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February 3, 2009

Guidelines on integrating complex unix command lines into programs

by Joe Kuan

unixNeedless to say, the power of combining unix commands together can break any complicated tasks into challenging tricks. These commands are a great help in programming to offer great short cuts. Less code means less bugs. However, I have seen some code embedded with complex unix command lines used slightly to the extreme.

That would be fine if that program is maintained solely by the author or a team of equal strength unix developers. Otherwise, there are drawbacks:

  • If problems arise in the middle of the complex command lines, it is difficult to interpret into meaningful error messages to the end users
  • Less obvious to the less experienced maintainers; it takes them much longer time to understand & fix the problem. They may even rewrite the whole thing out of frustration.

In general, I would keep the embedded unix command lines to no more than a sequence of 3 commands.

If it has to be a very complex command line, decouple it by

  1. breaking the long command into a well structured sequence of commands and storing them in a separate shell script
  2. keeping the information/debug messages to stdout, error messages to stderr
  3. making sure to exit with as many different values as possible to signal various stages of errors
  4. calling the script with popen (or anything similar) and pclose to get the exit status

This gives the benefits of :

  • extensible, easy to maintain
  • commenting the difficult parts more directly
  • providing unit tests for the script

I work for iTrinegy and here are my other technical blogs

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