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In some cases, no subject-matter expert may be available. In those cases, a review by an experienced developer is sufficient when coupled with appropriate testing. In some cases, no subject-matter expert may be available. In those cases, a review by an experienced developer is sufficient when coupled with appropriate testing.
[[commit-log-message]] [[commit-log-message]]
== Commit Log Messages == Commit Log Messages
This section contains some suggestions and traditions for how commit logs are formatted. This section contains some suggestions and traditions for how commit logs are formatted.
=== Why are commit messages important?
When you commit a change in Git, Subversion, or another version control system
(VCS), you're prompted to write some text describing the commit -- a commit
message. How important is this commit message? Should you spend some significant
effort writing it? Does it really matter if you write simply fixed a bug?
Most projects have more than one developer and last for some length of time.
Commit messages are a very important method of communicating with other
developers, in the present and for the future.
FreeBSD has hundreds of active developers and hundreds of thousands of commits
spanning decades of history. Over that time the developer community has learned
how valuable good commit messages are; sometimes these are hard-learned lessons.
Commit messages serve at least three purposes:
* Communicating with other developers
+
FreeBSD commits generate email to various mailing lists. These include the
commit message along with a copy of the patch itself. Commit messages are also
viewed through commands like git log. These serve to make other developers
aware of changes that are ongoing; that other developer may want to test the
change, may have an interest in the topic and will want to review in more
detail, or may have their own projects underway that would benefit from
interaction.
* Making Changes Discoverable
+
In a large project with a long history it may be difficult to find changes of
interest when investigating an issue or change in behaviour. Verbose, detailed
commit messages allow searches for changes that might be relevant. For example,
`git log --since 1year --grep 'USB timeout'`.
* Providing historical documentation
+
Commit messages serve to document changes for future developers, perhaps years
or decades later. This future developer may even be you, the original author.
A change that seems obvious today may be decidedly not so much later on.
The `git blame` command annotates each line of a source file with the change
(hash and subject line) that brought it in.
Having established the importance, here are elements of a good FreeBSD commit
message:
=== Start with a subject line
Commit messages should start with a single-line subject that briefly summarizes
the change. The subject should, by itself, allow the reader to quickly
determine if the change is of interest or not.
=== Keep subject lines short
The subject line should be as short as possible while still retaining the
required information. This is to make browsing git log more efficient, and so
that git log --oneline can display the short hash and subject on a single
80-column line. A good rule of thumb is to stay below 63 characters, and aim
for about 50 or fewer if possible.
=== Prefix the subject line with a component, if applicable
If the change relates to a specific component the subject line may be prefixed
with that component name and a colon (:).
✓ `foo: Add -k option to keep temporary data`
Include the prefix in the 63-character limit suggested above, so that
`git log --oneline` avoids wrapping.
=== Capitalize the first letter of the subject
Capitalize the first letter of the subject itself. The prefix, if any, is not
capitalized unless necessary (e.g., `USB:` is capitalized).
=== Do not end the subject line with punctuation
Do not end with a period or other punctuation. In this regard the subject line
is like a newspaper headline.
=== Separate the subject and body with a blank line
Separate the body from the subject with a blank line.
Some trivial commits do not require a body, and will have only a subject.
✓ `ls: fix typo in usage text`
=== Limit messages to 72 columns
`git log` and `git format-patch` indent the commit message by four spaces.
Wrapping at 72 columns provides a matching margin on the right edge. Limiting
messages to 72 characters also keeps the commit message in formatted patches
below RFC 2822's suggested email line length limit of 78 characters. This limit
works well with a variety of tools that may render commit messages; line
wrapping might be inconsistent with longer line length.
=== Use the present tense, imperative mood
This facilitates short subject lines and provides consistency, including with
automatically generated commit messages (e.g., as generated by git revert).
This is important when reading a list of commit subjects. Think of the subject
as finishing the sentence "when applied, this change will ...".
✓ `foo: Implement the -k (keep) option` +
✗ `foo: Implemented the -k option` +
✗ `This change implements the -k option in foo ✗ -k option added`
=== Focus on what and why, not how
Explain what the change accomplishes and why it is being done, rather than how.
Do not assume that the reader is familiar with the issue. Explain the
background and motivation for the change. Include benchmark data if you have it.
If there are limitations or incomplete aspects of the change, describe them in
the commit message.
=== Consider whether parts of the commit message could be code comments instead
Sometimes while writing a commit message you may find yourself writing a
sentence or two explaining some tricky or confusing aspect of the change. When
this happens consider whether it would be valuable to have that explanation as
a comment in the code itself.
=== Write commit messages for your future self
While writing the commit message for a change you have all of the context in
mind - what prompted the change, alternate approaches that were considered and
rejected, limitations of the change, and so on. Imagine yourself revisiting the
change a year or two in the future, and write the commit message in a way that
would provide that necessary context.
=== Commit messages should stand alone
You may include references to mailing list postings, benchmark result web
sites, or code review links. However, the commit message should contain all of
the relevant information in case these references are no longer available in
the future.
Similarly, a commit may refer to a previous commit, for example in the case of
a bug fix or revert. In addition to the commit identifier (revision or hash),
include the subject line from the referenced commit (or another suitable brief
reference). With each VCS migration (from CVS to Subversion to Git) revision
identifiers from previous systems may become difficult to follow.
=== Include appropriate metadata in a footer
As well as including an informative message with each commit, some additional information may be needed. As well as including an informative message with each commit, some additional information may be needed.
This information consists of one or more lines containing the key word or phrase, a colon, tabs for formatting, and then the additional information. This information consists of one or more lines containing the key word or phrase, a colon, tabs for formatting, and then the additional information.
The key words or phrases are: The key words or phrases are:
[.informaltable] [.informaltable]
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