Check output device is a terminal
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Demonstrate how to check whether the output device is a terminal or not.
- Related task
6502 Assembly
LDA $D011 ;screen control register 1
AND #%00100000 ;bit 5 clear = text mode, bit 5 set = gfx mode
BEQ isTerminalAda
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
begin
if Isatty(Fileno(Stdout)) = 0 then
Put_Line(Standard_Error, "stdout is not a tty.");
else
Put_Line(Standard_Error, "stdout is a tty.");
end if;
end Test_tty;
- Output:
$ ./test_tty stdout is a tty. $ ./test_tty > /dev/null stdout is not a tty.
C
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()
{
puts(isatty(fileno(stdout))
? "stdout is tty"
: "stdout is not tty");
return 0;
}
- Output:
$ ./a.out stdout is tty $ ./a.out > tmp $ cat tmp stdout is not tty $ ./a.out | cat stdout is not tty
C#
using System;
namespace CheckTerminal {
class Program {
static void Main(string[] args) {
Console.WriteLine("Stdout is tty: {0}", Console.IsOutputRedirected);
}
}
}
C++
#if _WIN32
#include <io.h>
#define ISATTY _isatty
#define FILENO _fileno
#else
#include <unistd.h>
#define ISATTY isatty
#define FILENO fileno
#endif
#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;
}
COBOL
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.
sample-main.
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
goback.
end program istty.
DISPLAY for fd 1 is directed to SYSERR to get some output during the various trials.
- Output:
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
(with-open-stream (s *standard-output*)
(format T "stdout is~:[ not~;~] a terminal~%"
(interactive-stream-p s)))
- Output:
$ 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
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));
}")
(ffi:def-function
("ttyPredicate" c-ttyp)
() :returning :int)
(defun tty-p()
(if (= 1 (c-ttyp))
t
nil))
(format T "stdout is~:[ not~;~] a terminal~%" (tty-p))
(quit)
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)'
- Output:
$ ./is-tty stdout is a terminal $ ./is-tty | cat - stdout is not a terminal
Crystal
File.new("testfile").tty? #=> false
File.new("/dev/tty").tty? #=> true
STDOUT.tty? #=> true
D
import std.stdio;
extern(C) int isatty(int);
void main() {
writeln("Stdout is tty: ", stdout.fileno.isatty == 1);
}
- Output:
prompt>a.out Stdout is tty: true prompt>a.out > out.txt Stdout is tty: false
ed
Given that ed is a non-graphical editor that's almost exclusively run in terminal, the program only needs to highlight this fact.
# by Artyom Bologov
H
a
Output to terminal!
.
p
Q
Factor
You have to know 1 is the correct file descriptor number:
IN: scratchpad USE: unix.ffi
IN: scratchpad 1 isatty
--- Data stack:
1
FreeBASIC
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."
Else
Print #1, "stdout is a tty."
End If
Close #1
Sleep
FutureBasic
include "NSLog.incl"
local fn IsOutputDeviceTerminal as BOOL
int fileDescriptor
cln fileDescriptor = fileno( stdout );
end fn = fn isatty( fileDescriptor ) != 0
BOOL isTerminal : isTerminal = fn IsOutputDeviceTerminal
if (isTerminal)
NSLog( @"Output is directed to a terminal." )
else
NSLog( @"Output is not directed to a terminal." )
end if
HandleEventsGo
Tells a terminal apart from a pipe on Linux and Mac, which is probably exactly what you need.
package main
import (
"os"
"fmt"
)
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")
}
}
- Output:
> hello Hello terminal > hello | cat Who are you? You're not a terminal.
Haskell
module Main where
-- requires the unix package
-- https://hackage.haskell.org/package/unix
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")
- Output:
$ runhaskell istty.hs stdout is tty $ runhaskell istty.hs | cat stdout is not tty
J
3=nc<'wd'
Explanation:
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...).
JavaScript
node -p -e "Boolean(process.stdout.isTTY)"
true
Julia
if isa(STDOUT, Base.TTY)
println("This program sees STDOUT as a TTY.")
else
println("This program does not see STDOUT as a TTY.")
end
- Output:
This program sees STDOUT as a TTY.
Kotlin
// Kotlin Native version 0.5
import platform.posix.*
fun main(args: Array<String>) {
if (isatty(STDOUT_FILENO) != 0)
println("stdout is a terminal")
else
println("stdout is not a terminal")
}
- Output:
stdout is a terminal
Lua
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
end
print( "stdin", isTTY( 0 ) )
print( "stdout", isTTY( 1 ) )
print( "stderr", isTTY( 2 ) )
- Output:
$ 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
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
end
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.
Nemerle
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;
Nim
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"
- Output:
Command: ./check_output_dev Result: stdout is a terminal
Command: ./check_output_dev >somefile Result: stdout is not a terminal
Nu
$'stdout is a terminal: (is-terminal -o)'OCaml
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 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.
Ol
(define (isatty? fd) (syscall 16 fd 19))
(print (if (isatty? stdout)
"stdout is a tty."
"stdout is not a tty."))
PascalABC.NET
##
uses system;
system.Console.IsOutputRedirected.println;
- Output:
C:\PABCWork.NET\Output>checkoutput.exe False C:\PABCWork.NET\Output>checkoutput.exe > test C:\PABCWork.NET\Output>type test True
Perl
The -t function on a filehandle tells you whether it's a terminal.
$ perl -e "warn -t STDOUT ? 'Terminal' : 'Other'"
Terminal
$ perl -e "warn -t STDOUT ? 'Terminal' : 'Other'" > x.tmp
Other
Phix
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)})
- Output:
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
PHP
if(posix_isatty(STDOUT)) {
echo "The output device is a terminal".PHP_EOL;
} else {
echo "The output device is NOT a terminal".PHP_EOL;
}
Python
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.'
else:
print 'The output device isn\'t like a teletype.'
Quackery
[ $ |from sys import stdout
to_stack( 1 if stdout.isatty() else 0)|
python ] is ttyout ( --> b )
ttyout if
[ say "Looks like a teletype." ]
else
[ say "Not a teletype." ]- Output:
Looks like a teletype.
Racket
(terminal-port? (current-output-port))
Raku
(formerly Perl 6)
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' True $ raku -e 'note $*OUT.t' >/dev/null False
REXX
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
Ruby
f = File.open("test.txt")
p f.isatty # => false
p STDOUT.isatty # => true
Rust
/* 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");
}
}
Scala
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 =>
ignore.printStackTrace()
false
}
}
println("tty " + apply(true))
}
Standard ML
val stdoutRefersToTerminal : bool = Posix.ProcEnv.isatty Posix.FileSys.stdout
Swift
print(isatty(STDOUT_FILENO) != 0 ? "TTY" : "Not 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:
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.
- 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.
set toTTY [expr {![catch {fconfigure stdout -mode}]}]
UNIX Shell
#!/bin/sh
if [ -t 1 ]
then
echo "Output is a terminal"
else
echo "Output is NOT a terminal" >/dev/tty
fi
Visual Basic .NET
Module Module1
Sub Main()
Console.WriteLine("Stdout is tty: {0}", Console.IsOutputRedirected)
End Sub
End Module
Wren
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_a_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);
break;
case WREN_ERROR_STACK_TRACE:
printf("[%s line %d] in %s\n", module, line, msg);
break;
case WREN_ERROR_RUNTIME:
printf("[Runtime Error] %s\n", msg);
break;
}
}
char *readFile(const char *fileName) {
FILE *f = fopen(fileName, "r");
fseek(f, 0, SEEK_END);
long fsize = ftell(f);
rewind(f);
char *script = malloc(fsize + 1);
fread(script, 1, fsize, f);
fclose(f);
script[fsize] = 0;
return script;
}
int main() {
WrenConfiguration config;
wrenInitConfiguration(&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_a_terminal.wren";
char *script = readFile(fileName);
WrenInterpretResult result = wrenInterpret(vm, module, script);
switch (result) {
case WREN_RESULT_COMPILE_ERROR:
printf("Compile Error!\n");
break;
case WREN_RESULT_RUNTIME_ERROR:
printf("Runtime Error!\n");
break;
case WREN_RESULT_SUCCESS:
break;
}
wrenFreeVM(vm);
free(script);
return 0;
}
- Output:
$ ./Check_output_device_is_a_terminal Output device is a terminal = true $ ./Check_output_device_is_a_terminal > tmp $ cat tmp Output device is a terminal = false $ ./Check_output_device_is_a_terminal | cat Output device is a terminal = false
zkl
On Unix, check to see if stdout's st_mode is a character device.
const S_IFCHR=0x2000;
fcn S_ISCHR(f){ f.info()[4].bitAnd(S_IFCHR).toBool() }
S_ISCHR(File.stdout).println();- Output:
$ zkl bbb # from the command line True $ zkl bbb | more False $ zkl bbb > foo.txt $ cat foo.txt False
- Hardware
- Terminal control
- Initialization
- Programming Tasks
- Solutions by Programming Task
- 6502 Assembly
- Ada
- C
- C sharp
- C++
- COBOL
- Common Lisp
- Crystal
- D
- Ed
- Factor
- FreeBASIC
- FutureBasic
- Go
- Haskell
- J
- JavaScript
- Julia
- Kotlin
- Lua
- Posix
- Nemerle
- Nim
- Nu
- OCaml
- Ol
- PascalABC.NET
- Perl
- Phix
- PHP
- Python
- Quackery
- Racket
- Raku
- REXX
- Ruby
- Rust
- Scala
- Standard ML
- Swift
- Tcl
- UNIX Shell
- Visual Basic .NET
- Wren
- Zkl
- TI-83 BASIC/Omit