isatty(3) - BetterMan

isatty(3)

test whether a file descriptor refers to a terminal

distropkg manpages-dev@6.9.1-1dataset 2026-04-01T06:19:02Z+debian+ec4110e+mandoc:1.14.6-4

Synopsis

#include <unistd.h>
int isatty(int 
fd
);

—

Options · 2

EBADF	fd is not a valid file descriptor.
ENOTTY	fd refers to a file other than a terminal. On some older kernels, some types of files resulted in the error EINVAL in this case (which is a violation of POSIX, which specifies the error ENOTTY).

NAME

isatty - test whether a file descriptor refers to a terminal

Standard C library (libc, -lc)

bash

#include <unistd.h>

bash

int isatty(int \nfd\n);

The isatty() function tests whether fd is an open file descriptor referring to a terminal.

isatty() returns 1 if fd is an open file descriptor referring to a terminal; otherwise 0 is returned, and errno is set to indicate the error.

EBADF: fd is not a valid file descriptor.
ENOTTY: fd refers to a file other than a terminal. On some older kernels, some types of files resulted in the error EINVAL in this case (which is a violation of POSIX, which specifies the error ENOTTY).

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
isatty ()	Thread safety	MT-Safe

POSIX.1-2008.

POSIX.1-2001, SVr4, 4.3BSD.