# Argh
**Argh is an opinionated Derive-based argument parser optimized for code size**
[](https://crates.io/crates/argh)
[](https://github.com/google/argh/LICENSE)
[](https://docs.rs/crate/argh/)
Derive-based argument parsing optimized for code size and conformance
to the Fuchsia commandline tools specification
The public API of this library consists primarily of the `FromArgs`
derive and the `from_env` function, which can be used to produce
a top-level `FromArgs` type from the current program's commandline
arguments.
## Basic Example
```rust,no_run
use argh::FromArgs;
#[derive(FromArgs)]
/// Reach new heights.
struct GoUp {
/// whether or not to jump
#[argh(switch, short = 'j')]
jump: bool,
/// how high to go
#[argh(option)]
height: usize,
/// an optional nickname for the pilot
#[argh(option)]
pilot_nickname: Option<String>,
}
fn main() {
let up: GoUp = argh::from_env();
}
```
`./some_bin --help` will then output the following:
```
Usage: cmdname [-j] --height <height> [--pilot-nickname <pilot-nickname>]
Reach new heights.
Options:
-j, --jump whether or not to jump
--height how high to go
--pilot-nickname an optional nickname for the pilot
--help display usage information
```
The resulting program can then be used in any of these ways:
- `./some_bin --height 5`
- `./some_bin -j --height 5`
- `./some_bin --jump --height 5 --pilot-nickname Wes`
Switches, like `jump`, are optional and will be set to true if provided.
Options, like `height` and `pilot_nickname`, can be either required,
optional, or repeating, depending on whether they are contained in an
`Option` or a `Vec`. Default values can be provided using the
`#[argh(default = "<your_code_here>")]` attribute, and in this case an
option is treated as optional.
```rust
use argh::FromArgs;
fn default_height() -> usize {
5
}
#[derive(FromArgs)]
/// Reach new heights.
struct GoUp {
/// an optional nickname for the pilot
#[argh(option)]
pilot_nickname: Option<String>,
/// an optional height
#[argh(option, default = "default_height()")]
height: usize,
/// an optional direction which is "up" by default
#[argh(option, default = "String::from(\"only up\")")]
direction: String,
}
fn main() {
let up: GoUp = argh::from_env();
}
```
Custom option types can be deserialized so long as they implement the
`FromArgValue` trait (automatically implemented for all `FromStr` types).
If more customized parsing is required, you can supply a custom
`fn(&str) -> Result<T, String>` using the `from_str_fn` attribute:
```rust
use argh::FromArgs;
#[derive(FromArgs)]
/// Goofy thing.
struct FiveStruct {
/// always five
#[argh(option, from_str_fn(always_five))]
five: usize,
}
fn always_five(_value: &str) -> Result<usize, String> {
Ok(5)
}
```
Positional arguments can be declared using `#[argh(positional)]`.
These arguments will be parsed in order of their declaration in
the structure:
```rust
use argh::FromArgs;
#[derive(FromArgs, PartialEq, Debug)]
/// A command with positional arguments.
struct WithPositional {
#[argh(positional)]
first: String,
}
```
The last positional argument may include a default, or be wrapped in
`Option` or `Vec` to indicate an optional or repeating positional argument.
Subcommands are also supported. To use a subcommand, declare a separate
`FromArgs` type for each subcommand as well as an enum that cases
over each command:
```rust
use argh::FromArgs;
#[derive(FromArgs, PartialEq, Debug)]
/// Top-level command.
struct TopLevel {
#[argh(subcommand)]
nested: MySubCommandEnum,
}
#[derive(FromArgs, PartialEq, Debug)]
#[argh(subcommand)]
enum MySubCommandEnum {
One(SubCommandOne),
Two(SubCommandTwo),
}
#[derive(FromArgs, PartialEq, Debug)]
/// First subcommand.
#[argh(subcommand, name = "one")]
struct SubCommandOne {
#[argh(option)]
/// how many x
x: usize,
}
#[derive(FromArgs, PartialEq, Debug)]
/// Second subcommand.
#[argh(subcommand, name = "two")]
struct SubCommandTwo {
#[argh(switch)]
/// whether to fooey
fooey: bool,
}
```
NOTE: This is not an officially supported Google product.
## How to debug the expanded derive macro for `argh`
The `argh::FromArgs` derive macro can be debugged with the [cargo-expand](https://crates.io/crates/cargo-expand) crate.
### Expand the derive macro in `examples/simple_example.rs`
See [argh/examples/simple_example.rs](./argh/examples/simple_example.rs) for the example struct we wish to expand.
First, install `cargo-expand` by running `cargo install cargo-expand`. Note this requires the nightly build of Rust.
Once installed, run `cargo expand` with in the `argh` package and you can see the expanded code.