The Universe of Disco

Addenda to recent articles 200811

• Earlier I discussed an interesting technique for flag variables in Bourne shell programs. I did a little followup research.

  I looked into several books on Unix shell programs, including:

  • Linux Shell Scripting with Bash (Burtch)
  • Unix Shell Programming 3ed. (Kochan and Wood)
  • Mastering UNIX Shell Scripting (Michael)
  All of these contained examples of flag variables in Bourne shell, and none used the technique I described. (In fact, most books wanted to switch to if [ ... ]; right away, or even to pretend that that was the only possible syntax.) So it may be obvious, but it doesn't seem to be widely used. I also looked into The Unix Programming Environment, by Kernighan and Pike, which is the book from which I learned shell programming, to see if it was there. I couldn't find any examples of boolean variables at all! But there were surprisingly few shell programs; they switched to awk rather quickly.

  But two readers sent me puzzled emails, to tell me that they had been using the true/false technique for years are were surprised that I found it surprising. Brooks Moses says that at his company they have a huge build system in Bourne shell, and they are trying to revise the boolean tests to the style I proposed. And Tom Limoncelli reports that code by Bill Cheswick and Hal Burch (Bell Labs guys) often use this technique. Tom speculates that it's common among the old farts from Bell Labs. Also, Adrián Pérez writes that he has known about this for years.

  It's tempting to write to Kernighan to ask about it, but so far I have been able to resist.

• My first meta-addendum: In October's addenda I summarized the results of a paper of Coquand, Hancock, and Setzer about the inductive strength of various theories. This summary was utterly wrong. Thanks to Charles Stewart and to Peter Hancock for correcting me.

  The topic was one I had hoped to get into anyway, so I may discuss it at more length later on.

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