If you place an executable named git-MYCOMMAND anywhere in your $PATH, then running `git MYCOMMAND` will call that script, just as if it were an official command underneath git. It can be anything from a shell script to a compiled C executable, it's totally up to you.

This is a good explanation of the details of this:

• Running `git MYCOMMAND --help` won't actually pass the --help parameter on to the executable, rather git will look for a man page called git-MYCOMMAND.1 in the manpath, and error out if one can't be found.
  • So it's a really good idea to create a man page for your custom command.
  • I dropped my man files in $HOME/man/man1/, and somehow they magically showed up in the manpath, but I'm not sure how.

Git aliases are nice, but they're nowhere near as powerful and flexible as custom git commands can be.

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Git commands useful for creating your own scripts

It seems like git has an endless number of commands. Most of them are user-oriented, but there are a few that are particularly useful for ingestion into your custom scripts:

• git rev-parse — a number of different "small utility functions" packaged together. Useful things you can do with it:
  • --git-dir for displaying the abs/relative path of the .git/ directory
  • --show-toplevel for displaying the top-level directory of the working tree (--show-cdup for the relative path)
  • Checking if you're currently within a repository using --is-inside-git-dir or within a work-tree using --is-inside-work-tree
  • Checking if the repo is a bare using --is-bare-repository
  • Printing SHA1 hashes of branches (--branches), tags (--tags) and the refs can also be filtered based on the remote (using --remote)
  • Use it as a resolver (to SHA1) of revision specifiers, like `git rev-parse HEAD^`
• git for-each-ref — output information on each ref
• git ls-remote — list references in a remote repository
• git ls-files — show information about files in the index and the working tree
• (anything mentioned under "Plumbing Commands" of the official Git reference)
• (anything mentioned under Appendix C of the Git Book — "Plumbing Commands")
• (anything else mentioned under Chapter 10 of the Git Book — "Git Internals")
• Other ways to accomplish some basic tasks:
  • Show a commit message — git log --format=%B -n 1 <revision>
  • Show the current branch name — git rev-parse --abbrev-ref HEAD

There are some Perl modules for accessing some of the Git plumbing:

• Git::Repository
• Git::Helpers

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Other people's custom git commands

While we're here, this is a list of custom commands that other people have published, that I particularly like:

• git activity — displays a list of all remote branches, sorted by which was most recently updated
• https://github.com/nvie/git-toolbelt — a long list of them
• https://github.com/wilsonmar/git-utilities/tree/master/git-custom-commands — a few
• https://github.com/matze/git-utils — a few