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. 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 (<). Similarly, to send data to a device or file other than standard output is the output 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 .mw-parser-output .monospaced{font-family:monospace,monospace}grep, which Doug McIlroy cites as what 'ingrained the tools outlook irrevocably' in the operating system, with later tools imitating it. grep at its simplest prints any lines containing a character string to its output. The following is an example: This finds all registered users that have 'foo' as part of their username by using the cut command to take the first field (username) of each line of the Unix system password file and passing them all as input to grep, which searches its input for lines containing the character string 'foo' and prints them on its output. Common Unix filter programs are: cat, cut, grep, head, sort, uniq, and tail. Programs like awk and sed can be used to build quite complex filters because they are fully programmable. Unix filters can also be used by Data scientists to get a quick overview about a file based dataset. Two standard filters from the early days of DOS-based computers are find and sort.