How can I redirect the output of a C program from /dev/tty to /dev/null?

Question

Consider this sample C program which writes to /dev/tty and doesn't have command line options to make it not do so.

#include <stdio.h>

int main (void) {
    FILE* fout = fopen("/dev/tty", "w");
    fprintf(fout, "Hello, World!\n");
    fclose(fout);
}

How could I redirect the output of it to /dev/null in a shell script?

P.S. I read this answer, but I didn't understand much. In any case, I'm expecting an answer that doesn't modify the code source of the program.

Answer 1

TL,DR:

script -c myprogram /dev/null </dev/null >/dev/null

---

You can't “redirect” /dev/tty in the same sense that you can redirect standard output. Standard output is defined as a file descriptor. Programs write to whatever file is already open on file descriptor 1 when they start. Some operating systems offer /dev/stdout as a file that's equivalent to standard output, but it's an “alias” for standard output. In contrast, /dev/tty is a file name, which refers to the process's controlling terminal. If a program opens /dev/tty, explicitly it opens /dev/tty, and that can't be redirected.

What you can do is run the program with a controlling terminal that isn't the same as the controlling terminal of the program that runs it. A simple way to do this is with the script command. In its simplest form:

script -c myprogram /dev/null >/dev/null

When myprogram runs and opens /dev/tty, this is a terminal provided by script, not the terminal in which script runs. What script does when it detects a write on the terminal is to both write to its own standard output and write to the indicated typescript file; hence I set both script's standard output and the typescript file to /dev/null.

If myprogram reads from the terminal, script reads from its own standard input, so you'll probably want to redirect this to /dev/null as well.

Note that script does not pass the exit status of myprogram to its caller. Some implementations (e.g. the one in Debian and derivatives) have a -e option to do that.

Answer 2

It's very unusual that a program would open /dev/tty itself. If you need to print something to the console, you'd just write to standard output, stdout, which you could redirect, as Gilles shows.

Your only choice here would be intercepting the libc fopen call, unless that's been inlined (which isn't that rare) or the underlying operating system open call and bend it.

It's not that hard. You'd need to write a small library that contains said open function, which in turn calls the actual open function after having "sanitized" its input (i.e., if the file name is /dev/tty, replace it with /dev/null).

Run program and intercept and redirect syscalls shows how that's done.