The Universe of Discourse

Thu, 30 Nov 2017

Another public service announcement about Git.

There are a number of commands everyone learns when they first start out using Git. And there are some that almost nobody learns right away, but that should be the first thing you learn once you get comfortable using Git day to day.

One of these has the uninteresting-sounding name git-rev-parse. Git has a bewildering variety of notations for referring to commits and other objects. If you type something like origin/master~3, which commit is that? git-rev-parse is your window into Git's understanding of names:

  % git rev-parse origin/master~3
37f2bc78b3041541bb4021d2326c5fe35cbb5fbb


A pretty frequent question is: How do I find out the commit ID of the current HEAD? And the answer is:

   % git rev-parse HEAD
2536fdd82332846953128e6e785fbe7f717e117a


or if you want it abbreviated:

   % git rev-parse --short HEAD
2536fdd


But more important than the command itself is the manual for the command. Whether you expect to use this command, you should read its manual. Because every command uses Git's bewildering variety of notations, and that manual is where the notations are completely documented.

When you use a ref name like master, Git finds it in .git/refs/heads/master, but when you use origin/master, Git finds it in .git/refs/remotes/origin/master, and when you use HEAD Git finds it in .git/HEAD. Why the difference? The git-rev-parse manual explains what Git is doing here.

Did you know that if you have an annoying long branch name like origin/martin/f42876-change-tracking you can create a short alias for it by sticking

    ref: origin/martin/f42876-change-tracking


into .git/CT, and from then on you can do git log CT or git rebase --onto CT or whatever?

Did you know that you can write topic@{yesterday} to mean “whatever commit topic was pointing to yesterday”?

Did you know that you can write ':/penguin system' to refer to the most recent commit whose commit message mentions the penguin system, and that 'HEAD:/penguin system' means the most recent such commit on the HEAD branch?

Did you know that there's a powerful sublanguage for ranges that you can give to git-log to specify all sorts of useful things about which commits you want to look at?

Once I got comfortable with Git I got in the habit of rereading the git-rev-parse manual every few months, because each time I would notice some new useful tool.

Check it out. It's an important next step.

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