system.os

system.os covers the process environment, the command line, and the host platform. Env and Args follow the same capability rule as Fs.

use system.io
use system.os

fn main()
    // platform() names the host: "macos", "linux" or "windows".
    println(f"platform: {platform()}")

    // Env is the capability for the process environment.
    let env = Env()
    match env.set("MIRI_GREETING", "hello")
        Result.Ok(replaced): println(f"set (replaced an existing value: {replaced})")
        Result.Err(e): println(f"could not set: {e}")

    // get returns String? — an unset variable is None, not an empty string.
    match env.get("MIRI_GREETING")
        Some(value): println(f"MIRI_GREETING={value}")
        None: println("MIRI_GREETING is unset")

    // Args is Iterable<String> over the command line.
    let args = Args()
    println(f"{args.length()} argument(s)")
    for arg in args
        println(f"  {arg}")

Member	Returns	Behaviour
`Env().get(name String)`	`String?`	An unset variable is `None`, distinct from a variable set to the empty string.
`Env().set(name String, value String)`	`Result<bool, EnvError>`	`true` when an existing value was replaced.
`Args().length()`	`int`	Number of command-line arguments.
`Args().element_at(index int)`	`String`	One argument; `Args` is `Iterable<String>`, so `for..in` works.
`platform()`	`String`	Free function naming the host: `macos`, `linux` or `windows`.
`exit(code int)`	—	Ends the process with a status code.