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The Art of Unix Programming
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Unix Programming - Unix Interface Design Patterns - The Cantrip Pattern

The Cantrip Pattern

The cantrip interface design pattern is the simplest of all. No input, no output, just an invocation and a numeric exit status. A cantrip's behavior is controlled only by startup conditions. Programs don't get any more scriptable than this.

Thus, the cantrip design pattern is an excellent default when the program doesn't require any runtime interaction with the user other than fairly simple setup of initial conditions or control information.

Indeed, because scriptability is important, Unix designers learn to resist the temptation to write more interactive programs when cantrips will do. A collection of cantrips can always be driven from an interactive wrapper or shell program, but interactive programs are harder to script. Good style therefore demands that you try to find a cantrip design for your tool before giving in to the temptation to write an interactive interface that will be harder to script. And when interactivity seems necessary, remember the characteristic Unix design pattern of separating the engine from the interface; often, the right thing is an interactive wrapper written in some scripting language that calls a cantrip to do the real work.

The console utility clear(1), which simply clears your screen, is the purest possible cantrip; it doesn't even take command-line options. Other classic simple examples are rm(1) and touch(1). The startx(1) program used to launch X is a complex example, typical of a whole class of daemon-summoning cantrips.

This interface design pattern, though fairly common, has not traditionally been named; the term “cantrip” is my invention. (In origin, it's a Scots-dialect word for a magic spell, which has been picked up by a popular fantasy-role-playing game to tag a spell that can be cast instantly, with minimal or no preparation.)


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The Art of Unix Programming
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