http://enos.itcollege.ee/~edmund/osadmin/eng/user-management/user-management.pdf
https://help.ubuntu.com/community/AddUsersHowto
http://askubuntu.com/questions/345974/what-is-the-difference-between-adduser-and-useradd
https://wiki.itcollege.ee/index.php/Gpasswd
https://wiki.itcollege.ee/index.php/Adduser_%26_useradd
https://help.ubuntu.com/lts/serverguide/user-management.html

configuration file:
/etc/adduser.conf

simulate root:
sudo -i

log in as another user:
su - username

current user ID:
id

who is logged in and what doing:
w

current user groups:
groups

which users are in the system:
ls /home
sudo getent passwd (will show also system users)

does not show system users:
sudo awk -F'[/:]' '{if ($3 >= 1000 && $3 != 65534) print $1}' /etc/passwd

create a user
sudo adduser username

there is also useradd but not suggested unless you know details

user: sysadmin

user: engineer, designer
group: specialists
- create group at first
- create user in that group
sudo groupadd specialists
sudo adduser --ingroup specialists engineer

change home folder permissions that only user can access
chmod 700 folder
chmod go-rwx folder

create a folder /var/data/specialists
and change group of that folder to specialists
make this folder accessible only for that group
sudo chmod 070 /var/data/specialists

make symlinks to user folders:
ln -s target linkname
ln -s /var/data/specialists /home/engineer/

add user to group:
sudo adduser user group
sudo gpasswd -a user group

change group:
sudo chgrp group file/folder

change ownership
----------------
sudo chown user: file/folder
sudo chown :group file/folder


remove from group
sudo gpasswd -d user group

remove group:
sudo delgroup groupname

creating e.g. FTP user without ability to log in using shell:
ls -l /bin/false
add /bin/false to /etc/shells
change /etc/passwd appropriate user shell to /bin/false

lock password (can still log in over SSH)
usermod -L <user>
passwd -l  <user>

unlock password
usermod -U <user>
passwd -u  <user>

lock user
chage -E 0 <user> #set expiry - cannot log in over SSH
chage -l <user> #check expiry status
chage -E -1 <user> #remove expiry

# # #

compgen -u #check users
compgen -g #check groups

more information:
https://unix.stackexchange.com/questions/151118/understand-compgen-builtin-command
https://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html