== A small script: _bsearch-nums_

Sometimes I write [[little scripts LittleScriptsV]] 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.)