commit-msg Hook
===============

NAME
----
commit-msg - Edit commit messages to insert a `Change-Id` tag.

DESCRIPTION
-----------

A Git hook automatically invoked by `git commit`, and most other
commit creation tools such as `git citool` or `git gui`.  The Gerrit
Code Review supplied implementation of this hook is a short shell
script which automatically inserts a globally unique Change-Id tag
in the footer of a commit message.  When present, Gerrit uses this
tag to track commits across cherry-picks and rebases.

After the hook has been installed in the user's local Git repository
for a project, the hook will modify a commit message such as:

----
  Improve foo widget by attaching a bar.

  We want a bar, because it improves the foo by providing more
  wizbangery to the dowhatimeanery.

  Signed-off-by: A. U. Thor <author@example.com>
----

by inserting a new `Change-Id: ` line in the footer:

----
  Improve foo widget by attaching a bar.

  We want a bar, because it improves the foo by providing more
  wizbangery to the dowhatimeanery.

  Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b
  Signed-off-by: A. U. Thor <author@example.com>
----

The hook implementation is reasonably intelligent at inserting the
Change-Id line before any Signed-off-by or Acked-by lines placed
at the end of the commit message by the author, but if no such
lines are present then it will just insert a blank line, and add
the Change-Id at the bottom of the message.

If a Change-Id line is already present in the message footer, the
script will do nothing, leaving the existing Change-Id unmodified.
This permits amending an existing commit, or allows the user to
insert the Change-Id manually after copying it from an existing
change viewed on the web.

OBTAINING
---------
To obtain the 'commit-msg' script use scp to copy it to your
local system:

  $ scp -p -P 29418 review.example.com:hooks/commit-msg .git/hooks/

SEE ALSO
--------

* link:user-changeid.html[Change-Id Lines]
* link:http://www.kernel.org/pub/software/scm/git/docs/git-commit.html[git-commit(1)]
* link:http://www.kernel.org/pub/software/scm/git/docs/githooks.html[githooks(5)]

IMPLEMENTATION
--------------

The hook generates unique Change-Id lines by creating a virtual
commit object within the local Git repository, and obtaining the
SHA-1 hash from it.  Like any other Git commit, the following
properties are included in the computation:

* SHA-1 of the tree being committed
* SHA-1 of the parent commit
* Name, email address, timestamp of the author
* Name, email address, timestamp of the committer
* Proposed commit message (before Change-Id was inserted)

Because the names of the tree and parent commit, as well as the
committer timestamp are included in the hash computation, the output
Change-Id is sufficiently unique.

GERRIT
------
Part of link:index.html[Gerrit Code Review]