Detect in C if outputting to a terminal

Question

I'm writing a C program for OS X and Linux, and I want to tweak the output based on whether or not it's going to a terminal. I know we've covered how to do this in a shell script, e.g. here:

Detecting the output stream type of a shell script

But how do I do it in a C program?

score 43 · Accepted Answer · edited Jan 22 '19 at 13:07

43

Use isatty():

$ man isatty
ISATTY(3)                  Linux Programmer's Manual                 ISATTY(3)

NAME
       isatty - does this descriptor refer to a terminal

SYNOPSIS
       #include <unistd.h>

       int isatty(int desc);

DESCRIPTION
       returns  1  if  desc is an open file descriptor connected to a terminal
       and 0 otherwise.

Since stdout is always file descriptor 1, you can do:

if(isatty(1))
    // stdout is a terminal

edited Jan 22 '19 at 13:07

Marco Bonelli

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answered Jun 30 '09 at 03:31

Laurence Gonsalves

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And could you explain a little bit deeper ? What is really checked by this function ? What really means `connected to a terminal` ? – xolodec Feb 08 '14 at 08:13
@PavelShvechikov It means the file descriptor is associated with a device that is considered to be a terminal. See http://en.wikipedia.org/wiki/POSIX_terminal_interface – Laurence Gonsalves Feb 08 '14 at 19:28

score 5 · Answer 2 · answered Jun 30 '09 at 03:34

5

if (isatty (1))
    fprintf (stdout, "Outputting to a terminal.");
else
    fprintf (stdout, "Not outputting to a terminal.");

answered Jun 30 '09 at 03:34

dreamlax

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Detect in C if outputting to a terminal

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