tl;dr: This Git alias lets you pull down a branch based on the GitHub pull request number, without worrying about the repo or name of the branch:

[alias]
    copr = "!f() { \
        git fetch -fu ${2:-origin} refs/pull/$1/head:pr/$1 && \
        git checkout pr/$1 && \
        git reset --hard @; \
    }; f"

You use it with the number of the pull request you want:

$ git copr 1234
...
Switched to branch 'pr/1234'
HEAD is now at a2d8da8ffa blah blah blah blah

•    •    •

When reviewing GitHub pull requests, I sometimes want to get the proposed code onto my own machine, for running it, or just reading it in my own editor. Here are a few ways to do it, with a digression into git/alias/shell weirdness tacked on the end.

If the pull request is from a branch in the same repo, you can just check out the branch by name:

$ git checkout joe/proposed-feature

But you might not remember the name of the branch, or it might be in a different fork. Better is to be able to request the code by the pull request number.

The first technique I found was to modify the repo’s .git/config file so that when you fetch code from the remote, it automatically pulls the pull request branches also. On GitHub, pull requests are at refspecs like “refs/pull/1234” (no, I don’t really know what refspecs are, but I look forward to the day when I do...) Bert Belder wrote up a description of how to tweak your repo to automatically pull down all the pull request branches. You add this line to the [remote “origin”] section of your .git/config:

fetch = +refs/pull/*/head:refs/remotes/origin/pr/*

Now when you “git fetch origin”, you’ll get all the pull request branches, and you can simply check out the one you want with “git checkout pr/1234”.

But this means having to edit your repo’s .git/config file before you can get the pull request code. If you have many repos, you’re always going to be finding the ones that haven’t been tweaked yet.

A technique I liked better is on Corey Frang’s gist, provided by Rico Sta. Cruz: Global .gitconfig aliases for pull request management. Here, you update your single ~/.gitconfig file to define a new command that will pull down a pull request branch when you need it:

[alias]
    copr = "!f() { \
        git fetch -fu ${2:-origin} refs/pull/$1/head:pr/$1 && \
        git checkout pr/$1 && \
        git reset --hard @; \
    }; f"

This gives us a new command, “git copr” (for CheckOut Pull Request) that gets branches from pull requests:

$ git copr 1234            # gets and switches to pr/1234 from origin
$ git copr 789 upstream    # gets and switches to pr/789 from upstream

This technique has the advantage that once you define the alias, it’s available in any repo, and also, it both fetches the branch and switches you to it.

BTW: finding and collecting these kinds of shortcuts can be daunting, because if you don’t understand every bit of them, then you’re in cargo-cult territory. “This thing worked for someone else, and if I copy it here, then it will work for me!”

In a few of the aliases on these pages, I see that the commands end with “&& :”. I asked about this in the #git IRC channel, and was told that it was pointless: “&&” joins two shell commands, and runs the second one if the first one succeeded, and “:” is a shell built-in that simply succeeds (it’s the same as “true”). So what does “&& :” add to the command? Seemed like it was pointless; we were stumped.

Then I also asked why other aliases took the form they did. Our copr alias has this form:

"!f() { command1; command2; }; f"

The bang character escapes from git syntax to the shell. Then we define a shell function called f with two commands in it, then we call the function. Why define the function? Why not just define the alias to be the two commands?

More discussion and experimentation turned up the answer. The way git invokes the shell, the arguments to the alias are available as $1, $2, etc, but they are also appended to the command line. As an example, let’s define three different git aliases, each of which uses two arguments:

[alias]
    ee1 = "!echo 1 is $1 stop; echo 2 is $2 stop"
    ee2 = "!echo 1 is $1 stop; echo 2 is $2 stop && :"
    ee3 = "!f() { echo 1 is $1 stop; echo 2 is $2 stop; }; f"

When we try these, the first does a bad thing, but the second and third are good:

$ git ee1 one two
1 is one stop
2 is two stop one two
$ git ee2 one two
1 is one stop
2 is two stop
$ git ee3 one two
1 is one stop
2 is two stop

The second one works because the “:” command eats up the extra arguments. The third one works because the eventual command run is “f one two”, so the values are passed to the function. So the “&& :” wasn’t pointless after all, it was needed to make the arguments work properly.

From previous cargo-cult expeditions, my ~/.gitconfig has other aliases using a different form:

[alias]
    ee4 = !sh -c 'echo 1 is $1 stop && echo 2 is $2 stop'
    ee5 = !sh -c 'echo 1 is $1 stop && echo 2 is $2 stop' -

These do this:

$ git ee4 one two
1 is two stop
2 is stop
$ git ee5 one two
1 is one stop
2 is two stop

(No, I have no idea why ee4 does what it does.) So we have three odd forms that all are designed to let you access arguments positionally, but not get confused by them:

[alias]
    cmd1 = "!command1 && command2 && :"
    cmd2 = "!f() { command1; command2; }; f"
    cmd3 = !sh -c 'command1 && command2' -

All of them work, I like the function-defining one best, it seems most programmery, and least shell-tricky. I’m sure there’s something here I’m misunderstanding, or a subtlety I’m overlooking, but I’ve learned stuff today.