\section isatty isatty - test if the specified file descriptor is a tty

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

\subsection isatty-description Description
<tt>isatty</tt> tests if a file descriptor is a tty.

<tt>FILE DESCRIPTOR</tt> may be either the number of a file descriptor, or one of the
strings <tt>stdin</tt>, \c stdout and <tt>stderr</tt>.

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