External Commands

Homebrew, like Git, supports external commands. This lets you create new commands that can be run like:

brew mycommand --option1 --option3 formula

without modifying Homebrew’s internals.

Command types

External commands come in two flavors: Ruby commands and shell scripts.

In both cases, the command file should be executable (chmod +x) and live somewhere in PATH.

Ruby commands

An external command extcmd implemented as a Ruby command should be named brew-extcmd.rb. The command is executed by doing a require on the full pathname. As the command is required, it has full access to the Homebrew “environment”, i.e. all global variables and modules that any internal command has access to.

The command may Kernel.exit with a status code if it needs to; if it doesn’t explicitly exit then Homebrew will return 0.

Shell scripts

A shell script for a command named extcmd should be named brew-extcmd. This file will be run via exec with some Homebrew variables set as environment variables, and passed any additional command-line arguments.

Variable	Description
`HOMEBREW_CACHE`	Where Homebrew caches downloaded tarballs to, by default `~/Library/Caches/Homebrew`.
`HOMEBREW_CELLAR`	The location of the Homebrew Cellar, where software is staged. This will be `HOMEBREW_PREFIX/Cellar` if that directory exists, or `HOMEBREW_REPOSITORY/Cellar` otherwise.
`HOMEBREW_LIBRARY_PATH`	The directory containing Homebrew’s own application code.
`HOMEBREW_PREFIX`	Where Homebrew installs software. This is always the grandparent directory of the `brew` executable, `/usr/local` by default.
`HOMEBREW_REPOSITORY`	If installed from a Git clone, the repository directory (i.e. where Homebrew’s .git directory lives).

Note that the script itself can use any suitable shebang (#!) line, so an external “shell script” can be written for sh, bash, Ruby, or anything else.

Providing `--help`

All internal and external Homebrew commands can provide styled --help output by using lines starting with #: (a comment then : character in both Bash and Ruby) which are then output by --help.

For example, see the header of brew services which is output with brew services --help.

User-submitted commands

These commands have been contributed by Homebrew users but are not included in the main Homebrew repository, nor are they installed by the installer script. You can install them manually, as outlined above.

Note they are largely untested, and as always, be careful about running untested code on your machine.

brew-livecheck

Check if there is a new upstream version of a formula. See the README for more info and usage.

Install using:

brew tap homebrew/livecheck

brew-gem

Install any gem package into a self-contained Homebrew Cellar location: https://github.com/sportngin/brew-gem

Note this can also be installed with brew install brew-gem.

brew-growl

Get Growl notifications for Homebrew: https://github.com/secondplanet/homebrew-growl

brew-services

Simple support for starting formulae using launchctl, has out of the box support for any formula which defines startup_plist (e.g. mysql, postgres, redis u.v.m.): https://github.com/Homebrew/homebrew-services