clap/README.md

clap

Command Line Argument Parser for Rust

Dual-licensed under Apache 2.0 or MIT.

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

About

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

Example

This uses our 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)]
    name: String,

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

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

    for _ in 0..args.count {
        println!("Hello {}!", args.name)
    }
}

Add this to Cargo.toml:

[dependencies]
clap = { version = "3.1.8", features = ["derive"] }

$ demo --help
clap [..]
Simple program to greet a person

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

OPTIONS:
    -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)

Aspirations

• 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 1.54.0)

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.

Related Projects

• wild for supporting wildcards (*) on Windows like you do Linux
• argfile for loading additional arguments from a file (aka response files)
• shadow-rs for generating Command::long_version
• clap_lex for a lighter-weight, battle-tested CLI parser
• clap_mangen for generating man page source (roff)
• clap_complete for shell completion support
• clap-verbosity-flag
• clap-cargo
• concolor-clap
• Command-line Apps for Rust book
• trycmd: Snapshot testing
  • Or for more control, assert_cmd and assert_fs

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

• 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.

• unstable-replace: Enable Command::replace
• unstable-multicall: Enable Command::multicall
• unstable-grouped: Enable ArgMatches::grouped_values_of
• unstable-v4: Preview features which will be stable on the v4.0 release

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