Git commit messages with attributes

While looking through the ConventionalCommit spec I stumbled upon a feature of Git I somehow always thought was there but hadn’t realize was actually part of the Git toolchain itself: Trailers!

In Git, trailers are key-value pairs written inside of commit messages that, for instance, make things like references to JIRA tickets programmatically accessible. Let’s take a look at the following commit message:

Awesome new feature

Some details about this commit

JIRA-Ref: PROJ-123

The last line is an example for a trailer. It consists of a token (JIRA-Ref), a separator (:), and a value (PROJ-123).

With the interpret-trailers the Git command offers a subcommand for extracting and manipulating these trailers. Using various configuration options, (among others) the following operations can be performed:

  • Add a trailer if it is not present (using a shell command)
  • Remove a trailer if present
  • Modify a trailer if present (using a shell command)

A call to git inspect-trailers could, for instance, be integrated into a commit-msg hook script in order to do some modifications or using pre-commit to prevent commit messages not including, for instance, certain trailers or explicitly adding trailers with information from the context of a commit.

In general, you’ll probably most often need to implement a bit of scripting around trailers, the git subcommand just makes it a bit easier so that you don’t have to extract the trailers yourself:

$ git config trailer.jiraref.key "JIRA-Ref"
$ git config trailer.jiraref.ifexists replace
$ git config trailer.jiraref.command "echo\$ARG"

Let’s now take the same commit message that we create above and interpret it. git inspect-trailers will then replace the JIRA-Ref line like this:

$ cat message.txt | git interpret-trailers
Awesome new feature

Some details about this commit.


Trailers only

You can also use inspect-trailers to get just the token/value pairs:

$ cat message.txt | git interpret-trailers --only-trailers

I haven’t yet tried to script my way around trailers but I’d assume I’d do something like this if I need more complex logic for updating the trailers:

  1. Extract just the trailers using the --only-trailers flag
  2. Massage the values into the for that I want
  3. Modify the commit message by passing the new trailers using something like cat message.txt | git interpret-trailers --trailer JIRA-Ref=my-new-ref

In such a scenario, I’d probably also remove the trailer.jiraref.command option that we set before simply to have all the processing logic inside (2) and not being split between that and the git configuration.

Use case

Coming to think of it, trailers might be a nice way to include a list of all the reviewers that checked a pull-request/merge-request before it was merged! Implementing that as some kind of hook/action for GitLab or GitHub might also be a nice coding dojo project 😄

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Alternatively, I'm also experimenting with Webmentions. If you write a post on a blog that supports this technique, I should get notified about your link 🙂