• Essential Commands:

  • useradd, groupadd, userdel, groupdel, usermod
  • Modify 3 files: /etc/passwd, /etc/group, /etc/shadow

Create User with Custom Options

useradd -G mygroup -s /bin/bash \
  -c "User Description" -m \
  -d "/home/myuser" myuser

Other Useful User Commands

• useradd myuser → create user
• id myuser → check if user exists
• groupadd → create group
• cat /etc/group → check group
• userdel -r myuser → delete user and home directory
• groupdel mygroup → delete group
• usermod -G mygroup myuser → change user's group
• chgrp -R mygroup myuser → change group ownership recursively
• passwd myuser → set or update user's password

Password Aging with chage

• chage → manage password aging

  • -m → minimum days
  • -M → maximum days
  • -d → last password change
  • -I → inactive days
  • -E → expiration date
  • -W → warning days

• Example:

  • chage -M 5 -M 90 -W 10 -I 10 username

Edit Password Policies

• File: /etc/login.defs

  • PASS_MAX_DAYS 9999
  • PASS_MIN_DAYS 0
  • PASS_MIN_LEN 5
  • PASS_WARN_AGE 7

Switch User Privileges

• visudo → edit sudo permissions config (/etc/sudoers)

  • Add user to wheel group for sudo access
• usermod -aG wheel myuser → grant sudo rights

User Info and Monitoring Commands

• who → see logged in users
• last → list last login records
• w → who is logged in with more details
• finger → show user details
• id → show UID, GID and groups
• id myuser → check user info

Send Messages to Users

• wall → broadcast message to all users

  • Type message, then Ctrl + D to send

• write myuser → send message to specific user

  • Type message, then Ctrl + D to send

• Basic Replace Syntax:

  • sed -i 's/KENNY/LENNY/g' filename

  • Substitute all occurrences of "KENNY" with "LENNY"

• Delete Line Containing String:

  • sed -i '/SEINFELD/d' filename

• Delete Empty Lines:

  • sed -i '/^$/d' filename

• Delete First Line:

  • sed '1d' filename

• sed '1,2d' filename → delete the first two lines

• sed 's/\t/ /g' filename → replace tabs with spaces

• sed -n '12,18p' filename → print only lines 12 to 18

• sed '12,18d' filename → delete lines 12 to 18

• sed G filename → insert an empty line after every line

• sed '8!s/seinfeld/S1/' filename → replace "seinfeld" with "S1" on every line except line 8