Check output device is a terminal

Check output device is a terminal is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

In this task, the job is to check whatever output filehandle exits to the terminal and mention whatever output goes to it or not.

C

Use isatty() on file descriptor to determine if it's a TTY. To get the file descriptor from a FILE* pointer, use fileno:

<lang c>#include <unistd.h> // for isatty()

1. include <stdio.h> // for fileno()

int main() {

   puts(isatty(fileno(stdout))
         ? "stdout is tty"
         : "stdout is not tty");
   return 0;

}</lang>

Output:

$ ./a.out
stdout is tty

$ ./a.out > tmp
$ cat tmp
stdout is not tty

$ ./a.out | cat
stdout is not tty

OCaml

<lang ocaml>let () =

 print_endline (
   if Unix.isatty Unix.stdout
   then "Output goes to tty."
   else "Output doesn't go to tty."
 )</lang>

Testing in interpreted mode:

$ ocaml unix.cma istty.ml
Output goes to tty.

$ ocaml unix.cma istty.ml > tmp
$ cat tmp
Output doesn't go to tty.

$ ocaml unix.cma istty.ml | cat
Output doesn't go to tty.

Tcl

To detect whether output is going to a terminal in Tcl, you check whether the stdout channel looks like a serial line (as those are indistinguishable from terminals). The simplest way of doing that is to see whether you can read the -mode channel option, which is only present on serial channels: <lang tcl>set toTTY [dict exists [fconfigure stdout] -mode] puts [expr {$toTTY ? "Output goes to tty" : "Output doesn't go to tty"}]</lang>

Demonstrating:

Assuming that the above script is stored in the file istty.tcl:

$ tclsh8.5 istty.tcl 
Output goes to tty
$ tclsh8.5 istty.tcl | cat
Output doesn't go to tty