1 2set -e 3cd /usr/bin 4[ "$PWD" = /usr/bin ] 5CDPATH=/: 6cd . 7[ "$PWD" = /usr/bin ] 8cd ./ 9[ "$PWD" = /usr/bin ] 10cd .. 11[ "$PWD" = /usr ] 12cd /usr/bin 13cd ../ 14[ "$PWD" = /usr ] 15