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 flavours: Ruby commands and shell scripts.

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

External commands can be added to a tap to allow easy distribution. See below for more details.

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. Be wary of using Homebrew internals; they may change at any time without warning.

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

Other executable scripts

An executable script for a command named extcmd should be named brew-extcmd. The script itself can use any suitable shebang (#!) line, so an external script can be written in Bash, Ruby, or even Python. Unlike the Ruby commands this file must not end with a language-specific suffix (.sh, or .py). 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_PREFIX Where Homebrew installs software. /usr/local by default for macOS Intel, /opt/homebrew for Apple Silicon and /home/linuxbrew/.linuxbrew for Linux.
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_REPOSITORY The Git repository directory (i.e. where Homebrew’s .git directory lives). Usually either the same as HOMEBREW_PREFIX or a Homebrew subdirectory.

Providing --help

All internal and external Homebrew commands can provide styled --help output by using Homebrew’s argument parser, as seen in the brew services command; or by including lines starting with #: (a comment then : character in both Bash and Ruby), as seen in the header of update.sh, which is printed with brew update --help.

Unofficial external commands

These commands have been contributed by Homebrew users but are not included in the main Homebrew organisation, 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.


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.

External commands in taps

External commands can be hosted in a tap to allow users to easily install and use them. See How to Create and Maintain a Tap for more details about creating and maintaining a tap.

External commands should be added to a cmd directory in the tap. An external command extcmd implemented as a Ruby command should live in cmd/extcmd.rb (don’t forget to chmod +x).

To easily use Homebrew’s argument parser, replicate the Ruby template below for external commands. Your implementation must include the following:

# frozen_string_literal: true

module Homebrew
  module Cmd
    class Foo < AbstractCommand
      cmd_args do
        description <<~EOS
          Do something. Place a description here.
        switch "-f", "--force",
               description: "Force doing something in the command."
        flag   "--file=",
               description: "Specify a file to do something with in the command."
        comma_array "--names",
                    description: "Add a list of names to the command."

        named_args [:formula, :cask], min: 1

      def run
        something if args.force?
        something_else if args.file == "file.txt"

Using the above will generate appropriate help text:

$ brew foo --help
Usage: brew foo [options] formula|cask [...]

Do something. Place a description here.

  -f, --force                      Force doing something in the command.
      --file                       Specify a file to do something with in the
      --names                      Add a list of names to the command.
  -d, --debug                      Display any debugging information.
  -q, --quiet                      Make some output more quiet.
  -v, --verbose                    Make some output more verbose.
  -h, --help                       Show this message.

The usage string is automatically generated based on the specified number and type of named arguments (see below for more details on specifying named arguments). The generated usage string can be overridden by passing the correct usage string to the usage_banner method (placed just before the description method). See the brew tap command for an example.

Use the named_args method to specify the type and number of named arguments that are expected. Pass either a symbol to indicate the type of argument expected, an array of symbols to indicate that multiple types should be expected, or an array of strings to specify which specific options should be expected (see the brew analytics command for an example of this).

Pass an integer to the number, min, or max parameter of named_args to specify the number of named arguments that are expected. See the following examples:

# Accept no named args
named_args :none

# Accept any number (including none) of formula arguments
named_args :formula

# Accept exactly one of the specified options as an argument
named_args %w[state off on], number: 1

# Accept at least one argument that is either a formula or a cask
named_args [:formula, :cask], min: 1

# Accept no more than one argument that is a tap
named_args :tap, max: 1

# Accept between one and two named args
named_args min: 1, max: 2

Named arguments can be accessed by calling args.named. Check out the internal commands and developer commands for more usage examples.

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