Redirection (Unix)

Redirection is a function common to most Unix shells which allow standard streams to be redirected to user-specified locations.

Redirection is usually implemented by placing certain characters between commands. Typically, the syntax of these characters is as follows:

command1 > file1  

execute 'command1', placing the output in file1

command1 < file1 

execute 'command1', using 'file1' as the source of input (as opposed to the keyboard).

command1 | command2   

execute 'command1', using its output as the input for 'command2' (commonly called 'piping', since the "|" character is known as a pipe.)

In sh based shells, the first two actions can be further modified by placing a number (the file descriptor) immediately before the character; this will affect which streamvis used for the redirection. In csh based shells, the syntax instead appends a & to the redirect characters which achieves a simiar result.

For example:

command1 2> file1

execute 'command1', directing the standard error stream to 'file1' (useful since standard error outputs always to the terminal and is unaffected by redirects unless so specified).

The three tokens can be chained together to create complex commands; for example:

ls | grep '.sh' | sort > shlist    

lists the contents of the current directory, where this output is filtered to only contain lines which contain '.sh', sort this resultant output alphabetically and place the final output in 'shlist'.

This type of construct is used very commonly in Unix shell scripts.