Can be useful to determine whether output is being piped elsewhere, for PHP shell scripts, for example;#!/usr/local/bin/php<?phpif ( !posix_isatty(STDOUT) ) { fwrite(STDOUT, "Invalid TTY\n"); exit(2);}fwrite(STDOUT, "Enter you name\n");$name = fgets(STDIN);fwrite(STDOUT,"Hello $name\n");exit(0);?>If this script is like:script.php > outfileoutfile will contain "Invalid TTY"