Portable mini-implementation of pushd/popd from the ksh docs

dirstack.sh

# These directory stack functions are based upon the versions in the Korn
# Shell documentation - http://docstore.mik.ua/orelly/unix3/korn/ch04_07.htm.
dirs() {
    echo "$_DIRSTACK"
}

pushd() {
    dirname=$1
    cd ${dirname:?"missing directory name."} || return 1
    _DIRSTACK="$PWD $_DIRSTACK"
    echo "$_DIRSTACK"
}

popd() {
    _DIRSTACK=${_DIRSTACK#* }
    top=${_DIRSTACK%% *}
    cd $top || return 1
    echo "$PWD"
}