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Command Line Argument Parser for Rust

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Dual-licensed under Apache 2.0 or MIT.

  1. About
  2. Tutorial: Builder API, Derive API
  3. Examples
  4. API Reference
  6. FAQ
  7. Questions & Discussions
  8. Contributing
  9. Sponsors


Create your command-line parser, with all of the bells and whistles, declaratively or procedurally.


Add clap as a dependency with the derive feature enabled:

cargo add clap -F derive

This allows using the Derive API which provides access to the Builder API as attributes on a struct:

use clap::Parser;

/// Simple program to greet a person
#[derive(Parser, Debug)]
#[clap(author, version, about, long_about = None)]
struct Args {
    /// Name of the person to greet
    #[clap(short, long, value_parser)]
    name: String,

    /// Number of times to greet
    #[clap(short, long, value_parser, default_value_t = 1)]
    count: u8,

fn main() {
    let args = Args::parse();

    for _ in 0..args.count {
        println!("Hello {}!", args.name)
$ demo --help
clap [..]
Simple program to greet a person

    demo[EXE] [OPTIONS] --name <NAME>

    -c, --count <COUNT>    Number of times to greet [default: 1]
    -h, --help             Print help information
    -n, --name <NAME>      Name of the person to greet
    -V, --version          Print version information

(version number and .exe extension on windows replaced by placeholders)


  • Out of the box, users get a polished CLI experience
    • Including common argument behavior, help generation, suggested fixes for users, colored output, shell completions, etc
  • Flexible enough to port your existing CLI interface
    • However, we won’t necessarily streamline support for each use case
  • Reasonable parse performance
  • Resilient maintainership, including
    • Willing to break compatibility rather than batching up breaking changes in large releases
    • Leverage feature flags to keep to one active branch
    • Being under WG-CLI to increase the bus factor
  • We follow semver and will wait about 6-9 months between major breaking changes
  • We will support the last two minor Rust releases (MSRV, currently 1.56.1)

While these aspirations can be at odds with fast build times and low binary size, we will still strive to keep these reasonable for the flexibility you get. Check out the argparse-benchmarks for CLI parsers optimized for other use cases.

Selecting an API

Why use the declarative Derive API:

  • Easier to read, write, and modify
  • Easier to keep the argument declaration and reading of argument in sync
  • Easier to reuse, e.g. clap-verbosity-flag

Why use the procedural Builder API:

  • Faster compile times if you aren’t already using other procedural macros
  • More flexible, e.g. you can look up how many times an argument showed up, what its values were, and what were the indexes of those values. The Derive API can only report presence, number of occurrences, or values but no indices or combinations of data.

Feature Flags

Default Features

  • std: Not Currently Used. Placeholder for supporting no_std environments in a backwards compatible manner.
  • color: Turns on colored error messages.
  • suggestions: Turns on the Did you mean '--myoption'? feature for when users make typos.
Optional features
  • deprecated: Guided experience to prepare for next breaking release (at different stages of development, this may become default)
  • derive: Enables the custom derive (i.e. #[derive(Parser)]). Without this you must use one of the other methods of creating a clap CLI listed above.
  • cargo: Turns on macros that read values from CARGO_* environment variables.
  • env: Turns on the usage of environment variables during parsing.
  • regex: Enables regex validators.
  • unicode: Turns on support for unicode characters (including emoji) in arguments and help messages.
  • wrap_help: Turns on the help text wrapping feature, based on the terminal size.
Experimental features

Warning: These may contain breaking changes between minor releases.






For tutorials, examples, etc, see either https://github.com/clap-rs/clap or build with --features unstable-doc


pub use crate::error::Error;
pub use crate::error::Result;


Error reporting

Command line argument parser


Deprecated, replaced with clap::command!

Create an Arg from a usage string.


Allows you to build the Command instance from your Cargo.toml at compile time.

Allows you to pull the authors for the command from your Cargo.toml at compile time in the form: "author1 lastname <author1@example.com>:author2 lastname <author2@example.com>"

Allows you to pull the description from your Cargo.toml at compile time.

Allows you to pull the name from your Cargo.toml at compile time.

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

Select a ValueParser implementation from the intended type


Deprecated, replaced with Command

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

Family of related arguments.

Container for parse results.

Iterate over indices for where an argument appeared when parsing, via ArgMatches::indices_of

Deprecated, replaced with ArgMatches::get_many()

A possible value of an argument.

Deprecated, replaced with ArgMatches::get_many()


Application level settings, which affect how Command operates

Behavior of arguments when they are encountered while parsing

Various settings that apply to arguments and may be set, unset, and checked via getter/setter methods Arg::setting, Arg::unset_setting, and Arg::is_set. This is what the Arg methods which accept a bool use internally.

Represents the color preferences for program output

Command line argument parser kind of error

Contains either a regular expression or a set of them or a reference to one.

Provide shell with hint on how to complete an argument.

Origin of the argument’s value


Parse arguments into enums.

Parse a set of arguments into a user-defined container.

Create a Command relevant for a user-defined container.

Converts an instance of ArgMatches to a user-defined container.

Create a Command relevant for a user-defined container.

Parse command-line arguments into Self.

Parse a sub-command into a user-defined enum.

Parse arguments into enums.

Type Definitions

Build a command-line interface.

Derive Macros

Deprecated, replaced with ValueEnum Generates the ValueEnum impl.