A ridiculously simple argument-parsing library for Swift.

Version 2.0.0

Command Line Interface


An option can have an unlimited number of long-form aliases and single-character shortcuts: --option, -o.

Option values can be separated by either a space, --opt 123, or an equals symbol, --opt=123. Either syntax can be used with shortcuts: -o 123, -o=123.

Multiple shortcuts can be condensed into a single block, e.g. -abc foo bar. Trailing arguments are consumed in sequence as required by the options.

Multivalued Options

Options can be treated as singular or multivalued as circumstances require. Each option maintains an internal list to which newly parsed values are appended; the (singular) value of the option is the final value in the list.

For example, in the command below:

$ myapp --foo abc --foo def

the value of the option foo is "def" but the array ["abc", "def"] is also available for use if required.


Flags are valueless options — they're either present or absent, but take no arguments. Like options, flags can have an unlimited number of long-form aliases and single-character shortcuts: --flag, -f.

Positional Arguments

Options and flags can be preceded by, followed by, or interspaced with positional arguments which are assembled by the parser into an array of strings.

The parser supports the standard -- switch for turning off option-parsing. All arguments following a -- will be treated as positional arguments, even if they begin with a single or double dash.


This library supports git-style command interfaces with arbitrarily-nested commands. Commands have builtin support for an automatic --help flag and an automatic help <cmd> command, i.e. the commands

$ myapp <cmd> --help


$ myapp help <cmd>

are functionally identical and will both print the help text registered with the command.

Non-Unicode Arguments

To keep its API simple this library uses Swift's native String type which can only represent valid unicode strings.

You should be aware that on Unix systems command line arguments can contain arbitrary byte values, not just valid unicode. Swift doesn't raise an error if this happens — instead it silently replaces unrepresentable byte values with the unicode replacement character (U+FFFD).

Negative Numbers

Some argument-parsing libraries struggle with negative numbers — for example, they will try to parse -3 as a flag or option named 3. This library always treats arguments beginning with a dash and a digit as positional arguments or option values, never as flag or option names.