Section (3) isatty
isatty — test whether a file descriptor refers to a terminal
isatty() function tests
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,
errno is set to indicate the
fdis not a valid file descriptor.
fdrefers 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).
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