clap-rs/clap

clap (Command Line Argument Parser) is a Rust library that makes it easier to build command-line tools, programs you run in a terminal. When you write a command-line program, you typically want users to be able to pass in options and values (like flags, file paths, or modes). Handling all of that input correctly from scratch is tedious and error-prone. clap solves that problem by giving you a ready-made system to define what arguments your program accepts and then automatically parse, validate, and document them. With clap (16,381 stars), a Rust developer can describe the full interface of their command-line tool, including positional arguments (values given by position, like a filename), optional flags (like --verbose), and subcommands (nested commands like "git commit" or "git push"), either declaratively (by describing the structure) or procedurally (by writing code step by step). Once the interface is described, clap handles all the hard work: parsing user input, showing helpful error messages for invalid input, and auto-generating a help page. You would use clap any time you are building a Rust command-line application and want users to pass arguments to it. It is dual-licensed under Apache 2.0 and MIT, meaning it can be freely used in both open-source and commercial projects. The README is brief, it points to full documentation and examples hosted separately.