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.

The task is check whether the output device is a terminal or not.

See also: Check input device is a terminal

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 or -xchar channel options, which are 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> At the system call level, when Tcl is setting up the channels that correspond to the underlying stdout (and stdin and stderr) file descriptors, it checks whether the channels are network sockets (with getsockname()) or serial lines (with isatty()). This allows Tcl scripts to find out information about their calling environment (e.g., when they are run from inetd) with minimal code.

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

Channel type discovery with older Tcl versions

Before Tcl 8.4, this discovery process is impossible; stdout always looks like it is going to a file. With 8.4, you can discover the channel type but you need slightly different (and less efficient, due to the thrown error in the non-tty case) code to do it. <lang tcl>set toTTY [expr {![catch {fconfigure stdout -mode}]}]</lang>