Most UNIX programs read their input from a `file descriptor' known as `standard input' (stdin) and send their output to `standard output' (stdout). Any error messages are sent to `standard error' (stderr). Usually stdin is the keyboard, and stdout and stderr are the screen. Sometimes it is useful to redirect these so that a program can take its input from somewhere else, or send its output elsewhere. You have already seen this used when we looked at printing files (see section 2.2.13, page ). There are more details about this in the manual pages for bash, but some of the possibilities are explained in the following sections.