It is a simple-to-use, efficient, and full-featured library for parsing command line arguments and subcommands when writing console/terminal applications.
> If you're looking for the readme for `clap v2.33` - find it on [github](https://github.com/clap-rs/clap/tree/v2.33.0), [crates.io](https://crates.io/crates/clap/2.33.0), [docs.rs](https://docs.rs/clap/2.33.0/clap/).
`clap` is used to parse *and validate* the string of command line arguments provided by a user at runtime. You provide the list of valid possibilities, and `clap` handles the rest. This means you focus on your *applications* functionality, and less on the parsing and validating of arguments.
`clap` provides many things 'for free' (with no configuration) including the traditional version and help switches (or flags) along with associated messages. If you are using subcommands, `clap` will also auto-generate a `help` subcommand and separate associated help messages.
Once `clap` parses the user provided string of arguments, it returns the matches along with any applicable values. If the user made an error or typo, `clap` informs them with a friendly message and exits gracefully (or returns a `Result` type and allows you to perform any clean up prior to exit). Because of this, you can make reasonable assumptions in your code about the validity of the arguments prior to your applications main execution.
First, let me say that these comparisons are highly subjective, and not meant in a critical or harsh manner. All the argument parsing libraries out there (to include `clap`) have their own strengths and weaknesses. Sometimes it just comes down to personal taste when all other factors are equal. When in doubt, try them all and pick one that you enjoy :) There's plenty of room in the Rust community for multiple implementations!
Simple! `clap`*is*`structopt`. With the 3.0 release, `clap` imported the `structopt` code into it's own codebase as the [`clap_derive`](https://github.com/clap-rs/clap_derive) crate. Since `structopt` already used `clap` under the hood, the transition was nearly painless, and is 100% feature compatible.
If you were using `structopt` before, the only thing you should have to do is change the attributes from `#[structopt(...)]` to `#[clap(...)]`.
Also the derive statements changed from `#[derive(Structopt)]` to `#[derive(Clap)]`. There is also some additional functionality that's been added to the `clap_derive` crate. See the documentation for that crate, for more details.
`getopts` is a very basic, fairly minimalist argument parsing library. This isn't a bad thing, sometimes you don't need tons of features, you just want to parse some simple arguments, and have some help text generated for you based on valid arguments you specify. The downside to this approach is that you must manually implement most of the common features (such as checking to display help messages, usage strings, etc.). If you want a highly custom argument parser, and don't mind writing the majority of the functionality yourself, `getopts` is an excellent base.
`getopts` also doesn't allocate much, or at all. This gives it a very small performance boost. Although, as you start implementing additional features, that boost quickly disappears.
Personally, I find many, many uses of `getopts` are manually implementing features that `clap` provides by default. Using `clap` simplifies your codebase allowing you to focus on your application, and not argument parsing.
I first want to say I'm a big a fan of BurntSushi's work, the creator of `Docopt.rs`. I aspire to produce the quality of libraries that this man does! When it comes to comparing these two libraries they are very different. `docopt` tasks you with writing a help message, and then it parsers that message for you to determine all valid arguments and their use. Some people LOVE this approach, others do not. If you're willing to write a detailed help message, it's nice that you can stick that in your program and have `docopt` do the rest. On the downside, it's far less flexible.
`docopt` is also excellent at translating arguments into Rust types automatically. There is even a syntax extension which will do all this for you, if you're willing to use a nightly compiler (use of a stable compiler requires you to somewhat manually translate from arguments to Rust types). To use BurntSushi's words, `docopt` is also a sort of black box. You get what you get, and it's hard to tweak implementation or customize the experience for your use case.
Because `docopt` is doing a ton of work to parse your help messages and determine what you were trying to communicate as valid arguments, it's also one of the more heavy weight parsers performance-wise. For most applications this isn't a concern and this isn't to say `docopt` is slow, in fact far from it. This is just something to keep in mind while comparing.
`clap` is as fast, and as lightweight as possible while still giving all the features you'd expect from a modern argument parser. In fact, for the amount and type of features `clap` offers it remains about as fast as `getopts`. If you use `clap` when just need some simple arguments parsed, you'll find it's a walk in the park. `clap` also makes it possible to represent extremely complex, and advanced requirements, without too much thought. `clap` aims to be intuitive, easy to use, and fully capable for wide variety use cases and needs.
#### All else being equal, what are some reasons *not* to use `clap`? (The Anti Pitch)
Depending on the style in which you choose to define the valid arguments, `clap` can be very verbose. `clap` also offers so many finetuning knobs and dials, that learning everything can seem overwhelming. I strive to keep the simple cases simple, but when turning all those custom dials it can get complex. `clap` is also opinionated about parsing. Even though so much can be tweaked and tuned with `clap` (and I'm adding more all the time), there are still certain features which `clap` implements in specific ways which may be contrary to some users use-cases. Finally, `clap` is "stringly typed" when referring to arguments which can cause typos in code. This particular paper-cut is being actively worked on, and should be gone in v3.x.
Below are a few of the features which `clap` supports, full descriptions and usage can be found in the [documentation](https://docs.rs/clap/) and [examples/](examples) directory
* **Groups**: Arguments can be made part of a group
- Fully compatible with other relational rules (requirements, conflicts, and overrides) which allows things like requiring the use of any arg in a group, or denying the use of an entire group conditionally
* **Specific Value Sets**: Positional or Option Arguments can 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`)
* **Automatic Version from Cargo.toml**: `clap` is fully compatible with Rust's `env!()` macro for automatically setting the version of your application to the version in your Cargo.toml. See [09_auto_version example](examples/09_auto_version.rs) for how to do this (Thanks to [jhelwig](https://github.com/jhelwig) for pointing this out)
* **Typed Values**: You can use several convenience macros provided by `clap` to get typed values (i.e. `i32`, `u8`, etc.) from positional or option arguments so long as the type you request implements `std::str::FromStr` See the [12_typed_values example](examples/12_typed_values.rs). You can also use `clap`s `arg_enum!` macro to create an enum with variants that automatically implement `std::str::FromStr`. See [13a_enum_values_automatic example](examples/13a_enum_values_automatic.rs) for details
* **Suggestions**: Suggests corrections when the user enters a typo. For example, if you defined a `--myoption` argument, and the user mistakenly typed `--moyption` (notice `y` and `o` transposed), they would receive a `Did you mean '--myoption'?` error and exit gracefully. This also works for subcommands and flags. (Thanks to [Byron](https://github.com/Byron) for the implementation) (This feature can optionally be disabled, see 'Optional Dependencies / Features')
* **Colorized Errors (Non Windows OS only)**: Error message are printed in in colored text (this feature can optionally be disabled, see 'Optional Dependencies / Features').
* **Global Arguments**: Arguments can optionally be defined once, and be available to all child subcommands. There values will also be propagated up/down throughout all subcommands.
* **Custom Validations**: You can define a function to use as a validator of argument values. Imagine defining a function to validate IP addresses, or fail parsing upon error. This means your application logic can be solely focused on *using* values.
* **POSIX Compatible Conflicts/Overrides** - In POSIX args can be conflicting, but not fail parsing because whichever arg comes *last* "wins" so to speak. This allows things such as aliases (i.e. `alias ls='ls -l'` but then using `ls -C` in your terminal which ends up passing `ls -l -C` as the final arguments. Since `-l` and `-C` aren't compatible, this effectively runs `ls -C` in `clap` if you choose...`clap` also supports hard conflicts that fail parsing). (Thanks to [Vinatorul](https://github.com/Vinatorul)!)
The following examples show a quick example of some of the very basic functionality of `clap`. For more advanced usage, such as requirements, conflicts, groups, multiple values and occurrences see the [documentation](https://docs.rs/clap/), [examples/](examples) directory of this repository or the [video tutorials](https://www.youtube.com/playlist?list=PLza5oFLQGTl2Z5T8g1pRkIynR3E0_pc7U).
**NOTE:** All of these examples are functionally the same, but show different styles in which to use `clap`. These different styles are purely a matter of personal preference.
The first example shows the simplest way to use `clap`, by defining a struct. If you're familiar with the `structopt` crate you're in luck, it's the same! (In fact it's the exact same code running under the covers!)
This second method shows a method using the 'Builder Pattern' which allows more advanced configuration options (not shown in this small example), or even dynamically generating arguments when desired. The downside is it's more verbose.
```rust
// (Full example with detailed comments in examples/01a_quick_example.rs)
//
// This example demonstrates clap's "builder pattern" method of creating arguments
// which the most flexible, but also most verbose.
The next example shows a far less verbose method, but sacrifices some of the advanced configuration options (not shown in this small example). This method also takes a *very* minor runtime penalty.
Since this feature requires additional dependencies that not everyone may want, it is *not* compiled in by default and we need to enable a feature flag in Cargo.toml:
If you were to compile any of the above programs and run them with the flag `--help` or `-h` (or `help` subcommand, since we defined `test` as a subcommand) the following would be output (except the first example where the help message sort of explains the Rust code).
(**note**: If you are concerned with supporting a minimum version of Rust that is *older* than the current stable Rust minus 2 stable releases, it's recommended to use the `~major.minor.patch` style versions in your `Cargo.toml` which will only update the patch version automatically. For more information see the [Compatibility Policy](#compatibility-policy))
* **derive**: Enables the custom derive (i.e. `#[derive(Clap)]`). Without this you must use one of the other methods of creating a `clap` CLI listed above
* **"vec_map"**: Use [`VecMap`](https://crates.io/crates/vec_map) internally instead of a [`BTreeMap`](https://doc.rust-lang.org/stable/std/collections/struct.BTreeMap.html). This feature provides a _slight_ performance improvement. (builds dependency `vec_map`)
`clap` will pin the minimum required version of Rust to the CI builds. Bumping the minimum version of Rust is considered a minor breaking change, meaning *at a minimum* the minor version of `clap` will be bumped.
In order to keep from being surprised of breaking changes, it is **highly** recommended to use the `~major.minor.patch` style in your `Cargo.toml` only if you wish to target a version of Rust that is *older* than current stable minus two releases:
This will cause *only* the patch version to be updated upon a `cargo update` call, and therefore cannot break due to new features, or bumped minimum versions of Rust.
Using `~` can cause issues in certain circumstances.
From @alexcrichton:
Right now Cargo's version resolution is pretty naive, it's just a brute-force search of the solution space, returning the first resolvable graph. This also means that it currently won't terminate until it proves there is not possible resolvable graph. This leads to situations where workspaces with multiple binaries, for example, have two different dependencies such as:
This is inherently an unresolvable crate graph in Cargo right now. Cargo requires there's only one major version of a crate, and being in the same workspace these two crates must share a version. This is impossible in this location, though, as these version constraints cannot be met.
`clap` will officially support current stable Rust, minus two releases, but may work with prior releases as well. For example, current stable Rust at the time of this writing is 1.38.0, meaning `clap` is guaranteed to compile with 1.36.0 and beyond.
There are several excellent crates which can be used with `clap`, I recommend checking them all out! If you've got a crate that would be a good fit to be used with `clap` open an issue and let me know, I'd love to add it!
* [`structopt`](https://github.com/TeXitoi/structopt) - This crate allows you to define a struct, and build a CLI from it! No more "stringly typed" and it uses `clap` behind the scenes! (*Note*: There is work underway to pull this crate into mainline `clap`).
* [`assert_cmd`](https://github.com/assert-rs/assert_cmd) - This crate allows you test your CLIs in a very intuitive and functional way!
`clap` follows semantic versioning, so breaking changes should only happen upon major version bumps. The only exception to this rule is breaking changes that happen due to implementation that was deemed to be a bug, security concerns, or it can be reasonably proved to affect no code. For the full details, see [CHANGELOG.md](./CHANGELOG.md).
* Argument values now take precedence over subcommand names. This only arises by using unrestrained multiple values and subcommands together where the subcommand name can coincide with one of the multiple values. Such as `$ prog <files>... <subcommand>`. The fix is to place restraints on number of values, or disallow the use of `$ prog <prog-args> <subcommand>` structure.
* struct field `ClapError::error_type` => `Error::kind`
*`ClapResult` => `Result`
*`ClapErrorType` => `ErrorKind`
* **Removed Deprecated Functions and Methods**
*`App::subcommands_negate_reqs`
*`App::subcommand_required`
*`App::arg_required_else_help`
*`App::global_version(bool)`
*`App::versionless_subcommands`
*`App::unified_help_messages`
*`App::wait_on_error`
*`App::subcommand_required_else_help`
*`SubCommand::new`
*`App::error_on_no_subcommand`
*`Arg::new`
*`Arg::mutually_excludes`
*`Arg::mutually_excludes_all`
*`Arg::mutually_overrides_with`
*`simple_enum!`
* **Renamed Error Variants**
*`InvalidUnicode` => `InvalidUtf8`
*`InvalidArgument` => `UnknownArgument`
* **Usage Parser**
* Value names can now be specified inline, i.e. `-o, --option <FILE> <FILE2> 'some option which takes two files'`
* **There is now a priority of order to determine the name** - This is perhaps the biggest breaking change. See the documentation for full details. Prior to this change, the value name took precedence. **Ensure your args are using the proper names (i.e. typically the long or short and NOT the value name) throughout the code**
*`ArgMatches::values_of` returns an `Values` now which implements `Iterator` (should not break any code)
*`crate_version!` returns `&'static str` instead of `String`
