Perl Programming/Keywords/-t

The -t keyword
-t is a file test that tests if the filehandle is opened to a TTY.

It takes one argument, either a FILENAME, a FILEHANDLE, or a DIRHANDLE to test the associated file to see, if something is true about it. If the argument is omitted, it tests STDIN. -t returns 1 for true and an empty string for false. If the file doesn't exist or can't be examined, it returns <tt>undef</tt> and sets <tt>$!</tt> (<tt>errno</tt>).