30.Mar.2009

The Importance Of Pipes

There’s a very subtle, often overlooked thing in Unix, the pipeline, or | character (often just called a pipe).

This is perhaps the most important thing in the entire operating system, with the possible exception of the “everything is a file” concept.

In case you’re unfamiliar (or didn’t feel like reading the wiki page above), here’s the basic concept:

A pipe allows you to link the output of one program to the input of another.

eg foo | bar – this takes the output from foo, and feeds whatever-it-is into bar – rather than, say, having to point bar at a specific file to make it do anything useful.

Why are pipes so awesome?

Well, the following reasons:

  1. Each program only has to do one thing, & do it well
  2. As such, development of those programs can be split up – even to the point where a thousand people can independently write a thousand programs, & they’ll all still be useful
  3. Each of those programs is very simple, thus faster to develop, easier to debug, etc
  4. Extremely complex behaviour can be created by linking different programs together in different ways
  5. None of that higher level behaviour has to be pre-thought or designed for

So, Unix has ended up with a ton of small but powerful programs. For example:

  • ls – lists a directory
  • cat – displays stuff
  • sort – sorts stuff
  • grep – finds things in stuff
  • tr – translates stuff (eg, upper to lower case)
  • wc – counts words or lines
  • less – pauses stuff, allowing forwards & backwards scrolling

I’ve been deliberately vague with the descriptions. Why? Because ‘stuff’ can mean a file – if we specify it, or, it can mean whatever we pass in by putting a pipe in front of it.

So here’s an example. The file we’ll use is /usr/share/dict/words – ie, the dictionary.

cat /usr/share/dict/words

displays the dictionary

cat /usr/share/dict/words | grep eft

displays dict, but only shows words with ‘eft’ in them

cat /usr/share/dict/words | grep eft | sort

displays ‘eft’ words, sorted

cat /usr/share/dict/words | grep eft | sort | less

displays sorted ‘eft’ words, but paused so we can see what the hell we’ve got before it scrolls madly off the screen

cat /usr/share/dict/words | grep eft | grep -ve ‘^[A-Z]’ | sort | less

displays paused sorted ‘eft’ words, but removes any that start with capital letters (ie, all the Proper Nouns)

cat /usr/share/dict/words | grep eft | grep -ve ‘^[A-Z]’ | wc -l

gives us the count of how many non proper-noun ‘eft’ words there are in the dictionary (in the huge british english dictionary? 149, since I know you’re curious)

So there’s an additional benefit which is probably obvious. Debugging a complex set of interactions with pipes is incredibly straightforward. You can simply build up what you think you need, experimenting a little at each stage, & viewing the output. When it looks like what you want, you just remove the output-to-screen, and voila!

For the end-users, this means that the operating cost of using the system in a complex manner is drastically reduced.

What would happen without pipes? You’d end up with monolithic programs for every imaginable combination of user need. Ie, an unmitigated disaster. You can see elements of this in, umm, ‘certain other’ operating systems. *cough*

Most importantly of all, there is a meta benefit. A combination of all of the above benefits.

Pipes enable incredibly complex higher level behaviours to emerge without being designed in. It’s a spontaneous emergent behaviour of the system. There’s no onus on the system development programmers to be demi-gods, all they need to do is tackle one simple problem at a time – display a file, sort a file, and so on. The system as a whole benefits exponentially from every small piece of added functionality, as pipes then enable them to be used in every possible permutation.

It’s as if an anthill full of differently talented ants was suddenly building space ships.

Perhaps a better bits-vs-atoms metaphor is of money. Specifically the exchange of goods (atoms) for money, allows the conversion of those atoms into other atoms, via money. In the same way, pipes allows different programs to seamlessly interact via streamed data, in infinitely variable ways.

You don’t need to know how to make a car, since you can do what you’re good at, get paid, & exchange that money for a car. Or a boat. Or a computer. Society as a whole is vastly better off as each person can specialize & everybody benefits. Think how basic our world would be if we only had things that everybody knew how to build or do. Same thing with computers & pipes.

What seems like an almost ridiculously simple concept, pipes, has allowed an unimaginably sophisticated system to emerge from simple, relatively easily built pieces.

It’s not quite the holy grail of systems design, but it’s bloody close.

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