isatty - test whether a file descriptor refers to a terminal
The isatty() function tests whether
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).
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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