clap 2.1.1

A simple to use, efficient, and full featured Command Line Argument Parser

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Command Line Argument Parser for Rust

It is a simple to use, efficient, and full featured library for parsing command line arguments and subcommands when writing console, or terminal applications.


Table of Contents

Created by gh-md-toc

What's New

In v2.1.1

New Features

  • Default Values: Args can now specify default values
  • Next Line Help: Args can have help strings on the line following the argument (useful for long arguments, or those with many values). This can be set command-wide or for individual args


  • Documenation Examples: The examples in the documentation have been vastly improved

For full details, see


clap is used to parse and validate the string of command line arguments provided by the user at runtime. You provide the list of valid possibilities, and clap handles the rest. This means you focus on your applications functionality, and less on the parsing and validating of arguments.

clap also provides the traditional version and help switches (or flags) 'for free' meaning automatically with no configuration. It does this by checking list of valid possibilities you supplied and adding only the ones you haven't already defined. If you are using subcommands, clap will also auto-generate a help subcommand for you in addition to the traditional flags.

Once clap parses the user provided string of arguments, it returns the matches along with any applicable values. If the user made an error or typo, clap informs them of the mistake and exits gracefully (or returns a Result type and allows you to perform any clean up prior to exit). Because of this, you can make reasonable assumptions in your code about the validity of the arguments.


For a full FAQ and more in depth details, see the wiki page


First, let me say that these comparisons are highly subjective, and not meant in a critical or harsh manner. All the argument parsing libraries out there (to include clap) have their own strengths and weaknesses. Sometimes it just comes down to personal taste when all other factors are equal. When in doubt, try them all and pick one that you enjoy :) There's plenty of room in the Rust community for multiple implementations!

How does clap compare to getopts?

getopts is a very basic, fairly minimalist argument parsing library. This isn't a bad thing, sometimes you don't need tons of features, you just want to parse some simple arguments, and have some help text generated for you based on valid arguments you specify. The downside to this approach is that you must manually implement most of the common features (such as checking to display help messages, usage strings, etc.). If you want a highly custom argument parser, and don't mind writing the majority of the functionality yourself, getopts is an excellent base.

getopts also doesn't allocate much, or at all. This gives it a very small performance boost. Although, as you start implementing additional features, that boost quickly disappears.

Personally, I find many, many uses of getopts are manually implementing features that clap provides by default. Using clap simplifies your codebase allowing you to focus on your application, and not argument parsing.

How does clap compare to

I first want to say I'm a big a fan of BurntSushi's work, the creator of I aspire to produce the quality of libraries that this man does! When it comes to comparing these two libraries they are very different. docopt tasks you with writing a help message, and then it parsers that message for you to determine all valid arguments and their use. Some people LOVE this approach, others do not. If you're willing to write a detailed help message, it's nice that you can stick that in your program and have docopt do the rest. On the downside, it's far less flexible.

docopt is also excellent at translating arguments into Rust types automatically. There is even a syntax extension which will do all this for you, if you're willing to use a nightly compiler (use of a stable compiler requires you to somewhat manually translate from arguments to Rust types). To use BurntSushi's words, docopt is also a sort of black box. You get what you get, and it's hard to tweak implementation or customize the experience for your use case.

Because docopt is doing a ton of work to parse your help messages and determine what you were trying to communicate as valid arguments, it's also one of the more heavy weight parsers performance-wise. For most applications this isn't a concern and this isn't to say docopt is slow, in fact from it. This is just something to keep in mind while comparing.

All else being equal, what are some reasons to use clap?

clap is as fast, and as lightweight as possible while still giving all the features you'd expect from a modern argument parser. In fact, for the amount and type of features clap offers it remains about as fast as getopts. If you use clap when just need some simple arguments parsed, you'll find its a walk in the park. clap also makes it possible to represent extremely complex, and advanced requirements, without too much thought. clap aims to be intuitive, easy to use, and fully capable for wide variety use cases and needs.


Below are a few of the features which clap supports, full descriptions and usage can be found in the documentation and examples/ directory

  • Auto-generated Help, Version, and Usage information
    • Can optionally be fully, or partially overridden if you want a custom help, version, or usage
  • Flags / Switches (i.e. bool fields)
    • Both short and long versions supported (i.e. -f and --flag respectively)
    • Supports combining short versions (i.e. -fBgoZ is the same as -f -B -g -o -Z)
    • Supports multiple occurrences (i.e. -vvv or -v -v -v)
  • Positional Arguments (i.e. those which are based off an index from the program name)
    • Supports multiple values (i.e. myprog <file>... such as myprog file1.txt file2.txt being two values for the same "file" argument)
    • Supports Specific Value Sets (See below)
    • Can set value parameters (such as the minimum number of values, the maximum number of values, or the exact number of values)
    • Can set custom validations on values to extend the argument parsing capability to truly custom domains
  • Option Arguments (i.e. those that take values)
    • Both short and long versions supported (i.e. -o value, -ovalue, -o=value and --option value or --option=value respectively)
    • Supports multiple values (i.e. -o <val1> -o <val2> or -o <val1> <val2>)
    • Supports delimited values (i.e. -o=val1,val2,val3, can also change the delimiter)
    • Supports Specific Value Sets (See below)
    • Supports named values so that the usage/help info appears as -o <FILE> <INTERFACE> etc. for when you require specific multiple values
    • Can set value parameters (such as the minimum number of values, the maximum number of values, or the exact number of values)
    • Can set custom validations on values to extend the argument parsing capability to truly custom domains
  • Sub-Commands (i.e. git add <file> where add is a sub-command of git)
    • Support their own sub-arguments, and sub-sub-commands independent of the parent
    • Get their own auto-generated Help, Version, and Usage independent of parent
  • Support for building CLIs from YAML - This keeps your Rust source nice and tidy and makes supporting localized translation very simple!
  • Requirement Rules: Arguments can define the following types of requirement rules
    • Can be required by default
    • Can be required only if certain arguments are present
    • Can require other arguments to be present
  • Confliction Rules: Arguments can optionally define the following types of exclusion rules
    • Can be disallowed when certain arguments are present
    • Can disallow use of other arguments when present
  • POSIX Override Rules: Arguments can define rules to support POSIX overriding
  • Groups: Arguments can be made part of a group
    • Fully compatible with other relational rules (requirements, conflicts, and overrides) which allows things like requiring the use of any arg in a group, or denying the use of an entire group conditionally
  • Specific Value Sets: Positional or Option Arguments can define a specific set of allowed values (i.e. imagine a --mode option which may only have one of two values fast or slow such as --mode fast or --mode slow)
  • Default Values: Although not specifically provided by clap you can achieve this exact functionality from Rust's Option<&str>.unwrap_or("some default") method (or Result<T,String>.unwrap_or(T) when using typed values)
  • Automatic Version from Cargo.toml: clap is fully compatible with Rust's env!() macro for automatically setting the version of your application to the version in your Cargo.toml. See 09_auto_version example for how to do this (Thanks to jhelwig for pointing this out)
  • Typed Values: You can use several convenience macros provided by clap to get typed values (i.e. i32, u8, etc.) from positional or option arguments so long as the type you request implements std::str::FromStr See the 12_typed_values example. You can also use claps arg_enum! macro to create an enum with variants that automatically implement std::str::FromStr. See 13a_enum_values_automatic example for details
  • Suggestions: Suggests corrections when the user enters a typo. For example, if you defined a --myoption argument, and the user mistakenly typed --moyption (notice y and o transposed), they would receive a Did you mean '--myoption' ? error and exit gracefully. This also works for subcommands and flags. (Thanks to Byron for the implementation) (This feature can optionally be disabled, see 'Optional Dependencies / Features')
  • Colorized Errors (Non Windows OS only): Error message are printed in in colored text (this feature can optionally be disabled, see 'Optional Dependencies / Features').
  • Global Arguments: Arguments can optionally be defined once, and be available to all child subcommands.
  • Custom Validations: You can define a function to use as a validator of argument values. Imagine defining a function to validate IP addresses, or fail parsing upon error. This means your application logic can be solely focused on using values.
  • POSIX Compatible Conflicts - In POSIX args can be conflicting, but not fail parsing because whichever arg comes last "wins" to to speak. This allows things such as aliases (i.e. alias ls='ls -l' but then using ls -C in your terminal which ends up passing ls -l -C as the final arguments. Since -l and -C aren't compatible, this effectively runs ls -C in clap if you choose...clap also supports hard conflicts that fail parsing). (Thanks to Vinatorul!)
  • Supports the unix -- meaning, only positional arguments follow

Quick Example

The following examples show a quick example of some of the very basic functionality of clap. For more advanced usage, such as requirements, conflicts, groups, multiple values and occurrences see the documentation, examples/ directory of this repository or the video tutorials (which are quite outdated by now).

NOTE: All these examples are functionally the same, but show three different styles in which to use clap

The following example is show a method that allows more advanced configuration options (not shown in this small example), or even dynamically generating arguments when desired. The downside is it's more verbose.

// (Full example with detailed comments in examples/
// This example demonstrates clap's full 'builder pattern' style of creating arguments which is
// more verbose, but allows easier editing, and at times more advanced options, or the possibility
// to generate arguments dynamically.
extern crate clap;
use clap::{Arg, App, SubCommand};

fn main() {
    let matches = App::new("My Super Program")
                          .author("Kevin K. <>")
                          .about("Does awesome things")
                               .help("Sets a custom config file")
                               .help("Sets the input file to use")
                               .help("Sets the level of verbosity"))
                                      .about("controls testing features")
                                      .author("Someone E. <>")
                                          .help("print debug information verbosely")))

    // Gets a value for config if supplied by user, or defaults to "default.conf"
    let config = matches.value_of("config").unwrap_or("default.conf");
    println!("Value for config: {}", config);

    // Calling .unwrap() is safe here because "INPUT" is required (if "INPUT" wasn't
    // required we could have used an 'if let' to conditionally get the value)
    println!("Using input file: {}", matches.value_of("INPUT").unwrap());

    // Vary the output based on how many times the user used the "verbose" flag
    // (i.e. 'myprog -v -v -v' or 'myprog -vvv' vs 'myprog -v'
    match matches.occurrences_of("v") {
        0 => println!("No verbose info"),
        1 => println!("Some verbose info"),
        2 => println!("Tons of verbose info"),
        3 | _ => println!("Don't be crazy"),

    // You can handle information about subcommands by requesting their matches by name
    // (as below), requesting just the name used, or both at the same time
    if let Some(matches) = matches.subcommand_matches("test") {
        if matches.is_present("debug") {
            println!("Printing debug info...");
        } else {
            println!("Printing normally...");

    // more program logic goes here...

The following example is functionally the same as the one above, but shows a far less verbose method but sacrifices some of the advanced configuration options (not shown in this small example).

// (Full example with detailed comments in examples/
// This example demonstrates clap's "usage strings" method of creating arguments which is less
// less verbose
extern crate clap;
use clap::{Arg, App, SubCommand};

fn main() {
    let matches = App::new("myapp")
                          .author("Kevin K. <>")
                          .about("Does awesome things")
                              "-c, --config=[FILE] 'Sets a custom config file'
                              <INPUT>              'Sets the input file to use'
                              -v...                'Sets the level of verbosity'")
                                      .about("controls testing features")
                                      .author("Someone E. <>")
                                      .arg_from_usage("-d, --debug 'Print debug information'"))

    // Same as previous example...

The following combines the previous two examples by using the less verbose from_usage methods and the performance of the Builder Pattern.

// (Full example with detailed comments in examples/
// Must be compiled with `--features unstable`
// This example demonstrates clap's "usage strings" method of creating arguments which is less
// less verbose
extern crate clap;

fn main() {
    let matches = clap_app!(myapp =>
        (version: "1.0")
        (author: "Kevin K. <>")
        (about: "Does awesome things")
        (@arg config: -c --config +takes_value "Sets a custom config file")
        (@arg INPUT: +required "Sets the input file to use")
        (@arg verbose: -v ... "Sets the level of verbosity")
        (@subcommand test =>
            (about: "controls testing features")
            (version: "1.3")
            (author: "Someone E. <>")
            (@arg verbose: -d --debug "Print debug information")

// Same as previous examples...

This final method shows how you can use a YAML file to build your CLI and keep your Rust source tidy or support multiple localized translations by having different YAML files for each localization. First, create the cli.yml file to hold your CLI options, but it could be called anything we like (we'll use the same both examples above to keep it functionally equivalent):

name: myapp
version: 1.0
author: Kevin K. <>
about: Does awesome things
    - config:
        short: c
        long: config
        value_name: FILE
        help: Sets a custom config file
        takes_value: true
    - INPUT:
        help: Sets the input file to use
        required: true
        index: 1
    - verbose:
        short: v
        multiple: true
        help: Sets the level of verbosity
    - test:
        about: controls testing features
        version: 1.3
        author: Someone E. <>
            - debug:
                short: d
                help: print debug information

Now we create our file just like we would have with the previous two examples:

// (Full example with detailed comments in examples/
// This example demonstrates clap's building from YAML style of creating arguments which is far
// more clean, but takes a very small performance hit compared to the other two methods.
extern crate clap;
use clap::App;

fn main() {
    // The YAML file is found relative to the current file, similar to how modules are found
    let yaml = load_yaml!("cli.yml");
    let matches = App::from_yaml(yaml).get_matches();

    // Same as previous examples...

NOTE: The YAML and macro builder options require adding a special features flag when compiling clap because they are not compiled by default. Simply change your clap = "2" to clap = {version = "2", features = ["yaml"]} for YAML, or features = ["unstable"] for the macro builder, in your Cargo.toml.

If you were to compile any of the above programs and run them with the flag --help or -h (or help subcommand, since we defined test as a subcommand) the following would be output

$ myprog --help
My Super Program 1.0
Kevin K. <>
Does awesome things


    -h, --help       Prints this message
    -v               Sets the level of verbosity
    -V, --version    Prints version information

    -c, --config <FILE>    Sets a custom config file

    INPUT    The input file to use

    help    Prints this message
    test    Controls testing features

NOTE: You could also run myapp test --help to see similar output and options for the test subcommand.

Try it!

Pre-Built Test

To try out the pre-built example, use the following steps:

  • Clone the repository $ git clone && cd clap-rs/clap-tests
  • Compile the example $ cargo build --release
  • Run the help info $ ./target/release/claptests --help
  • Play with the arguments!

BYOB (Build Your Own Binary)

To test out clap's default auto-generated help/version follow these steps:

  • Create a new cargo project $ cargo new fake --bin && cd fake
  • Add clap to your Cargo.toml
clap = "2"
  • Add the following to your src/
extern crate clap;
use clap::App;

fn main() {
  • Build your program $ cargo build --release
  • Run with help or version $ ./target/release/fake --help or $ ./target/release/fake --version


For full usage, add clap as a dependency in your Cargo.toml file to use from

clap = "2"

Or track the latest on the master branch at github:

git = ""

Add extern crate clap; to your crate root.

Define a list of valid arguments for your program (see the documentation or examples/ directory of this repo)

Then run cargo build or cargo update && cargo build for your project.

Optional Dependencies / Features

If you'd like to keep your dependency list to only clap, you can disable any features that require an additional dependency. To do this, add this to your Cargo.toml:

version = "2"
default-features = false

You can also selectively enable only the features you'd like to include, by adding:

version = "2"
default-features = false

# Cherry-pick the features you'd like to use
features = [ "suggestions", "color" ]

The following is a list of optional clap features:

  • "suggestions": Turns on the Did you mean '--myoption' ? feature for when users make typos.
  • "color": Turns on colored error messages. This feature only works on non-Windows OSs.
  • "lints": This is not included by default and should only be used while developing to run basic lints against changes. This can only be used on Rust nightly.
  • "debug": This is not included by default and should only be used while developing to display debugging information.
  • "yaml": This is not included by default. Enables building CLIs from YAML documents.
  • "unstable": This is not included by default. Enables unstable features, unstable refers to whether or not they may change, not performance stability.

Dependencies Tree

The following graphic depicts claps dependency graph (generated using cargo-graph).

  • Dashed Line: Optional dependency
  • Red Color: NOT included by default (must use cargo features to enable)

clap dependencies

More Information

You can find complete documentation on the github-pages site for this project.

You can also find usage examples in the examples/ directory of this repo.

Video Tutorials

There's also the video tutorial series Argument Parsing with Rust that I've been working on.

Note: Two new videos have just been added (08 From Usage, and 09 Typed Values), if you're already familiar with clap but want to know more about these two details you can check out those videos without watching the previous few.

Note: Apologies for the resolution of the first video, it will be updated to a better resolution soon. The other videos have a proper resolution.

How to Contribute

Contributions are always welcome! And there is a multitude of ways in which you can help depending on what you like to do, or are good at. Anything from documentation, code cleanup, issue completion, new features, you name it, even filing issues is contributing and greatly appreciated!

Another really great way to help is if you find an interesting, or helpful way in which to use clap. You can either add it to the examples/ directory, or file an issue and tell me. I'm all about giving credit where credit is due :)

Please read before you start contributing.

Running the tests

If contributing, you can run the tests as follows (assuming you're in the clap-rs directory)

$ cargo test && make -C clap-tests test
$ cargo test --features yaml

# Only on nightly compiler:
$ cargo build --features lints


There are a few goals of clap that I'd like to maintain throughout contributions. If your proposed changes break, or go against any of these goals we'll discuss the changes further before merging (but will not be ignored, all contributes are welcome!). These are by no means hard-and-fast rules, as I'm no expert and break them myself from time to time (even if by mistake or ignorance :P).

  • Remain backwards compatible when possible
    • If backwards compatibility must be broken, use deprecation warnings if at all possible before removing legacy code
    • This does not apply for security concerns
  • Parse arguments quickly
    • Parsing of arguments shouldn't slow down usage of the main program
    • This is also true of generating help and usage information (although slightly less stringent, as the program is about to exit)
  • Try to be cognizant of memory usage
    • Once parsing is complete, the memory footprint of clap should be low since the main program is the star of the show
  • panic! on developer error, exit gracefully on end-user error


clap is licensed under the MIT license. Please read the LICENSE-MIT file in this repository for more information.

Recent Breaking Changes

clap follows semantic versioning, so breaking changes should only happen upon major version bumps. The only exception to this rule is breaking changes that happen due to implementation that was deemed to be a bug, security concerns, or it can be reasonably proved to affect no code. For the full details, see

As of 2.0.0 (From 1.x)

  • Fewer lifetimes! Yay!
  • App<'a, 'b, 'c, 'd, 'e, 'f> => App<'a, 'b>
  • Arg<'a, 'b, 'c, 'd, 'e, 'f> => Arg<'a, 'b>
  • ArgMatches<'a, 'b> => ArgMatches<'a>
  • Simply Renamed
  • App::arg_group => App::group
  • App::arg_groups => App::groups
  • ArgGroup::add => ArgGroup::arg
  • ArgGroup::add_all => ArgGroup::args
  • ClapError => Error
  • struct field ClapError::error_type => Error::kind
  • ClapResult => Result
  • ClapErrorType => ErrorKind
  • Removed Deprecated Functions and Methods
  • App::subcommands_negate_reqs
  • App::subcommand_required
  • App::arg_required_else_help
  • App::global_version(bool)
  • App::versionless_subcommands
  • App::unified_help_messages
  • App::wait_on_error
  • App::subcommand_required_else_help
  • SubCommand::new
  • App::error_on_no_subcommand
  • Arg::new
  • Arg::mutually_excludes
  • Arg::mutually_excludes_all
  • Arg::mutually_overrides_with
  • simple_enum!
  • Renamed Error Variants
  • InvalidUnicode => InvalidUtf8
  • InvalidArgument => UnknownArgument
  • Usage Parser
  • Value names can now be specified inline, i.e. -o, --option <FILE> <FILE2> 'some option which takes two files'
  • There is now a priority of order to determine the name - This is perhaps the biggest breaking change. See the documentation for full details. Prior to this change, the value name took precedence. Ensure your args are using the proper names (i.e. typically the long or short and NOT the value name) throughout the code
  • ArgMatches::values_of returns an Values now which implements Iterator (should not break any code)
  • crate_version! returns &'static str instead of String
  • Using the clap_app! macro requires compiling with the unstable feature because the syntax could change slightly in the future


Old method names will be left around for several minor version bumps, or one major version bump.

As of 2.1.1:

  • None!