While reading Adam Johnson's book Boost Your Git DX I discovered that there is a seting which is not on by default in GitHub called "Automatically delete head branches" in the settings of any repository.
This setting, when enabled, will delete a branch after it's been merged so you don't have to clean up after yourself. Brilliant!
You can either enable the setting in the UI of Github on a per repository basis OR you can use the following command
gh api 'repos/{owner}/{repo}' --method PATCH --field delete_branch_on_merge=true
This is great, but what if you want to update ALL of your existing, active repositories?
You can if you run this command:
gh repo list --no-archived --source --limit 100 --json name --jq '.[] | .name' | xargs -I{} gh api "repos/username/{}" --silent --method PATCH --field delete_branch_on_merge=true
A quick breakdown
gh repo list --no-archived --source --limit 100 --json name --jq '.[] | .name will get up to 100 of your active repositories (change this number for your use case) and return the name as a json array
This is then piped to xargs and will loop over each of the names that were piped from the first command into the gh command to set the delete_branch_on_merge to be true, i.e. programatically check the box for you for all of your repositories.
If you want to be a bit more consiervative and only update the repositories that were last updated in 2017 you could run this:
gh repo list --no-archived --source --limit 100 --json name,updatedAt --jq '.[] | select(.updatedAt|test("^2017.")) | .name' | xargs -I{} gh api "repos/username/{}" --silent --method PATCH --field delete_branch_on_merge=true
In both cases be sure to replace username with your github user name