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The Art of Unix Programming
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Unix Programming - Unix Interface Design Patterns - The ed pattern

The ed pattern

All the previous patterns have very low interactivity; they use only control information passed in at startup time, and separate from the data. Many programs, of course, need to be driven by a continuing dialog with the user after startup time.

In the Unix tradition, the simplest interactive design pattern is exemplified by ed(1), the Unix line editor. Other classic examples of this pattern include ftp(1) and sh(1), the Unix shell. The ed(1) program takes a filename argument; it modifies that file. On its input, it accepts command lines. Some of the commands result in output to standard output, which is intended to be seen immediately by the user as part of the dialog with the program.

An actual sample ed(1) session will be included in Chapter13.

Many browserlike and editorlike programs under Unix obey this pattern, even when the named resource they edit is something other than a text file. Consider gdb(1), the GNU symbolic debugger, as an example.

Programs obeying the ed interface design pattern are not quite so scriptable as would be the simpler interface types resembling filters. You can feed them commands on standard input, but it is trickier to generate sequences of commands (and interpret any output they might ship back) than it is to just set environment variables and command-line options. If the action of the commands is not so predictable that they can be run blind (e.g., with a here-document as input and ignoring output), driving ed-like programs requires a protocol, and a corresponding state machine in the calling process. This raises the problems we noted in Chapter7 during the discussion of slave process control.

Nevertheless, this is the simplest and most scriptable pattern that supports fully interactive programs. Accordingly, it is still quite useful as a component of the “separated engine and interface” pattern we'll describe below.


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The Art of Unix Programming
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