By default, Ubuntu requires a minimum password length of 6 characters, as well as some basic entropy checks. These values are controlled in the file /etc/pam.d/common-password, which is outlined below.

password        [success=1 default=ignore]      pam_unix.so obscure use_authtok try_first_pass sha512
等都注释了，完后添加：

password    [success=1 default=ignore]  pam_unix.so minlen=1 sha512

执行命令：

sudo passwd houlin

Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
成了！