External Commands

Overview

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

brew mycommand --option1 --option2 formula

without modifying Homebrew’s internals.

Note: External commands are a new and somewhat experimental feature. Feedback is appreciated.

Command Types

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

In both cases, the command file should be +x (executable) and live somewhere in your $PATH. (Homebrew internally finds commands with which.)

Ruby Commands

An external command foo implemented as a Ruby command should be named brew-foo.rb. The command is executed by doing a require on the full pathname. As the command is require’d, it has full access to the Homebrew “environment”, ie, 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 an command named foo should be named brew-foo. This file will be run via exec with some Homebrew variables set as environmental variables, and passed any additional command-line arguments.

Variable	Description
HOMEBREW_CACHE	Where Homebrew caches downloaded tarballs to, typically ~/Library/Caches/Homebrew.
HOMEBREW_CELLAR	The location of the Homebrew Cellar, where software is built to.
HOMEBREW_LIBRARY_PATH	The folder containing Homebrew’s own application code.
HOMEBREW_PREFIX	Where Homebrew installs software to, usually /usr/local.
HOMEBREW_REPOSITORY	If installed from a Git clone, the repo folder. (That is, where Homebrew’s .git folder lives.)

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

Examples

Homebrew comes with some sample commands in Library/Contributions/examples. To run these commands, that folder should be added to your $PATH.