Pipes and filters
Pipes and filters is a software design pattern invented by Douglas McIlroy. Its inclusion on the Unix operating system was the first implementation of his vision of software componentry. The main difference between pipes and filters and newer software component technologies is that it only allows information to flow in one direction, like water flows in a pipe.
In the context of Unix operating systems, a pipe signifies that the output of one program feeds directly as input to another program.
A file can be used to provide input or to hold the output (in the pipe metaphor, a file would be a container).
The Unix shell uses the pipe character (|) to join programs together. A sequence of commands joined together by pipes is known as a pipeline. For creating this mechanism, all Unix tools have access to three distinct special files:
- stdin—the standard input file,
- stdout—the standard output file, and
- stderr—the standard error file.
By joining one tool's stdout to another tool's stdin, a pipeline is formed. Errors are sent to a side track and accumulated.
This pattern is extensively used in Unix, but was also ported to DOS, OS/2, Windows NT, BeOS, MacOS X and other operating systems with a text console.
Often filter programs form the constituent programs in a pipeline, performing their bits of functional programming, using byte streams as data objects.
This pattern encourages the use of text streams as the input and output of programs.
This reliance on text has to be accounted when creating graphic shells to text programs.
Tools like netcat can connect pipes to TCP/IP sockets, following the Unix philosophy of "everything is a file".
An attempt to bring together the text-stream interface and hypermedia is XMLTerm, where the input and the output is XML and can be interpreted graphically.
