shell script how to user my absolute path name

file_name=`basename "$0"`
directory=$(pwd)

echo "same as $(realpath)/$(basename "$0")"
echo "this is $(pwd)/$(basename "$0")"
echo "also in $(cd "$(dirname "$1")" && pwd -P)/$(basename "$1")$(basename "$0")"
echo "my name is ${directory}/${file_name}"

##

test/answer.sh:4: command not found: realpath
same as /answer.sh
this is /Users/giljaeeun/Documents/work_dir/answer.sh
also in /Users/giljaeeun/Documents/work_dir/answer.sh
my name is /Users/giljaeeun/Documents/work_dir/answer.sh

# realpath가 맥에서 안먹음