Linux.com: Shell Skills, Part 2: I/O, Redirection, and Pipes

“In this installment of the “Shell Skills” series, you’ll
receive the background you need before you can start stringing
commands together like a pro. Specifically, we’re going to talk
about terms such as “standard input”, “standard output”,
“redirection”, and “piping”. Don’t be intimidated – we’ll give you
some examples, and soon you’ll be one step closer to mastering the
shell.“

“Often, a command needs someplace to send information, often in
the form of text. This place is called “standard output”. So,
“where” exactly is that? Is it a text file, or the terminal screen,
or a printer? Well, it can be any of those places. The key is
something called “redirection.”

“By default, the shell “redirects” any output from a command to
a special file that represents your terminal. (In Linux, all
devices are represented by special files under the “/dev”
directory.) Your terminal is represented by “/dev/ttyn”, where n is
a unique identifier for that particular terminal. So, by default,
all information from a command will be sent to the terminal window.
For example, in the case of the “ls” command, this would be a
directory listing. There is also a second type of output, known as
“standard error”. It can be used by programs to separate regular
output from error messages, so that they don’t get mixed up. For
example, you might want to save a log of only the error messages
generated by a program. Later in this article, we’ll show you how
you can redirect standard output and/or standard error to someplace
other than the terminal.”

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