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0.5.0 Jun 4, 2022
0.4.2 Jun 3, 2021
0.4.1 May 3, 2021
0.4.0 Jan 3, 2021
0.2.0 Jul 26, 2019

#38 in Command-line interface

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385,519 downloads per month
Used in 532 crates (113 directly)

MIT license

666 lines


Build Status Crates.io Documentation Rust 1.32+

An ultra simple CLI arguments parser.

If you think that this library doesn't support some feature, it's probably intentional.

  • No help generation
  • Only flags, options, free arguments and subcommands are supported
  • Options can be separated by a space, = or nothing. See build features
  • Arguments can be in any order
  • Non UTF-8 arguments are supported

Build features

  • eq-separator

    Allows parsing arguments separated by =
    This feature adds about 1KiB to the resulting binary

  • short-space-opt

    Makes the space between short keys and their values optional (e.g. -w10)
    If eq-separator is enabled, then it takes precedence and the '=' is not included.
    If eq-separator is disabled, then -K=value gives an error instead of returning "=value".
    The optional space is only applicable for short keys because --keyvalue would be ambiguous

  • combined-flags

    Allows combination of flags, e.g. -abc instead of -a -b -c
    If short-space-opt or eq-separator are enabled, you must parse flags after values, to prevent ambiguities


The main fundamental limitation of pico-args is that it parses arguments in an arbitrary order. This is because we have a sort of "steaming" API and we don't know all the keys/arguments beforehand. This could lead to some unexpected behaviors. Specifically, let's say you have a following arguments:

--arg1 --arg2 value

If your parser tries to parse --arg1 as key-value first, than its value would be --arg2 and not value, because the parser simply takes the "next" argument. A properer parser would knew that --arg2 is a key and will return an error, since the value is missing.

If your parser tries to parse --arg2 as a flag first and then --arg1 as key-value, than its value would be value, because --arg2 was already removed by the parser and the arguments list looks like --arg1 value to the parser.

If such behavior is unacceptable to your application, then you have to use a more high-level arguments parsing library.


The core idea of pico-args is to provide some "sugar" for arguments parsing without a lot of overhead (binary or compilation time wise). There are no point in comparing parsing features since pico-args supports only the bare minimum. Here is a great comparison of various arguments parsing libraries.



No runtime deps