#!/bin/sh
#  append -x to above to debug this script
# Illustrates: error handling and primitive argument handling.
# Strip alphabetics or nonalphabetics from (contents of) files.

usage() 
{  ## via echos
	 exec >&2
         echo "Usage: `basename $0` -d|-k [file ...]"
         echo "   -d      delete alphabetic characters"
         echo "   -k      keep only alphabetic characters and spaces"
         echo "   file... files to process"
   ## via here document.  "-" strips off leading tabs
         cat >&2 <<-EOF
		Usage: `basename $0` -d|-k [file ...]
		   -d      delete alphabetic characters
		   -k      keep only alphabetic characters and spaces
		   file... files to process
		EOF
}

if [ $# = 0 ] ; then
  echo >&2 "`basename $0`: option required"
  usage ; exit 1
fi

case $1 in   ## not very good - better to use "getopts"
    -d)  opt=d  ;;
    -k)  opt=k  ;;
    *)   echo >&2 "`basename $0`: illegal option -- $1"; 
	 usage ; exit 1 ;;
esac
shift

TMPFILE=/tmp/`basename $0`.$LOGNAME.$$

## Simple signal handling example (also see "man kill"):
trap 'echo "Trap 0."      ; /bin/rm "$TMPFILE" 2>/dev/null' 0
trap 'echo "Trap 1,2,15." ; /bin/rm "$TMPFILE" 2>/dev/null; exit 1' 1 2 15
  ## Signals:  1=hangup  2=interrupt(^C)  3=quit(^\)  15=kill
  ## Instead of terminating immediately, we clean up first.
  ## Signal 3 not used so as to give an escape hatch.
  ## To ignore signal 2:    trap '' 2
  ## To reset to default:   trap 2 
echo 'Sleeping for 5 seconds - try a signal if you wish.' ; sleep 5
echo Woke up

MV="echo +++ mv "  ## Defang this script while testing.
# MV=mv            ## What to use in the final production version!

for i
do
	if [ ! -f $i ] ; then 
		echo >&2 "`basename $0`: file $i does not exist"
	elif [ $opt = d ] ; then
		tr -d '[:alpha:]'      < $i > $TMPFILE  &&  $MV $TMPFILE $i 
	else
		tr -cd '[:alpha:]\n'   < $i > $TMPFILE  &&  $MV $TMPFILE $i 
	fi
done