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Unix Filters -- Classic "Pipable" Utilities

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Unix created a new style of computing, a new way of thinking of how to attack a problem with a computer. This style was essentially the first successful component model in programming. As Frederick P. Brooks Jr (another computer pioneer who early recognized the importance of pipes) noted, the creators of Unix "...attacked the accidental difficulties that result from using individual programs together, by providing integrated libraries, unified file formats, and pipes and filters."

It is important fully understand full the scope and limitations of this component programming model. The key concept of pipe model is a concept of filters, a self-contained programs that programs that accept standard input and produce standard output for processing by the next stage of the pipe. Filters can be standard or custom. In the latter case they can be written in any language supported by the OS by typically are written in shell, awk or Perl.

In a way operation of filters are similar to operations of relational database. We can talk about horizontal filters that just change composition of each line without changing the total number of lines (cut, tr expand ,  etc) and vertical filters which change the order of the output line or their number or both  (grep, sort, uniq, head, tail, split, etc). 

The first stage of the pipe does not need to accept standard input and thus can he half-filer -- a program that just produces record oriented output that is suitable for processing by pipes (for example who, ls, df, last, find,  etc). Similarly the last stage of the pipe does not need to produce standard output in form processing by pipe. It can write record to other files like logger, calculate some stats like wc,  invoke other programs like xargs, create some kind of archives like cpio, drive the GUI-based programs like expect, send mail like mailx, etc

There are also one debugging filter called pv - pipe viewer. Actually this component model allows a specific type of debugging: you can debug you complex pipe one stage at a time feeding it with the file that is the output of previous stage on particular test data. See Pipe Debugging

Pipethink

Pipeline programming involves applying special style of componentization  that allows to break a problem into a number of small steps, each of which can then be performed by a simple program. We will call this type of componentization "pipethink" in which wherever possible, programmer  relies on preexisting collection of useful "stages" implemented by what is called Unix filters.

Traditionally, Unix utilities that are used as stages in pipelines are small and perform a single well-defined function. At the same time they are generalized to allow re-use in different situations. Because they are so small and well-defined, it is possible to make them very reliable. In other words, Unix filters are "little gems".  Over years Unix accumulated a rich collection of such gems and shell programmers often find that some of requires text transformations can be written using only filters. Sometimes adding one of two custom stages typically written in shell, awk or Perl.

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