Redirections

Redirection

Capturing output from a file or a command (or program or script) and sending it as input to another file or command (or program or script).

|

(pipe) redirects standard output of a program as standard input of another.

>

redirects standard output to a file (and overwrites the file if it already exists).

<

uses a file as standard input.

>>

Appends standard output to a file.

< Input_data_file  program1 | program2 > Output_data_file

A "filter" is a program which reads data from standard input, processes it in some way, and sends the processed data to standard output.