What are filters?
A filter is a specific type of UNIX command. It is primarily used to do some manipulation in the text of the file. It changes plain text into something that can be used together with the other filters and pipes. All of which together form a series of operations that can be utilized to produce highly specific results.
The filter gets most of its data from its standard input, which is main input stream. It then writes its main results to its standard output, i.e. the main output stream. The two most popular UNIX filters are the sed and awk commands. Both are extremely powerful and yet are somewhat complex to use.