GitHubのPull RequestをチェックアウトするGitのalias

普通にかくとサブコマンドとして解釈されてしまうが!を使うとふつうの外部コマンドが書けてべんり¹

However, maybe you want to run an external command, rather than a Git subcommand. In that case, you start the command with a ! character.
https://git-scm.com/book/tr/v2/Git-Basics-Git-Aliases

~/.gitconfig

[alias]
  gh = ! sh -c 'git fetch origin pull/$1/head:GH-$1 && git checkout GH-$1' --

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1. この例で!抜かすとgit-shコマンドが実行されてしまう ↩