How to redirect child process output to stderr?

Question

I am trying to start a process with the Command API and redirect its standard output to standard error. The following fails:

Command::new("tput")
    .arg("rc")
    .stdout(io::stderr())
    .status()
    .expect("failed to run tput");

because Command::new("tput").arg("rc").stdout(<XXX>) expects a std::process::Stdio:

expected struct `std::process::Stdio`, found struct `std::io::Stderr`

The equivalent in Bash would probably be tput rc > /dev/stderr.

I would like to know how to do this properly.

Deduct

Briefly looking at the API I'd say this is not possible. Perhaps an issue should be raised for this where the stdout method could accept a Writer instance instead? – Smut

Answer 1

As of Rust 1.74 which contains rust-lang/rust#114590, the code works exactly as you originally tried it. This didn't used to work in older versions of Rust.

Command::new("tput")
    .arg("rc")
    .stdout(io::stderr())
    .status()
    .expect("failed to run tput");

Ulcer

Answer 2

As of Rust 1.15.0, Stdio doesn't expose this functionality in a portable API, but there are platform-specific extension traits that you can use for this purpose.

On Unix-like platforms, the std::os::unix::io::FromRawFd trait is implemented on Stdio. This trait provides a single method, from_raw_fd, that can turn a file descriptor into the type that implements the trait. Since standard error is defined as file descriptor 2, you could simply use .stdout(Stdio::from_raw_fd(2)).

On Windows, there's a similar trait called FromRawHandle implemented on Stdio. Unfortunately, it's not listed in the online documentation; it only contains the Unix-specific variants. You would call GetStdHandle(STD_ERROR_HANDLE) to obtain a handle to the standard error.

Clinical

Answer 3

This worked on linux. Pulling in the AsFd trait allows us to get a BorrowedFd from stderr. The try_clone_to_owned method was hard to find, but it gives a OwnedFd which can automatically convert into a process::Stdio, the type required to configure stderr of child processes.

use std::os::fd::AsFd;

let stderr = std::io::stderr().as_fd().try_clone_to_owned()?;
std::process::Command::new("terraform")
    .arg("fmt")
    .arg(&outpath)
    .stdout(stderr)
    .status()
    .unwrap_or_else(|_| panic!("terraform fmt failed: {:?}", outpath));

---

Compacted into a function:

fn stderr() -> Result<std::process::Stdio, Error> {
    use std::os::fd::AsFd;
    let stderr = std::io::stderr().as_fd().try_clone_to_owned()?;
    Ok(stderr.into())
}

Melcher