What is filter in Unix?

A filter in UNIX is a program that gets most of its data from its standard input (the main input stream) and writes its main results to its standard output (the main output stream).

Auxiliary data may come from command line flags or configuration files, while auxiliary output may go to standard error. The command format for getting data from a device or file other than standard input is the input operator (<). Likewise, to send data to a device or file other than standard output is the output operator (>). To append data to an existing output file, one can use the append operator (>>). Filters may be strung together into a pipeline with a pipe operator ("|"). This operator highlights that the main output of the command to the left is passed as the main input to the command on the right.