Redirection

Redirection is a method of connecting files to a command. This mechanism is used to capture the output of a command or to feed input to it.

Note

During this section, we will introduce a few commands briefly, in order to illustrate some concepts. The commands are only used as examples, and their usage does not have any connection to the main topics being covered here. The detailed descriptions of all the features and uses of those commands will be covered in the topic on text-processing commands.

Input and Output Streams

Every command that is run has a channel for data input, termed standard input (stdin), data output, termed standard output (stdout) and standard error (stderr). A command reads data from stdin and writes its results to stdout. If any error occurs, the error messages are written to stderr. These channels can also be thought of as streams through which data flows.

By convention, stdin, stdout, and stderr are assigned the numbers 0, 1, and 2...