`where` in the $PATH is an executable found. Bash/shell/unix script. Extends `which` to also print the $PATH index (GNU General Public License)

where.sh

#! /bin/sh
set -ef

if test -n "$KSH_VERSION"; then
	puts() {
		print -r -- "$*"
	}
else
	puts() {
		printf '%s\n' "$*"
	}
fi

ALLMATCHES=0

while getopts a whichopts
do
        case "$whichopts" in
                a) ALLMATCHES=1 ;;
                ?) puts "Usage: $0 [-a] args"; exit 2 ;;
        esac
done
shift $(($OPTIND - 1))

if [ "$#" -eq 0 ]; then
 ALLRET=1
else
 ALLRET=0
fi
case $PATH in
	(*[!:]:) PATH="$PATH:" ;;
esac
for PROGRAM in "$@"; do
 RET=1
 IFS_SAVE="$IFS"
 IFS=:
 case $PROGRAM in
  */*)
   if [ -f "$PROGRAM" ] && [ -x "$PROGRAM" ]; then
    puts "$PROGRAM"
    RET=0
   fi
   ;;
  *)
    j=0
   for ELEMENT in $PATH; do
    if [ -z "$ELEMENT" ]; then
     ELEMENT=.
    fi
    if [ -f "$ELEMENT/$PROGRAM" ] && [ -x "$ELEMENT/$PROGRAM" ]; then
     puts "$ELEMENT/$PROGRAM    $j"
     RET=0
     [ "$ALLMATCHES" -eq 1 ] || break
    fi
    j=$((j+1))
   done
   ;;
 esac
 IFS="$IFS_SAVE"
 if [ "$RET" -ne 0 ]; then
  ALLRET=1
 fi
done

exit "$ALLRET"

where_example.sh

#! /bin/sh

# Where in the $PATH is `cp` picked up from?: entry 11 "/bin"
./where.sh cp
# > /bin/cp    11

# Same input format as `which`: can take multipel inputs
./where.sh cp scp which
# > /bin/cp    11
# > /usr/bin/scp    9
# > /usr/bin/which    9

# For comparision:
./which.sh cp scp which
# > /bin/cp
# > /usr/bin/scp
# > /usr/bin/which

Slight modification of the which command source.