# Pipes

The `std::Child` struct represents a running child process, and exposes the
`stdin`, `stdout` and `stderr` handles for interaction with the underlying
process via pipes.

```rust,ignore
use std::io::prelude::*;
use std::process::{Command, Stdio};

static PANGRAM: &'static str =
"the quick brown fox jumps over the lazy dog\n";

fn main() {
    // Spawn the `wc` command
    let mut cmd = if cfg!(target_family = "windows") {
        let mut cmd = Command::new("powershell");
        cmd.arg("-Command").arg("$input | Measure-Object -Line -Word -Character");
        cmd
    } else {
        Command::new("wc")
    };
    let process = match cmd
                                .stdin(Stdio::piped())
                                .stdout(Stdio::piped())
                                .spawn() {
        Err(why) => panic!("couldn't spawn wc: {}", why),
        Ok(process) => process,
    };

    // Write a string to the `stdin` of `wc`.
    //
    // `stdin` has type `Option<ChildStdin>`, but since we know this instance
    // must have one, we can directly `unwrap` it.
    match process.stdin.unwrap().write_all(PANGRAM.as_bytes()) {
        Err(why) => panic!("couldn't write to wc stdin: {}", why),
        Ok(_) => println!("sent pangram to wc"),
    }

    // Because `stdin` does not live after the above calls, it is `drop`ed,
    // and the pipe is closed.
    //
    // This is very important, otherwise `wc` wouldn't start processing the
    // input we just sent.

    // The `stdout` field also has type `Option<ChildStdout>` so must be unwrapped.
    let mut s = String::new();
    match process.stdout.unwrap().read_to_string(&mut s) {
        Err(why) => panic!("couldn't read wc stdout: {}", why),
        Ok(_) => print!("wc responded with:\n{}", s),
    }
}
```
