Check output device is a terminal

From Rosetta Code
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.

Demonstrate how to check whether the output device is a terminal or not.

Related task


Works with: GNAT

We use the interface to C library functions isatty() and fileno().

with Ada.Text_IO;          use Ada.Text_IO;
with Interfaces.C_Streams; use Interfaces.C_Streams;
procedure Test_tty is
if Isatty(Fileno(Stdout)) = 0 then
Put_Line(Standard_Error, "stdout is not a tty.");
Put_Line(Standard_Error, "stdout is a tty.");
end if;
end Test_tty;
$ ./test_tty 
stdout is a tty.
$ ./test_tty > /dev/null
stdout is not a tty.


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

#include <unistd.h>   // for isatty()
#include <stdio.h> // for fileno()
int main()
? "stdout is tty"
: "stdout is not tty");
return 0;
$ ./a.out
stdout is tty

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

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

Common Lisp[edit]

Works with: SBCL
(with-open-stream (s *standard-output*)
(format T "stdout is~:[ not~;~] a terminal~%"
(interactive-stream-p s)))
$ sbcl --script rc.lisp
stdout is a terminal
$ sbcl --script rc.lisp | cat
stdout is not a terminal
$ sbcl --script rc.lisp > foo.txt
$ cat foo.txt
stdout is not a terminal

Crystal[edit]"testfile").tty?   #=> false"/dev/tty").tty? #=> true
STDOUT.tty? #=> true


You have to know 1 is the correct file descriptor number:

IN: scratchpad USE: unix.ffi
IN: scratchpad 1 isatty
--- Data stack:


package main
import (
func main() {
if terminal.IsTerminal(int(os.Stdout.Fd())) {
fmt.Println("Hello terminal")
} else {
fmt.Println("Who are you? You're not a terminal.")
> hello
Hello terminal
> hello | cat
Who are you?  You're not a terminal.


module Main where
-- requires the unix package
import System.Posix.Terminal (queryTerminal)
import System.Posix.IO (stdOutput)
main :: IO ()
main = do
istty <- queryTerminal stdOutput
putStrLn (if istty
then "stdout is tty"
else "stdout is not tty")
$ runhaskell istty.hs
stdout is tty
$ runhaskell istty.hs | cat
stdout is not tty


node -p -e "Boolean(process.stdout.isTTY)"




J does not have a concept of an "output device", so we approximate that by seeing whether we have bothered to define a the code which typically does graphical output.

The use of the phrase "output device" suggests that we are thinking about something like the unix `isatty` command. Here, stdout might be a file or might be a terminal. But in J we are often hosting our own user interaction environment. It's not uncommon for a J user to be on a web page where hitting enter sends a form request to a J interpreter which in turn composes an updated html presentation of current state which it sends to the browser. Or, the J user might be talking to a Java program which similarly wraps the J session (though this is older technology at this point). Or, the J user might be interacting with Qt. Or, sure, we might be talking to a tty and J might be sending its output straight to the tty. (But we can't know if that tty is hosted in emacs, running under control of a script on a remote machine via ssh, talking directly to a human user who happens to be in direct control of the session, or whatever else...)

The point being that in the general case the J programmer cannot know whether the concept of "terminal" has any relevance to the user.

But, like everyone else, we can certainly use heuristics.

But, correctness requires us to keep in mind that these will only be heuristics, and will sometimes be incorrect (hopefully not often enough to matter a lot...).


if isa(STDOUT, Base.TTY)
println("This program sees STDOUT as a TTY.")
println("This program does not see STDOUT as a TTY.")
This program sees STDOUT as a TTY.


There is no explicit way (ie isatty())to do this; however, if we assume that standard out is a terminal, we can check if the output stream has been redirected (presumably to something other than a terminal).

def isTerm = System.Console.IsOutputRedirected;


let () =
print_endline (
if Unix.isatty Unix.stdout
then "Output goes to tty."
else "Output doesn't go to tty."

Testing in interpreted mode:

$ ocaml unix.cma
Output goes to tty.

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

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


The -t function on a filehandle tells you whether it's a terminal.

$ perl -e "warn -t STDOUT ? 'Terminal' : 'Other'"
$ perl -e "warn -t STDOUT ? 'Terminal' : 'Other'" > x.tmp

Perl 6[edit]

Works with: Rakudo version 2015.12

The .t method on a filehandle tells you whether it's going to the terminal. Here we use the note function to emit our result to standard error rather than standard out.

$ perl6 -e 'note $*OUT.t'
$ perl6 -e 'note $*OUT.t' >/dev/null


Pretty much the same as Check input device is a terminal#Python.

from sys import stdout
if stdout.isatty():
print 'The output device is a teletype. Or something like a teletype.'
print 'The output device isn\'t like a teletype.'


(terminal-port? (current-output-port))


Programming note:   The comment about the REXX statements have to be on one line isn't quite true,
but because the REXX special variable SIGL is defined where it's executed, it makes coding simpler.

SIGL   is set to the REXX statment number where:

  •   a CALL statement is used
  •   a function is invoked
  •   a SIGNAL statement is used

Method used:   since REXX has no direct way of determining if the STDIN is a terminal or not, the REXX code (below)
actually raises (which is no way to run a railroad) a syntax error when attempting to read the 2nd line from   STDIN,
which causes a routine (named syntax:) to get control, determines where the syntax error occurred, and returns an
appropriate string indicating if STDIN is a terminal (or other).

Note that under VM/CMS, this can be accomplished with a (host) command within REXX and then examining the results.
On IBM mainframes, a user can have STDIN defined, but the terminal can be disconnected.

/*REXX program determines if the   STDIN   is a   terminal   or  other. */
signal on syntax /*if syntax error, jump──► SYNTAX*/
say 'output device:' testSTDIN() /*displays terminal ──or── other */
exit /*stick a fork in it, we're done.*/
/*──────────────────────────────────TESTSTDIN subroutine────────────────*/
testSTDIN: syntax.=1; signal .; .: z.=sigl; call linein ,2; ..: syntax.=0
return z.. /* [↑] must all be on one line.*/
/*──────────────────────────────────SYNTAX subroutine───────────────────*/
syntax: z..='other' /*when SYNTAX occur, come here. */
if syntax. then do /*handling STDIN thingy error? */
if sigl==z. then z..='terminal'; signal .. /*stdin ?*/
end /* [↑] can't use a RETURN here.*/
/* ··· handle other REXX syntax errors here ··· */


output device: terminal


f ="test.txt")
p f.isatty # => false
p STDOUT.isatty # => true


/* Uses C library interface */
extern crate libc;
fn main() {
let istty = unsafe { libc::isatty(libc::STDOUT_FILENO as i32) } != 0;
if istty {
println!("stdout is tty");
} else {
println!("stdout is not tty");


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:

set toTTY [dict exists [fconfigure stdout] -mode]
puts [expr {$toTTY ? "Output goes to tty" : "Output doesn't go to tty"}]

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.


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[edit]

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.

set toTTY [expr {![catch {fconfigure stdout -mode}]}]

UNIX Shell[edit]

if [ -t 1 ]
echo "Output is a terminal"
echo "Output is NOT a terminal" >/dev/tty


On Unix, check to see if stdout's st_mode is a character device.

const S_IFCHR=0x2000;
fcn S_ISCHR(f){[4].bitAnd(S_IFCHR).toBool() }
$ zkl bbb  # from the command line
$ zkl bbb | more
$ zkl bbb > foo.txt
$ cat foo.txt