Command manuals for authentication

adduser, addgroup - add or manipulate users or groups

chsh - change login shell

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

groupadd - create a new group

groupdel - delete a group

groupmod - modify a group definition on the system

pam - Netpbm common 2-dimensional bitmap format

passwd - change user password

sshd — OpenSSH daemon

su - run a command with substitute user and group ID

sudo, sudoedit — execute a command as another user

useradd - create a new user or update default new user information

userdel - delete a user account and related files

usermod - modify a user account

xauth - X authority file utility

xhost - server access control program for X