useradd
passwd
su(substitute user)
/etc/passwd
/etc/shadow
/etc/default/useradd
/etc/login.defs
/etc/skel
usermod
userdel
passwd
chage
chpasswd
/etc/group
/ect/gshadow
groupadd
groupmod
groupdel
gpasswd
newgrp
users
who
whoami
w
logname
id
groups
lslogins
write
wall
mesg
mail