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Synopsis

Casks Are Ruby Blocks

Each Cask is a Ruby block, beginning with a special header line. The Cask definition itself is always enclosed in a do … end block. Example:

cask 'alfred' do
  version '2.7.1_387'
  sha256 'a3738d0513d736918a6d71535ef3d85dd184af267c05698e49ac4c6b48f38e17'

  url "https://cachefly.alfredapp.com/Alfred_#{version}.zip"
  name 'Alfred'
  homepage 'https://www.alfredapp.com/'

  app 'Alfred 2.app'
  app 'Alfred 2.app/Contents/Preferences/Alfred Preferences.app'
end

The Cask Language Is Declarative

Each Cask contains a series of stanzas (or “fields”) which declare how the software is to be obtained and installed. In a declarative language, the author does not need to worry about order. As long as all the needed fields are present, Homebrew Cask will figure out what needs to be done at install time.

To make maintenance easier, the most-frequently-updated stanzas are usually placed at the top. But that’s a convention, not a rule.

Exception: do blocks such as postflight may enclose a block of pure Ruby code. Lines within that block follow a procedural (order-dependent) paradigm.

Conditional Statements

Efficiency

Conditional statements are permitted, but only if they are very efficient. Tests on the following values are known to be acceptable:

value examples
MacOS.version coconutbattery.rb, yasu.rb

Version Comparisons

Tests against MacOS.version may use either symbolic names or version strings with numeric comparison operators:

if MacOS.version <= :mavericks     # symbolic name
if MacOS.version <= '10.9'         # version string

The available symbols for macOS versions are: :mavericks, :yosemite, :el_capitan, :sierra, :high_sierra, and :mojave. The corresponding numeric version strings should be given as major releases containing a single dot.

Always Fall Through to the Newest Case

Conditionals should be constructed so that the default is the newest OS version. When using an if statement, test for older versions, and then let the else statement hold the latest and greatest. This makes it more likely that the Cask will work without alteration when a new OS is released. Example (from coconutbattery.rb):

if MacOS.version <= :sierra
  # ...
elsif MacOS.version <= :mojave
  # ...
else
  # ...
end

Switch Between Languages or Regions

If a cask is available in multiple languages, you can use the language stanza to switch between languages or regions based on the system locale.

Arbitrary Ruby Methods

In the exceptional case that the Cask DSL is insufficient, it is possible to define arbitrary Ruby variables and methods inside the Cask by creating a Utils namespace. Example:

cask 'myapp' do
  module Utils
    def self.arbitrary_method
      ...
    end
  end

  name 'MyApp'
  version '1.0'
  sha256 'a32565cdb1673f4071593d4cc9e1c26bc884218b62fef8abc450daa47ba8fa92'

  url "https://#{Utils.arbitrary_method}"
  homepage 'https://www.example.com/'
  ...
end

This should be used sparingly: any method which is needed by two or more Casks should instead be rolled into the core. Care must also be taken that such methods be very efficient.

Variables and methods should not be defined outside the Utils namespace, as they may collide with Homebrew Cask internals.

Header Line Details

The first non-comment line in a Cask follows the form:

cask '<cask-token>' do

<cask-token> should match the Cask filename, without the .rb extension, enclosed in single quotes.

There are currently some arbitrary limitations on Cask tokens which are in the process of being removed. The Travis bot will catch any errors during the transition.

Stanza order

Having a common order for stanzas makes Casks easier to update and parse. Below is the complete stanza sequence (no Cask will have all stanzas). The empty lines shown here are also important, as they help to visually delineate information.

version
sha256

language

url
appcast
name
homepage

auto_updates
conflicts_with
depends_on
container

suite
app
pkg
installer
binary
colorpicker
dictionary
font
input_method
internet_plugin
prefpane
qlplugin
screen_saver
service
audio_unit_plugin
vst_plugin
vst3_plugin
artifact, target: # target: shown here as is required with `artifact`
stage_only

preflight

postflight

uninstall_preflight

uninstall_postflight

uninstall

zap

caveats

Note that every stanza that has additional parameters (:symbols after a ,) shall have them on separate lines, one per line, in alphabetical order. An exception is target: which typically consists of short lines.