Utilities

adduser, addgroup – add a user or group to the system

--gecos GECOS	Set the gecos field for the new entry generated. adduser will not ask for finger information if this option is given.
--system	Create a system user or group.
--shell SHELL	Use SHELL as the user’s login shell, rather than the default specified by the configuration file.
--no-create-home	Do not create the home directory, even if it doesn’t exist.
--quiet	Suppress informational messages, only show warnings and errors.

adduser --system --no-create-home proxmoxbackup
adduser --no-create-home --shell /usr/sbin/nologin proxmoxbackup

adduser --disabled-password --gecos "" username

deluser, delgroup – remove a user or group from the system

--system	Only delete if user/group is a system user/group.
--remove-home	Remove the home directory of the user and its mailspool.
--remove-all-files	Remove all files from the system owned by this user.

deluser --system --remove-home proxmoxbackup

usermod – modify a user account

-a, --append	Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]	A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.

Add current user to the nut group (works after logoff):

usermod -aG nut `whoami`

gpasswd – administer /etc/group and /etc/gshadow

-a, –add user	Add the user to the named group.
-d, –delete user	Remove the user from the named group.

Add a user to a group:

gpasswd -a $USER timekpr

pgrep, pkill, pidwait – look up, signal, or wait for processes based on name and other attributes

Unlog a user

pkill -U leopardb

id – print real and effective user and group IDs

who – show who is logged on

Random

List local users or groups

getent passwd

getent group

useradd,adduser