1.centos系统默认是没有tree命令的，需要安装。tree的安装命令：
sudo yum -y install tree
2.tree -L N 其中N表示展示几级目录
[root@ipha-dev71-1 docker]# tree -L 1 /var/lib/docker/ /var/lib/docker/ ├── builder ├── buildkit ├── containers ├── image ├── network ├── overlay2 ├── plugins ├── runtimes ├── swarm ├── tmp ├── trust └── volumes 12 directories, 0 files