args library

This library lets you define parsers for parsing raw command-line arguments into a set of options and values using GNU and POSIX style options.


Use pub to install this package. Add the following to your pubspec.yaml file.

  args: any

Then run pub install.

For more information, see the args package on

Defining options

To use this library, you create an ArgParser object which will contain the set of options you support:

var parser = new ArgParser();

Then you define a set of options on that parser using addOption() and addFlag(). The minimal way to create an option is:


This creates an option named "name". Options must be given a value on the command line. If you have a simple on/off flag, you can instead use:


Flag options will, by default, accept a 'no-' prefix to negate the option. This can be disabled like so:

parser.addFlag('name', negatable: false);

(From here on out "option" will refer to both "regular" options and flags. In cases where the distinction matters, we'll use "non-flag option".)

Options may have an optional single-character abbreviation:

parser.addOption('mode', abbr: 'm');
parser.addFlag('verbose', abbr: 'v');

They may also specify a default value. The default value will be used if the option isn't provided:

parser.addOption('mode', defaultsTo: 'debug');
parser.addFlag('verbose', defaultsTo: false);

The default value for non-flag options can be any String. For flags, it must be a bool.

To validate non-flag options, you may provide an allowed set of values. When you do, it will throw a FormatException when you parse the arguments if the value for an option is not in the allowed set:

parser.addOption('mode', allowed: ['debug', 'release']);

You can provide a callback when you define an option. When you later parse a set of arguments, the callback for that option will be invoked with the value provided for it:

parser.addOption('mode', callback: (mode) => print('Got mode $mode));
parser.addFlag('verbose', callback: (verbose) {
  if (verbose) print('Verbose');

The callback for each option will always be called when you parse a set of arguments. If the option isn't provided in the args, the callback will be passed the default value, or null if there is none set.

Parsing arguments

Once you have an ArgParser set up with some options and flags, you use it by calling ArgParser.parse() with a set of arguments:

var results = parser.parse(['some', 'command', 'line', 'args']);

These will usually come from new Options().arguments, but you can pass in any list of strings. It returns an instance of ArgResults. This is a map-like object that will return the value of any parsed option.

var parser = new ArgParser();
parser.addFlag('verbose', defaultsTo: true);
var results = parser.parse('['--mode', 'debug', 'something', 'else']);

print(results['mode']); // debug
print(results['verbose']); // true

The parse() method will stop as soon as it reaches -- or anything that it doesn't recognize as an option, flag, or option value. If there are still arguments left, they will be provided to you in

print(; // ['something', 'else']

Specifying options

To actually pass in options and flags on the command line, use GNU or POSIX style. If you define an option like:

parser.addOption('name', abbr: 'n');

Then a value for it can be specified on the command line using any of:

--name somevalue
-n somevalue

Given this flag:

parser.addFlag('name', abbr: 'n');

You can set it on using one of:


Or set it off using:


Multiple flag abbreviation can also be collapsed into a single argument. If you define:

parser.addFlag('verbose', abbr: 'v');
parser.addFlag('french', abbr: 'f');
parser.addFlag('iambic-pentameter', abbr: 'i');

Then all three flags could be set using:


By default, an option has only a single value, with later option values overriding earlier ones; for example:

var parser = new ArgParser();
var results = parser.parse(['--mode', 'on', '--mode', 'off']);
print(results['mode']); // prints 'off'

If you need multiple values, set the allowMultiple flag. In that case the option can occur multiple times and when parsing arguments a List of values will be returned:

var parser = new ArgParser();
parser.addOption('mode', allowMultiple: true);
var results = parser.parse(['--mode', 'on', '--mode', 'off']);
print(results['mode']); // prints '[on, off]'

Defining commands

In addition to options, you can also define commands. A command is a named argument that has its own set of options. For example, when you run:

$ git commit -a

The executable is git, the command is commit, and the -a option is an option passed to the command. You can add a command like so:

var parser = new ArgParser();
var command = parser.addCommand('commit');

It returns another ArgParser which you can then use to define options specific to that command. If you already have an ArgParser for the command's options, you can pass it to addCommand:

var parser = new ArgParser();
var command = new ArgParser();
parser.addCommand('commit', command);

The ArgParser for a command can then define whatever options or flags:

command.addFlag('all', abbr: 'a');

You can add multiple commands to the same parser so that a user can select one from a range of possible commands. When an argument list is parsed, you can then determine which command was entered and what options were provided for it.

var results = parser.parse(['commit', '-a']);
print(; // "commit"
print(results.command['a']); // true

Options for a command must appear after the command in the argument list. For example, given the above parser, "git -a commit" is not valid. The parser will try to find the right-most command that accepts an option. For example:

var parser = new ArgParser();
parser.addFlag('all', abbr: 'a');
var command = new ArgParser().addCommand('commit');
parser.addFlag('all', abbr: 'a');
var results = parser.parse(['commit', '-a']);
print(results.command['a']); // true

Here, both the top-level parser and the "commit" command can accept a "-a" (which is probably a bad command line interface, admittedly). In that case, when "-a" appears after "commit", it will be applied to that command. If it appears to the left of "commit", it will be given to the top-level parser.

Displaying usage

This library can also be used to automatically generate nice usage help text like you get when you run a program with --help. To use this, you will also want to provide some help text when you create your options. To define help text for the entire option, do:

parser.addOption('mode', help: 'The compiler configuration',
    allowed: ['debug', 'release']);
parser.addFlag('verbose', help: 'Show additional diagnostic info');

For non-flag options, you can also provide detailed help for each expected value using a map:

parser.addOption('arch', help: 'The architecture to compile for',
    allowedHelp: {
      'ia32': 'Intel x86',
      'arm': 'ARM Holding 32-bit chip'

If you define a set of options like the above, then calling this:


Will display something like:

--mode            The compiler configuration
                  [debug, release]

--[no-]verbose    Show additional diagnostic info
--arch            The architecture to compile for

      [arm]       ARM Holding 32-bit chip
      [ia32]      Intel x86

To assist the formatting of the usage help, single line help text will be followed by a single new line. Options with multi-line help text will be followed by two new lines. This provides spatial diversity between options.


A class for taking a list of raw command line arguments and parsing out options and flags from them.
The results of parsing a series of command line arguments using ArgParser.parse(). Includes the parsed options and any remaining unparsed command line arguments.
A command-line option. Includes both flags and options which take a value.