clap 2.9.3

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

Crate clap [] [src]

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


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.

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 * toml [dependencies] 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.

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.

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

$ cargo build --features lints


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



Counts the number of comma-delimited expressions passed to it. The result is a compile-time evaluable expression, suitable for use as a static array size, or the value of a const.


Convenience macro to generate more complete enums with variants to be used as a type when parsing arguments. This enum also provides a variants() function which can be used to retrieve a Vec<&'static str> of the variant names, as well as implementing FromStr and Display automatically.


Allows you to pull the authors for the app from your Cargo.toml at compile time as "author1 lastname.", "author2 lastname."


Allows you to pull the version from your Cargo.toml at compile time as MAJOR.MINOR.PATCH_PKGVERSION_PRE


Convenience macro getting a typed value T where T implements std::str::FromStr from an argument value. This macro returns a Result<T,String> which allows you as the developer to decide what you'd like to do on a failed parse. There are two types of errors, parse failures and those where the argument wasn't present (such as a non-required argument). You can use it to get a single value, or a iterator as with the ArgMatches::values_of


Convenience macro getting a typed value T where T implements std::str::FromStr or exiting upon error, instead of returning a Result type.


Convenience macro getting a typed value Vec<T> where T implements std::str::FromStr This macro returns a clap::Result<Vec<T>> which allows you as the developer to decide what you'd like to do on a failed parse.


Convenience macro getting a typed value Vec<T> where T implements std::str::FromStr or exiting upon error.



Used to create a representation of a command line program and all possible command line arguments. Application settings are set using the "builder pattern" with the App::get_matches family of methods being the terminal methods that starts the runtime-parsing process. These methods then return information about the user supplied arguments (or lack there of).


The abstract representation of a command line argument. Used to set all the options and relationships that define a valid argument for the program.


ArgGroups are a family of related arguments and way for you to express, "Any of these arguments". By placing arguments in a logical group, you can create easier requirement and exclusion rules instead of having to list each argument individually, or when you want a rule to apply "any but not all" arguments.


Used to get information about the arguments that where supplied to the program at runtime by the user. New instances of this struct are obtained by using the App::get_matches family of methods.


Command Line Argument Parser Error


An iterator for getting multiple values out of an argument via the ArgMatches::values_of_os method. Usage of this iterator allows values which contain invalid UTF-8 code points unlike Values.


The abstract representation of a command line subcommand.


An iterator for getting multiple values out of an argument via the ArgMatches::values_of method.



Application level settings, which affect how App operates


Various settings that apply to arguments and may be set, unset, and checked via getter/setter methods Arg::set, Arg::unset, and Arg::is_set


Command line argument parser kind of error


Describes which shell to produce a completions file for

Type Definitions


Short hand for Result type