Checkout PR, using GitHub API to determine what to track

checkout_pr.sh

#!/bin/sh
set -o errexit
[ $# != 1 ] && { >&2 echo 'Argument: <pull request id>'; exit 1; }

# Get origin remote
origin=$(git remote get-url origin)

# Find target ("owner/repo") of PR
temp=${origin#*github.com?}
[ $origin = $temp ] && { >&2 echo Unrecognized origin.; exit 1; }
target=${temp%.git}

# Find source ("owner/repo") of PR and remote name and branch name
temp=$(curl --silent https://github.com/$target/pull/$1 | grep --max-count=1 head-ref | cut --delimiter=\" --fields=2)
source=${temp%%:*}
remote=$(echo $origin | sed "s $target $source ")
branch=${temp#*:}

# Set up branch and checkout
git fetch --no-tags $remote $branch:pr/$1
git config branch.pr/$1.remote $remote
git config branch.pr/$1.merge refs/heads/$branch
git checkout pr/$1

This checks out the branch in a fork associated with a given PR number in a local branch called pr/123 and sets it up to track that branch on that fork.

Some notes:

• You'll need to create a token which permits read access to the repos in question.
• This was written for Linux, and requires a rather more recent of Bash than macOS currently comes with.
• You'll also need to set git config push.default upstream to be able to easily push back to the fork, because your branch name will differ from that in the fork.

Usage:

• ./checkout_pr.sh 123
• do whatever
• git push (assuming you've set push.default=upstream)

I've just updated this to make it easier to use in a few ways:

• It no longer uses the GraphQL API, so you don't need a token anymore. (It instead scrapes the HTML, which is a bit more brittle. If it becomes a problem, I'll probably rework it to use the real REST API instead. The main reason I'm not doing that now is to avoid dealing with JSON or depending on something like jq.)
• If your repo was cloned with SSH, the new branches pointing at the forks will also be cloned with SSH rather than HTTP. (When I first wrote this, I recall that pushing as a maintainer to a branch of a PR didn't work when it was cloned via SSH. That seems to be working now, so no reason not to support this.)
• It no longer uses Bash. It works for me on Ubuntu with Dash, so presumably this will also work as-is on macOS without installing anything else.

Usage is still the same:

• ./checkout_pr.sh 123
• do whatever
• git push (assuming you've set push.default=upstream)

Here's a hacky way of using the REST API without jq (which is probably not much better than scraping the HTML like I'm doing now), but which I wanted to stick somewhere for posterity, in case it can become the basis for something useful later:

# Find source ("owner/repo") of PR and remote name and branch name
temp="$(curl --silent https://api.github.com/repos/$target/pulls/$1 | tr --delimiter \\n\\r | tr \" \\n)"
source=$(echo "$temp" | grep --after-context=2 --max-count=1 ^full_name$ | tail --lines=1)
remote=$(echo $origin | sed "s $target $source ")
branch=$(echo "$temp" | grep --after-context=2 --max-count=1 ^ref$ | tail --lines=1)

I needed to make the following change to make this work on MacOS:

-temp=$(curl --silent https://github.com/$target/pull/$1 | grep --max-count=1 head-ref | cut --delimiter=\" --fields=2)
+temp=$(curl --silent https://github.com/$target/pull/$1 | grep --max-count=1 head-ref | cut -d\" -f2)

ChatGPT promises me that this should work cross-platform.

Interesting. I've gotten into a habit of using long GNU-style command line options in all my scripts because it's generally a little more clear about what it's actually doing. I did know there were some situations/environments that only support short/POSIX-style switches, and apparently we've run into one here. If I'm going to publish this as a standalone thing, it might make sense to switch everything over to POSIX switches. I'm not sure.