Identifying output stream type
posted by Robbie Shade on 15 Feb 2011

The default output of the ls command is one line with space separated results. Pipe the output into cat and you find that each entry appears on its own line. This is why ls | wc -l doesn’t just return 1 and you can do things like ls | head -n 5.

What devilry is this? Surely what happens to a program’s output is up to the shell? Actually not quite - there is a system call isatty which returns true if the file descriptor passed to it is a terminal and false otherwise. By getting the file descriptor for stdout or stderr you can use this to behave differently depending on where the output is going.

Here’s it is in action - note that you need to use fileno to get an integer descriptor of the output stream before passing it to isatty:

#include <unistd.h>
#include <stdio.h>

int main()
    int fd = fileno( stdout );

    int ret = isatty( fd );

    if( ret )
    { fprintf(stderr, "Terminal output...\n"); }
    { fprintf(stderr, "Pipes ahoy!\n"); }

If this hasn’t satisfied your unhealthy appetite for file streams and descriptors, then you can gorge yourself here: