Filter (software)

A filter is a computer program or subroutine to process a stream, producing another stream. While a single filter can be used individually, they are frequently strung together to form a pipeline. Some operating systems such as Unix are rich with filter programs. Windows 7 and later are also rich with filters, as they include Windows PowerShell. In comparison, however, few filters are built into cmd.exe (the original command-line interface of Windows), most of which have significant enhancements relative to the similar filter commands that were available in MS-DOS. OS X includes filters from its underlying Unix base but also has Automator, which allows filters (known as "Actions") to be strung together to form a pipeline. In Unix and Unix-like operating systems, a filter 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 input may come from command line flags or configuration files, while auxiliary output may go to standard error. The command syntax for getting data from a device or file other than standard input is the input operator (). To append data lines to an existing output file, one can use the append operator (>>). Filters may be strung together into a pipeline with the pipe operator ("|"). This operator signifies that the main output of the command to the left is passed as main input to the command on the right. The Unix philosophy encourages combining small, discrete tools to accomplish larger tasks. The classic filter in Unix is Ken Thompson's , which Doug McIlroy cites as what "ingrained the tools outlook irrevocably" in the operating system, with later tools imitating it. at its simplest prints any lines containing a character string to its output.