Redirection (Unix)
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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.)
A good example for command piping is combining echo with another command to achieve something interactive in a non-interactive shell , ex. echo -e "user\npass" | ftp localhost , this would run the ftp client and enter user, press return then pass .
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 stream is used for the redirection. In csh based shells, the syntax instead appends an & to the redirect characters which achieves a similar 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.