Writing reasonable git commit messages helps fellow team members to read and understand a project's development history at a glance.
Following this facilitates several other activities, such as documentation, root-cause analysis or reverting changes.
MAGNOLIA-6223 Add improved API for resource change observation
- Separate subject from body with a blank line
- Limit the subject line to 50 characters
- Capitalize the subject line
- Do not end the subject line with a period
- Use the imperative mood in the subject line
- Wrap the body at 72 characters
- Use the body to explain what and why vs. how
On top of these rules, mind the following Magnolia-specific guidelines:
- The subject should start with the Jira ticket number, in the appropriate Jira project corresponding to the repository at hand.
- All code changes must belong to a Jira ticket, even if they are "only" considered refactoring. If a ticket does not exist in the current project, please create it and link it as appropriate.
- Please avoid cross-referencing Jira projects from other repositories. This makes it hard to track changes, know when a module should be released, or which ticket/version introduced or fixed an issue.
- Commits only updating internal versions in webapps/bundles may use the motivating Jira ticket from that module, or simply no prefix at all, e.g. "Bump personalization to 2.1-SNAPSHOT".
- QA prefix may be used, but exclusively for 100% cosmetic changes, e.g. formatting, Javadoc.
- Make sure the subject describes and summarizes well what you have done with the commit. The title/issue summary in Jira may be a good starting point, however do not just copy paste!
The seven rules of a great git commit message
1. Separate subject from body with a blank line
git commit manpage:
Though not required, it's a good idea to begin the commit message with a single short (less than 50 character) line summarizing the change, followed by a blank line and then a more thorough description. The text up to the first blank line in a commit message is treated as the commit title, and that title is used throughout Git. For example, git-format-patch(1) turns a commit into email, and it uses the title on the Subject line and the rest of the commit in the body.
Firstly, not every commit requires both a subject and a body. Sometimes a single line is fine, especially when the change is so simple that no further context is necessary. For example:
Fix typo in introduction to user guide
Nothing more need be said; if the reader wonders what the typo was, she can simply take a look at the change itself, i.e. use
git show or
git diff or
git log -p.
If you're committing something like this at the command line, it's easy to use the
-m switch to
$ git commit -m"Fix typo in introduction to user guide"
However, when a commit merits a bit of explanation and context, you need to write a body. For example:
Derezz the master control program MCP turned out to be evil and had become intent on world domination. This commit throws Tron's disc into MCP (causing its deresolution) and turns it back into a chess game.
This is not so easy to commit this with the
-m switch. You really need a proper editor. If you do not already have an editor set up for use with git at the command line, read this section of Pro Git.
In any case, the separation of subject from body pays off when browsing the log. Here's the full log entry:
$ git log commit 42e769bdf4894310333942ffc5a15151222a87be Author: Kevin Flynn <firstname.lastname@example.org> Date: Fri Jan 01 00:00:00 1982 -0200 Derezz the master control program MCP turned out to be evil and had become intent on world domination. This commit throws Tron's disc into MCP (causing its deresolution) and turns it back into a chess game.
git log --oneline, which prints out just the subject line:
$ git log --oneline 42e769 Derezz the master control program
git shortlog, which groups commits by user, again showing just the subject line for concision:
$ git shortlog Kevin Flynn (1): Derezz the master control program Alan Bradley (1): Introduce security program "Tron" Ed Dillinger (3): Rename chess program to "MCP" Modify chess program Upgrade chess program Walter Gibbs (1): Introduce protoype chess program
There are a number of other contexts in git where the distinction between subject line and body kicks in—but none of them work properly without the blank line in between.
2. Limit the subject line to 50 characters
The advice to limit the subject line to 50 characters is well-intentioned - it seems very hard to follow. Check the git log of main, ui and other Magnolia projects and try to find commit message subjects which use not more than 50 characters.
To keep the commit message subject clean and short is very important, but very often it seems impossible to follow the "50 characters rule".
If you can manage not to use more than 72 (see rule #6) - seems fine.
(Personal note from Christoph Meier)
50 characters is not a hard limit, just a rule of thumb. Keeping subject lines at this length ensures that they are readable, and forces the author to think for a moment about the most concise way to explain what's going on.
Tip: If you're having a hard time summarizing, you might be committing too many changes at once. Strive for atomic commits (a topic for a separate post).
GitHub's UI is fully aware of these conventions. It will warn you if you go past the 50 character limit:
And will truncate any subject line longer than 69 characters with an ellipsis:
So shoot for 50 characters, but consider 69 the hard limit.
3. Capitalize the subject line
This is as simple as it sounds. Begin all subject lines with a capital letter.
- Accelerate to 88 miles per hour
- accelerate to 88 miles per hour
4. Do not end the subject line with a period
Trailing punctuation is unnecessary in subject lines. Besides, space is precious when you're trying to keep them to 50 chars or less.
- Open the pod bay doors
- Open the pod bay doors.
5. Use the imperative mood in the subject line
Imperative mood just means "spoken or written as if giving a command or instruction". A few examples:
- Clean your room
- Close the door
- Take out the trash
Each of the seven rules you're reading about right now are written in the imperative ("Wrap the body at 72 characters", etc).
The imperative can sound a little rude; that's why we don't often use it. But it's perfect for git commit subject lines. One reason for this is that git itself uses the imperative whenever it creates a commit on your behalf.
For example, the default message created when using
git merge reads:
Merge branch 'myfeature'
And when using
Revert "Add the thing with the stuff" This reverts commit cc87791524aedd593cff5a74532befe7ab69ce9d.
Or when clicking the "Merge" button on a GitHub pull request:
Merge pull request #123 from someuser/somebranch
So when you write your commit messages in the imperative, you're following git's own built-in conventions. For example:
- Refactor subsystem X for readability
- Update getting started documentation
- Remove deprecated methods
- Release version 1.0.0
Writing this way can be a little awkward at first. We're more used to speaking in the indicative mood, which is all about reporting facts. That's why commit messages often end up reading like this:
- Fixed bug with Y
- Changing behavior of X
And sometimes commit messages get written as a description of their contents:
- More fixes for broken stuff
- Sweet new API methods
To remove any confusion, here's a simple rule to get it right every time.
A properly formed git commit subject line should always be able to complete the following sentence:
- If applied, this commit will your subject line here
- If applied, this commit will refactor subsystem X for readability
- If applied, this commit will update getting started documentation
- If applied, this commit will remove deprecated methods
- If applied, this commit will release version 1.0.0
- If applied, this commit will merge pull request #123 from user/branch
Notice how this doesn't work for the other non-imperative forms:
- If applied, this commit will fixed bug with Y
- If applied, this commit will changing behavior of X
- If applied, this commit will more fixes for broken stuff
- If applied, this commit will sweet new API methods
Remember: Use of the imperative is important only in the subject line. You can relax this restriction when you're writing the body.
6. Wrap the body at 72 characters
Git never wraps text automatically. When you write the body of a commit message, you must mind its right margin, and wrap text manually.
The recommendation is to do this at 72 characters, so that git has plenty of room to indent text while still keeping everything under 80 characters overall.
A good text editor can help here. It's easy to configure Vim, for example, to wrap text at 72 characters when you're writing a git commit. Traditionally, however, IDEs have been terrible at providing smart support for text wrapping in commit messages (although in recent versions, IntelliJ IDEA has finally gotten better about this).
7. Use the body to explain what and why vs. how
This commit from Bitcoin Core is a great example of explaining what changed and why:
commit eb0b56b19017ab5c16c745e6da39c53126924ed6 Author: Pieter Wuille <email@example.com> Date: Fri Aug 1 22:57:55 2014 +0200 Simplify serialize.h's exception handling Remove the 'state' and 'exceptmask' from serialize.h's stream implementations, as well as related methods. As exceptmask always included 'failbit', and setstate was always called with bits = failbit, all it did was immediately raise an exception. Get rid of those variables, and replace the setstate with direct exception throwing (which also removes some dead code). As a result, good() is never reached after a failure (there are only 2 calls, one of which is in tests), and can just be replaced by !eof(). fail(), clear(n) and exceptions() are just never called. Delete them.
Take a look at the full diff and just think how much time the author is saving fellow and future committers by taking the time to provide this context here and now. If he didn't, it would probably be lost forever.
In most cases, you can leave out details about how a change has been made. Code is generally self-explanatory in this regard (and if the code is so complex that it needs to be explained in prose, that's what source comments are for). Just focus on making clear the reasons you made the change in the first place—the way things worked before the change (and what was wrong with that), the way they work now, and why you decided to solve it the way you did.