lizzy是我们新建的用户
添加lizzy到work中：
Now to add the user lizzy to the work group with the following command:
$ sudo usermod -aG work lizzy
查看所有用户组：
cat etc/group
grep出hadoop用户组：
cat etc/group |grep hadoop
hadoop:x:1000:hadoop
添加用户：
add another user ash.
$ sudo useradd -m ash
Also set password for ash.
$ sudo passwd ash