跟user和group有关的命令和文件

commands:
useradd
userdel
groupadd
groupdel
passwd
chfn
finger
chsh
chage
usermod
groupmod

files:
/etc/passwd
/etc/shadow
/etc/group
/etc/gshadow
/etc/login.defs
/etc/default/useradd
/etc/skel