How to write reasonable git commit message

This page describes how to phrase reasonable git commit messages. We assume that you have already read and agreed in Commit rules!

Intro

How to start the message

Below you will find seven good rules to write great commit messages. But before, please remember this specific Magnolia rule(s)

Use a well descriptive subject

Make sure that the subject of the message describes and summarizes well what you have done with the commit.

The title / summary of the JIRA ticket usually is a good starting point - however, do not just copy paste! When writing the message really rethink about the work you commit.

The seven rules of a great git commit message

1. Separate subject from body with a blank line

From the 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:

$ 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 <kevin@flynnsarcade.com>
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.

And now git log --oneline, which prints out just the subject line:

$ git log --oneline
42e769 Derezz the master control program

Or, 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

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.

For example:

• Accelerate to 88 miles per hour

Instead of:

• 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.

Example:

• Open the pod bay doors

Instead of:

• 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 git revert:

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

For example:

• 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 <pieter.wuille@gmail.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.

References

http://chris.beams.io/posts/git-commit/