> I would like to check for a prior instance of a script, and if
> running, exit.

I would use a lockfile, somewhat along the lines of:

#!/bin/sh

myLOCKFILE="$HOME/.`basename $0`..LCK"

if [ -f $myLOCKFILE ]; then
  echo "Killroy woz 'ere"
  exit 1
fi
trap "rm -f $myLOCKFILE" 0 1 2 3 15

touch $myLOCKFILE >/dev/null 2>&1
echo "Just me and my shadow..."

exit 0
# eof

Klaus