clap 0.5.9

A simple and efficient Command Line Argument Parser



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.

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 be fully, or partially overridden if you wish to roll your own 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)
    • Also supports multiple occurrences (i.e. myprog -vvv or myprog -v -v -v)
  • Positional Arguments (i.e. those which are based off an index)
    • Also supports multiple values (i.e. myprog <file>...
    • 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)
    • Also supports multiple values (i.e. myprog --option <value> --option <othervalue>)
    • 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
    • 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
  • Auto Version from Cargo.toml: clap is fully compatible with Rust's env!() macro for achieving this functionality. 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 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 an optional output file")
                               .help("Turn debugging information on"))
                                      .about("controls testing features")
                                          .help("print test information verbosely")))

    if let Some(o) = matches.value_of("output") {
        println!("Value for output: {}", o);

    if let Some(c) = matches.value_of("CONFIG") {
        println!("Value for config: {}", c);

    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(ref 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 output woud be presented

$ myprog --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

    output            Sets an optional output file

    help            Prints this message
    test            Controls testing features


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)

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 full usage examples in the examples/ directory of this repo.

How to build and 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$USER/clap-rs && cd clap-rs)
  3. Create new branch (git checkout -b your-branch)
  4. Make your changes, and commit (git commit -am "your message")
  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 create the pull request right away, and we'll merge when read. 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 already cloned the repo to clap-rs/

cd clap-rs
cargo test
cd claptests
make test

Building the documentation

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

make doc

Then browse to clap-rs/docs/clap/index.html in your web-browser of choice to check it out. You can then create a PR on the gh-pages branch


There are a few goals of clap that I'd like to maintain. 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 just 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
  • Try not 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