Sometimes I write little scripts partly in order to play around with a command I haven't used before. Such is the case with today's script, which I have unimaginatively called bsearch-nums. Here it is:

#!/bin/sh
# usage: bsearch-nums START END

lo=$1; hi=$2
while [ $lo -lt $hi ]; do
  mid=`echo $lo $hi +2/p | dc`
  echo "try: $mid"
  echo -n "u/d? "
  read a || exit 0
  case "$a" in
    [uU+]*) lo=`echo $mid 1+p | dc`;;
    [dD-]*) hi=$mid;;
  esac
done
echo
echo "result: $lo"

Every so often, I find myself running down something where the best way to find the answer is a binary search; today's example was finding out which revision of our aliases file introduced a particular alias. But what usually happens is that only the first few steps are a true binary search and then I resort to jumping around to numbers that look vaguely right, because keeping track of the low and high points and cranking the math gets too annoying.

Enter bsearch-nums, which walks you through the numbers for a manual binary search. You get to supply the actual testing (and then tell the program whether the search should go up or down at each step).

As you might guess, the command I was finally trying out was dc, which is one of those hoary old Unix utilities that barely gets any love and attention these days. (Please don't bother to tell me that a modern shell could do all the math with builtins. I'm too old fashioned, and besides, Solaris 8's /bin/sh isn't a modern shell. Although it doesn't do 'echo -n' either, so I guess this script shows where I actually wrote it.)