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Environment predicate

An environment predicate is a small grammar that describes conditions on an environment. In a manifest file, environment predicates can be used in the when field of the moniker section. Here are some examples of different environment predicates. One predicate per line:

os = linux
arch = "x86_64"
os in (linux, freebsd, macos)
arch not in (x86, "x86_64")
kernel-release ^= "4.1"
!(os in (linux, freebsd, macos) && arch = "x86_64" || os = openbsd)

Left-hand side

There are three fields that are subject to predication:

  • os: operating system
  • arch: processor architecture
  • kernel: kernel
  • kernel-release: kernel version
  • moniker: moniker

Comparison operators

There are four operators that can be used to compare against a target of a predicate:

  • =: is …
  • !=: is not …
  • ^=: starts with … (only available for kernel-release field)
  • $=: ends with … (only available for kernel-release field)
  • in: is one of …
  • not in`: not any of …

Comparison operators are always preceded by fields, followed by string literals after =/!=/^=/$=, and lists after in and not in.


All comparison operators are case-insensitive.

Right-hand side

String literals

String literals are representations of string values, such as linux and "macos", and are of three types:

  • Bare string literals: These are not enclosed in any quotation marks and start with an alphabet, and can only contain alphabets and numbers. Most values can be represented this way, such as linux, windows, and aarch64.
  • Double-quoted string literal: A string enclosed in double quotes, supporting C/Python-style escape-column syntax (e.g., \0, \n, \", \xff, \uffff). Double-quotes can be written literally without escape sequences.
  • Single-quote string literals: strings enclosed in single-quotes, supporting C/Python-style escape-column syntax (e.g. \0, \n, \', \xff, \uffff). Single-quotes can be written literally without escape sequences.

In most cases, bare string literals will be sufficient, except when representing values like "x86_64".


A list is an expression that contains zero or more string literals, and is a comma-separated list of string literals in parentheses. For example, () is an empty list, and ("x86_64", aarch64) is a list of two strings.

Boolean constants

Although you'll never actually use them, there are two keywords for boolean constants:

  • always: true
  • never: false

These boolean constants are never used with comparison operators, and always stand alone as predicates.


Predicates can be enclosed in parentheses to give them operational precedence.

Boolean operators

There are three types of boolean operators:

  • &&: An infix binary operator that requires both predicates to be true, otherwise the whole predicate is false.
  • ||: An infix binary operator, when only one of the predicates on either side is true, the whole predicate is true, otherwise false.
  • !: A prefix unary operator, used immediately before an opening parenthesis, that negates the predicate inside the parenthesis.