pushdpopd（在堆栈上存储当前 dir 并转到 dest pop prev 目录并转到它）

$ pwd
/home/bob/somedir1/somedir2/somedir3

$ pushd /home/bob/otherdir1/otherdir2
/home/bob/otherdir1/otherdir2 /home/bob/somedir1/somedir2/somedir3

$ popd
/home/bob/somedir1/somedir2/somedir3

$ pushd /usr
/usr /home/bob/somedir1/somedir2/somedir3

$ pushd /var
/var /usr /home/bob/somedir1/somedir2/somedir3

$ popd
/usr /home/bob/somedir1/somedir2/somedir3

$ pwd
/usr

$ popd
/home/bob/somedir1/somedir2/somedir3

$ pwd
/home/bob/somedir1/somedir2/somedir3