ISATTY(3) Linux Programmer's Manual ISATTY(3)

isatty - test whether a file descriptor refers to a terminal

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

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.

fd is not a valid file descriptor.
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.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

fstat(2), ttyname(3)
2019-03-06 Linux