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.

Answer 1

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.