Contributed by Stephane CHAZELAS on c.u.s. Assuming POSIX shell:
prg=$0
if [ ! -e "$prg" ]; then
case $prg in
(*/*) exit 1;;
(*) prg=$(command -v -- "$prg") || exit;;
esac
fi
dir=$(
cd -P -- "$(dirname -- "$prg")" && pwd -P
) || exit
prg=$dir/$(basename -- "$prg") || exit
printf '%s\n' "$prg"