clap 0.5.14

A simple and efficient Command Line Argument Parser


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Command Line Argument Parser written in Rust

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

Video Tutorials

I've been working on a few short video tutorials about using clap. They're located on youtube.

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.


You can use clap to lay out a list of possible valid command line arguments and subcommands, then let clap parse and validate the string given by the user at runtime. This means you focus on your applications functionality, not parsing and validating arguments.

What is different about clap from other options available is the very simple almost 'Pythonic' style in which you define the valid available arguments for your program, while still giving advanced features. clap allows you express complex relationships between arguments in a very simple manner. This means you don't have to spend tons time learning an entirely new library's structures and use. The basics of clap can be learned almost intuitively.

clap also provides all the traditional version and help switches (or flags) 'for free' by parsing the list of developer supplied arguments. If the developer hasn't defined them already (or only defined some of them), clap will auto-generate the applicable "help" and "version" switches (as well as a "help" subcommand so long as other subcommands have been manually defined as well).

After defining a list of possible valid arguments and subcommands, clap parses the string given by the end-user at runtime then gives you a list of the valid matches and their values. If the user made an error or typo, clap informs them and exits gracefully. This means that you can simply use these matches and values to determine the functioning of your program.


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)
    • Optionally supports multiple occurrences (i.e. myprog -vvv or myprog -v -v -v)
  • Positional Arguments (i.e. those which are based off an index from the program name)
    • Optionally supports multiple values (i.e. myprog <file>... such as myprog file1.txt file2.txt being two values for the same "file" argument)
    • Optionally supports Specific Value Sets (See below)
  • Option Arguments (i.e. those that take values as options)
    • Both short and long versions supported (i.e. -o value and --option value or --option=value respectively)
    • Optionally supports multiple values (i.e. myprog --option <value> --option <other_value>)
    • Optionally supports Specific Value Sets (See below)
  • Sub-Commands (i.e. git add <file> where add is a sub-command of git)
    • Support their own sub-arguments, and sub-commands independant of the parent
    • Get their own auto-generated Help, Version, and Usage independant of parent
  • Requirement Rules: Arguments can optionally define the following types of requirement rules
    • Required by default
    • Required only if certain arguments are present
    • Can require other arguments to be present
  • Exclusion 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
  • Specific Value Sets: Positional or Option Arguments can optionally 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
  • Get Version from Cargo.toml: clap is fully compatible with Rust's env!() macro for automatically getting the version from your Cargo.toml. See examples/ for how to do this (Thanks to jhelwig for pointing this out)

Quick Example

The following shows a quick example of some of the basic functionality of clap. For more advanced usage, such as requirements, exclusions, multiple values and occurrences see the video tutorials, documentation, or examples/ directory of this repository.

// (Full example with comments in examples/
extern crate clap;
use clap::{Arg, App, SubCommand};

fn main() {
    let matches = App::new("myapp")
                          .author("Kevin K. <>")
                          .about("Does awesome things")
                               .help("Sets a custom config file")
                               .help("Sets the input file to use")
                               .help("Turn debugging information on"))
                                      .about("controls testing features")
                                      .author("Someone E. <>")
                                          .help("print test information verbosely")))

    // 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());
    // 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);
    // Vary the output based on how many times the user used the "debug" flag
    // (i.e. 'myapp -d -d -d' or 'myapp -ddd' vs 'myapp -d'   
    match matches.occurrences_of("debug") {
        0 => println!("Debug mode is off"),
        1 => println!("Debug mode is kind of on"),
        2 => println!("Debug mode is on"),
        3 | _ => println!("Don't be crazy"),

    if let Some(matches) = matches.subcommand_matches("test") {
        if matches.is_present("verbose") {
            println!("Printing verbosely...");
        } else {
            println!("Printing normally...");

    // more porgram logic goes here...

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

$ myapp --help
myapp 1.0
Kevin K. <>
Does awesome things


    -d               Turn debugging information on
    -h, --help       Prints this message
    -v, --version    Prints version information
    -c, --config=CONFIG    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.


Add clap as a dependecy in your Cargo.toml file to use from

clap = "*"

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.

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.

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!

  1. Fork the project
  2. Clone your fork (git clone$YOUR_USERNAME/clap-rs && cd clap-rs)
  3. Create new branch (git checkout -b new-branch)
  4. Make your changes, and commit (git commit -am "your message") (I try to use a conventional changelog format so I can update it using clog)
  5. If applicable, run the tests (See below)
  6. Push your changes back to your fork (git push origin your-branch)
  7. Create a pull request! (You can also create the pull request right away, and we'll merge when ready. This a good way to discuss proposed changes.)

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 :)

Running the tests

If contributing, you can run the tests as follows (assuming you've cloned the repo to clap-rs/

cd clap-rs && cargo test
cd clap-tests && make test

Building the documentation

If your changes require re-building the documentation, run this instead of cargo doc to generate the proper module docstring:

cd clap-rs && make doc

Then browse to clap-rs/docs/clap/index.html in your web-browser of choice to check it out.


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 the LICENSE-MIT file in this repository for more information.