Another way with Bash (also two steps).
IFS=/ read -a array <<<'/dir0/dir1/dir2/filename.ext'
echo ${array[ $(( ${#array[@]}-2 )) ]}
dir2
Another way with Bash (also two steps).
IFS=/ read -a array <<<'/dir0/dir1/dir2/filename.ext'
echo ${array[ $(( ${#array[@]}-2 )) ]}
dir2