这样的需求利用sed或bash自身处理会更容易懂：

sed:

path_old=$(pwd)
path_new=$(sed 's#/root##' <<<${path_old})

bash:

path_old=$(pwd)
path_new=${path_old/\/root/}

当然cut和awk也能办到：

cut：

path_old=$(pwd)
path_new=$(echo $path_old|cut -c6-)

awk:

path_old=$(pwd)
path_new=$(echo $path_old|awk 'sub(/\/root/,"")')