How to capture the output of a process piped into a Rust program?

Question

I know how to read the command line arguments, but I am having difficulties reading the command output from a pipe.

1. Connect a program (A) that outputs data to my Rust program using a pipe:

   A | R
   

2. The program should consume the data line by line as they come.

   $ pwd | cargo run should print the pwd output.

   OR

   $ find . | cargo run should output the find command output which is more than 1 line.

Answer 1

Use BufRead::lines on a locked handle to standard input:

use std::io::{self, BufRead};

fn main() {
    let stdin = io::stdin();
    for line in stdin.lock().lines() {
        let line = line.expect("Could not read line from standard in");
        println!("{}", line);
    }
}

If you wanted to reuse the allocation of the String, you could use the loop form:

use std::io::{self, Read};

fn main() {
    let stdin = io::stdin();
    let mut stdin = stdin.lock(); // locking is optional

    let mut line = String::new();

    // Could also `match` on the `Result` if you wanted to handle `Err` 
    while let Ok(n_bytes) = stdin.read_to_string(&mut line) {
        if n_bytes == 0 { break }
        println!("{}", line);
        line.clear();
    }
}