ISATTY(3) Linux Programmers Manual ISATTY(3)
isatty - does this descriptor refer to a terminal
int isatty(int desc);
returns 1 if desc is an open file descriptor connected to a terminal
and 0 otherwise.
SVr4, 4.3BSD, POSIX.1-2001.
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Linux 1995-04-20 ISATTY(3)