Is it possible to connect more than the two standard streams to a terminal in Linux?

Question

Consider the following simple program, and suppose it is in a file called Test.c.

#include <stdio.h>

int main(){
    fprintf(stdout, "Hello stdout\n");
    fprintf(stderr, "Hello stderr\n");
}

Suppose I compile this program into an executable called Test and run it as follows.

./Test > Out 2> Err

After this run, I will have two files Out and Err, which will contain the two messages respectively.

This is wonderful because I can have two different types of messages normally printed to the console, and then filter one or both by using bash redirection. However, the fact that I can only do this kind of filtering with two file descriptors seems very limiting.

Is there some way to open a third or nth file descriptor which points at the terminal output, so I could filter it separately?

The syntax might be something like this.

./Test > Out 2> Err 3> Err2

I speculate that bash may have some rudimentary support for this because of the following test, which seems to imply that bash will treat the number after a & as a file descriptor.

$ ./Test >&2
Hello stdout
Hello stderr
$ ./Test >&3
bash: 3: Bad file descriptor

Answer 1

At a shell, running either

exec 3>/dev/tty

...or...

exec 3>&1

...will open file descriptor 3, pointing it to your TTY explicitly (in the first case), or to the location at which stdout is currently writing to (in the second).

---

If you want to use this in a program, I would strongly suggest taking an FD number to write extra logs to as an optional argument:

yourprogram --extra-logs-fd=3

...combining that output with stderr, or suppressing it entirely (as appropriate) if no such option is given. (A user who wanted extra logging to go to stdout could thus use --extra-logs-fd=1, or --extra-logs-fd=2 for stderr).

Even better, if your only target OS is Linux, is just to accept a filename to write to:

# to write to a file
yourprogram --extra-logs=extra_logs.txt

# to write to FD 3
yourprogram --extra-logs=/dev/fd/3

# to write to a tee program, then to stderr (in ksh or bash)
yourprogram --extra-logs=>(tee extra_logs.txt >&2)

...of course, you can do all that with the FD mode (just redirect 3>extra_logs.txt in your shell in the first case, and 3> >(tee extra_logs.txt >&2) in the third), but this makes you do the work of managing FD numbers by hand, and with what advantage?