does not seem right and when the team grew to beyond myself I worked on moving all of the automation scripts I had written in JS over to dart and while there are still a few node dependencies left, they are purely for use in the CI pipeline.

For the purpose of executing git-hooks across the team, I added Lefthook to the project and initially only used pre-push for executing the test scripts, but only recently had an idea for how to execute the commit-msg hook.

The Hook

I found out that the commit message is stored in a plain text file COMMIT_EDITMSG inside the ./git folder, which would be easy to read up into a string within Dart.

I added a git_hook folder with a commit_message.dart file, which I could later run from Lefthook.

Within our project, and I suspect many at this point follow the semantic commit message approach, you can read more about that HERE.

Our commit message file looks like this:

*import *'dart:io';

*dynamic *main() {
  *final *rootDir = Directory.*current*;
  *final *commitFile = File(rootDir.path + "/.git/COMMIT_EDITMSG");
  *final *commitMessage = commitFile.readAsStringSync();

  *final *regExp = RegExp(

  *final *valid = regExp.hasMatch(commitMessage);
  *if *(!valid) exitCode = 1;

Let's break it down

*final *rootDir = Directory.*current*;
*final *commitFile = File(rootDir.path + "/.git/COMMIT_EDITMSG");
*final *commitMessage = commitFile.readAsStringSync();

The first few lines are quite simple, here we are getting the current directory and with that creating a File reference to COMMIT_EDITMSG, which is then read up to a string, which can be validated with the following Regular Expression.

*final *regExp = RegExp(

This validates our commit message format, which starts with a prefix, these are most important for the automation scripts used to generate our version number as well as the changelog.

The Expression


Each message has to begin with one of the above strings, after that we have our scope:


This is normally used as a module or feature reference, just to provide a small context on where the change is happening.

After that we have the optional space and Jira ticket references, all our Jira issues begin with DEV.


Finally, any characters after that,


In our case a valid commit message would look like this:

feat(video): [DEV-1223] added support for leading text
feat(video): added support for leading text

The Validation

After that, we can validate our commit message against our Regular Expression using:

*final *valid = regExp.hasMatch(commitMessage);
*if *(!valid) exitCode = 1;

If not valid, we simply set the exitCode to 1, anything other than 0 is an error, which would terminate the script and prevent you from commit with an incorrect message.

Adding in Lefthook

Now that our script is complete, we can update Lefthook so that this script can be executed whenever one of us attempts a git commit.

      run: flutter pub run ./git_hooks/commit_message.dart

After adding the above snippet to lefthook.yaml, one simply needs to run lefthook install to activate the hook.

Lefthook v0.7.2

EXECUTE > validate
The message: none(lefthook): added commit validation

SUMMARY: (done in 1.13 seconds)
✔️  validate

If everything has worked, after your next commit, assuming your message met the required format, you should see something similar to the above message.

I hope you found this interesting, and if you have any questions, comments, or improvements, feel free to drop a comment. Enjoy your Flutter development journey :D

If you liked it, a like would be awesome, and if you really liked it, a cup of coffee would be great.

Thanks for reading.

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