\section isatty isatty - Test if the specidied file descriptor is a tty

\subsection isatty-synopsis Synopsis
 <tt>isatty [FILE DESCRIPTOR]</tt>

where FILE DESCRIPTOR may be either the number of a file descriptor, or one
of the strings stdin, stdout and stderr.

If he specified file descriptor is a tty, the exit status of the
command is zero, otherwise, it is non-zero.