Check output device is a terminal

From Rosetta Code
(Redirected from Detect output to terminal)
Check output device is a terminal
You are encouraged to solve this task according to the task description, using any language you may know.


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

Related task

6502 Assembly[edit]

Works with: Commodore 64
LDA $D011                ;screen control register 1
AND #%00100000           ;bit 5 clear = text mode, bit 5 set = gfx mode
BEQ isTerminal


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


using System;

namespace CheckTerminal {
    class Program {
        static void Main(string[] args) {
            Console.WriteLine("Stdout is tty: {0}", Console.IsOutputRedirected);


Translation of: C
#if _WIN32
#include <io.h>
#define ISATTY _isatty
#define FILENO _fileno
#include <unistd.h>
#define ISATTY isatty
#define FILENO fileno

#include <iostream>

int main() {
    if (ISATTY(FILENO(stdout))) {
        std::cout << "stdout is a tty\n";
    } else {
        std::cout << "stdout is not a tty\n";

    return 0;


Works with GnuCOBOL.

      *> istty, check id fd 0 is a tty
      *> Tectonics: cobc -xj istty.cob
      *>            echo "test" | ./istty
       identification division.
       program-id. istty.

       data division.
       working-storage section.
       01 rc usage binary-long.

       procedure division.

       call "isatty" using by value 0 returning rc
       display "fd 0 tty: " rc

       call "isatty" using by value 1 returning rc
       display "fd 1 tty: " rc upon syserr

       call "isatty" using by value 2 returning rc
       display "fd 2 tty: " rc

       end program istty.

DISPLAY for fd 1 is directed to SYSERR to get some output during the various trials.

prompt$ cobc -xj istty.cob
fd 0 tty: +0000000001
fd 1 tty: +0000000001
fd 2 tty: +0000000001
prompt$ echo "test" | ./istty
fd 0 tty: +0000000000
fd 1 tty: +0000000001
fd 2 tty: +0000000001
prompt$ echo "test" | ./istty >/dev/null
fd 1 tty: +0000000000
prompt$ echo "test" | ./istty 2>/dev/tty
fd 0 tty: +0000000000
fd 1 tty: +0000000001
fd 2 tty: +0000000001
prompt$ echo "test" | ./istty 2>/dev/null
fd 0 tty: +0000000000
fd 2 tty: +0000000000

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
Works with: ECL

We use the interface to C library functions isatty() and fileno(). It needs to be compiled to be executed.

(ffi:clines "
    #include <sys/ioctl.h>
    #include <unistd.h>
    int ttyPredicate() {
      return isatty(fileno(stdout)); 

    ("ttyPredicate" c-ttyp)
    () :returning :int)

(defun tty-p()
  (if (= 1 (c-ttyp))

(format T "stdout is~:[ not~;~] a terminal~%" (tty-p))

Compilation can be done with the following commands :

ecl --eval '(compile-file "file.lisp" :system-p t)' --eval '(quit)'

ecl --eval '(c:build-program "is-tty" :lisp-files (list "file.o"))' --eval '(quit)'

$ ./is-tty 
stdout is a terminal
$ ./is-tty  | cat -
stdout is not a terminal

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


import std.stdio;

extern(C) int isatty(int);

void main() {
    writeln("Stdout is tty: ", stdout.fileno.isatty == 1);
Stdout is tty: true
prompt>a.out > out.txt
Stdout is tty: false


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

IN: scratchpad USE: unix.ffi
IN: scratchpad 1 isatty

--- Data stack:


Open Cons For Output As #1
' Open Cons abre los flujos de entrada (stdin) o salida (stdout) estándar 
' de la consola para leer o escribir. 

If Err > 0 Then
    Print #1, "stdout is not a tty."
    Print #1, "stdout is a tty."
End If  
Close #1


Tells a terminal apart from a pipe on Linux and Mac, which is probably exactly what you need.

package main

import (

func main() {
    if fileInfo, _ := os.Stdout.Stat(); (fileInfo.Mode() & os.ModeCharDevice) != 0 {
        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
    (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




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...).


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


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.


Works with: Ubuntu version 14.04
// Kotlin Native version 0.5

import platform.posix.*

fun main(args: Array<String>) {
    if (isatty(STDOUT_FILENO) != 0)
        println("stdout is a terminal")
        println("stdout is not a terminal") 
stdout is a terminal


Works with: Lua version 5.1+

Using pure Lua, assuming a *NIX-like runtime environment ...

local function isTTY ( fd )
    fd = tonumber( fd ) or 1
    local ok, exit, signal = os.execute( string.format( "test -t %d", fd ) )
    return (ok and exit == "exit") and signal == 0 or false

print( "stdin", isTTY( 0 ) )
print( "stdout", isTTY( 1 ) )
print( "stderr", isTTY( 2 ) )
$ lua istty.lua
stdin   true
stdout  true
stderr  true
$ cat /dev/null | lua istty.lua
stdin   false
stdout  true
stderr  true
$ lua istty.lua | tee
stdin   true
stdout  false
stderr  true
$ lua istty.lua 2>&1 | tee
stdin   true
stdout  false
stderr  false
Works with: Lua version 5.1+
Library: posix

You can accomplish the same results using the luaposix [1] library:

local unistd = require( "posix.unistd" )

local function isTTY ( fd )
    fd = tonumber( fd ) or 1
    local ok, err, errno = unistd.isatty( fd )
    return ok and true or false

print( "stdin", isTTY( 0 ) )
print( "stdout", isTTY( 1 ) )
print( "stderr", isTTY( 2 ) )

The output of this version is identical to the output of the first version.


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;


Using function "isatty" of standard module "terminal" which accepts a File as argument. As we want to redirect stdout, we write the messages on stderr.

import terminal

stderr.write if stdout.isatty: "stdout is a terminal\n" else: "stdout is not a terminal\n"
Command: ./check_output_dev
Result: stdout is a terminal
Command: ./check_output_dev >somefile
Result: stdout is not a terminal


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.


(define (isatty? fd) (syscall 16 fd 19))
(print (if (isatty? stdout)
   "stdout is a tty."
   "stdout is not a 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


without js -- (no input or output redirection in a browser!)
printf(1,"stdin:%t, stdout:%t, stderr:%t\n",{isatty(0),isatty(1),isatty(2)})
C:\Program Files (x86)\Phix>p test
stdin:true, stdout:true, stderr:true

C:\Program Files (x86)\Phix>p test > test.txt; type test.txt
stdin:true, stdout:false, stderr:true

C:\Program Files (x86)\Phix>p test 2> test.txt
stdin:true, stdout:true, stderr:false

C:\Program Files (x86)\Phix>type test.txt | p test
stdin:false, stdout:true, stderr:true


if(posix_isatty(STDOUT)) {
    echo "The output device is a terminal".PHP_EOL;
} else {
    echo "The output device is NOT a terminal".PHP_EOL;


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.'


Translation of: Python
  [ $ |from sys import stdout
to_stack( 1 if stdout.isatty() else 0)|
    python ]                            is ttyout ( --> b )     

  ttyout if 
    [ say "Looks like a teletype." ] 
    [ say "Not a teletype." ]
Looks like a teletype.


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


(formerly Perl 6)

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.

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


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

                                 /*    ···  handle other REXX syntax errors here  ···   */
output   when using the default input:
output device: terminal

The following is the output when used with R4 REXX:

output   when using the default input:
Reading console input (Press Ctrl-Z to quit):
                                   ◄■■■■■■■■ user input (pressed ENTER)  
                                   ◄■■■■■■■■ user input (pressed ENTER a 2nd time)                          
output device: 6


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");


Works with: Ubuntu version 14.04
import org.fusesource.jansi.internal.CLibrary._

object IsATty  extends App {

  var enabled = true

  def apply(enabled: Boolean): Boolean = {
    // We must be on some unix variant..
    try {
      enabled && isatty(STDOUT_FILENO) == 1
    catch {
      case ignore: Throwable =>


    println("tty " + apply(true))

Standard ML[edit]

val stdoutRefersToTerminal : bool = Posix.ProcEnv.isatty Posix.FileSys.stdout


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

Visual Basic .NET[edit]

Translation of: C#
Module Module1

    Sub Main()
        Console.WriteLine("Stdout is tty: {0}", Console.IsOutputRedirected)
    End Sub

End Module


Translation of: C

As there is currently no way to obtain this information via Wren CLI, we instead embed a Wren script in a C application and ask the host program to get it for us.

/* check_output_device_is_terminal.wren */

class C {
    foreign static isOutputDeviceTerminal

System.print("Output device is a terminal = %(C.isOutputDeviceTerminal)")

We now embed this Wren script in the following C program, compile and run it.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include "wren.h"

void C_isOutputDeviceTerminal(WrenVM* vm) {
    bool isTerminal = (bool)isatty(fileno(stdout));
    wrenSetSlotBool(vm, 0, isTerminal);

WrenForeignMethodFn bindForeignMethod(
    WrenVM* vm,
    const char* module,
    const char* className,
    bool isStatic,
    const char* signature) {
    if (strcmp(module, "main") == 0) {
        if (strcmp(className, "C") == 0) {
            if (isStatic && strcmp(signature, "isOutputDeviceTerminal") == 0) {
                return C_isOutputDeviceTerminal;
    return NULL;

static void writeFn(WrenVM* vm, const char* text) {
    printf("%s", text);

void errorFn(WrenVM* vm, WrenErrorType errorType, const char* module, const int line, const char* msg) {
    switch (errorType) {
        case WREN_ERROR_COMPILE:
            printf("[%s line %d] [Error] %s\n", module, line, msg);
            printf("[%s line %d] in %s\n", module, line, msg);
        case WREN_ERROR_RUNTIME:
            printf("[Runtime Error] %s\n", msg);

char *readFile(const char *fileName) {
    FILE *f = fopen(fileName, "r");
    fseek(f, 0, SEEK_END);
    long fsize = ftell(f);
    char *script = malloc(fsize + 1);
    fread(script, 1, fsize, f);
    script[fsize] = 0;
    return script;

int main() {
    WrenConfiguration config;
    config.writeFn = &writeFn;
    config.errorFn = &errorFn;
    config.bindForeignMethodFn = &bindForeignMethod;
    WrenVM* vm = wrenNewVM(&config);
    const char* module = "main";
    const char* fileName = "check_output_device_is_terminal.wren";
    char *script = readFile(fileName);
    WrenInterpretResult result = wrenInterpret(vm, module, script);
    switch (result) {
            printf("Compile Error!\n");
            printf("Runtime Error!\n");
    return 0;
$ ./check_output_device_is_terminal
Output device is a terminal = true

$ ./check_output_device_is_terminal > tmp
$ cat tmp
Output device is a terminal = false

$ ./check_output_device_is_terminal | cat
Output device is a terminal = false


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