Finding System Information

• cat /etc/redhat-release → Red Hat version
• uname -a → Linux hostname, kernel, architecture
• dmidecode → hardware, BIOS, system info
• arch → x86_64 or i386/32-bit

Common Keyboard Shortcuts

• Ctrl + U → erase command line
• Ctrl + C → stop/kill current command
• Ctrl + Z → suspend command
• Ctrl + D → exit interactive session

Record Terminal Session

• script mylog.log → record shell session

  • Use exit to stop recording

Root Password Recovery

1. Reboot → enter GRUB menu
2. Press e → edit boot params
3. Add rd.break at end of kernel line
4. Ctrl + X to boot
5. Mount root with chroot /sysroot
6. passwd root → change password
7. touch /.autorelabel → SELinux relabeling on reboot

View Environment Variables

• printenv, echo $PATH, echo $SHELL

Set Environment Variables

• Temporary: export TEST=1, echo $TEST

• Permanent:

  vi ~/.bashrc
  TEST=123
  export TEST

Global → vi /etc/profile or /etc/bashrc

• 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

Crontab Basics

• crontab -e → edit crontab
• crontab -l → list crontab entries
• crontab -r → remove crontab entries
• systemctl status crond → check crond service status

AT: One-time Scheduled Tasks

• Used to schedule jobs only once.

Scheduling Examples:

• at HH:MM PM → schedule a job interactively

• at 11:23AM 12/24/25

  • Enter job (e.g., echo "test") and press Ctrl + D to finish
• at 2:45PM 01/30/25
• at 4PM + 4 days
• at now + 5 hours
• at 8:00AM Sun
• at 10:00AM next month

Managing AT Jobs

• atq → list the scheduled at jobs
• atrm # → remove a specific at job (use number from atq)
• systemctl status atd → check atd daemon status

Disk and Memory Info

• df → disk partition info
• df -h → human-readable format
• du → size of file/directory
• free → memory info
• cat /proc/cpuinfo → CPU info
• cat /proc/meminfo → memory info

System Logs & Diagnostics

• dmesg → check kernel ring buffer logs
• iostat → check disk I/O stats
• ip route | column -t → check routing table
• ss → view open ports/sockets

Log Directory Structure

• Logs in /var/log/

  • boot → startup records
  • chronyd → NTP info
  • cron → cron jobs
  • maillog → mail daemon
  • secure → login/logout tracking
  • messages → general trace logs
  • httpd → web server errors

Log Monitoring

• tail -f filename → follow log file in real time

System Shutdown & Boot Control

• shutdown → graceful shutdown
• init 0-6 → runlevels (0=off, 3=multi-user, 6=reboot)
• reboot → reboot system
• halt → force shutdown immediately

Hostname Management

• cat /etc/hostname → current hostname
• hostnamectl set-hostname myhostname → change hostname

• 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 System Info Commands

• uptime → time, uptime, users, load average
• hostname → current hostname
• ip hostname → show IP & hostname
• uname -a → current OS and kernel info
• which command → path to a command

• cal → calendar for current month/year

  • cal 2025 → full year
• bc → basic calculator

System Monitoring

• systemctl → interact with systemd system manager
• ps → view current processes
• top → dynamic view of process usage

Systemctl Usage

• systemctl start|stop|status servicename.service → manage service state
• systemctl enable servicename.service → start service at boot
• systemctl restart|reload servicename → restart or reload service
• systemctl list-units --all → list all units

• To add a service:

  • Create a unit file in /etc/systemd/system/servicename.service

Process Management

• Start a process in the background:

  • Ctrl + Z then jobs → suspend and background
  • bg → continue in background
  • fg → bring back to foreground

• Run process detached from terminal:

  • nohup process & → keep running after logout
  • nohup process > /dev/null 2>&1 & → suppress all logs/output

Killing and Prioritizing

• pkill name → kill process by name

• nice → start process with a priority (range -20 to 19)

  • lower = higher priority
  • nice -20 top

Jobs Monitoring

• jobs → list background/foreground job states

Vi Editor Commands

• i → insert mode
• I → insert at line start
• ESC → escape to command mode
• R → replace mode
• x → delete character
• dd → delete line
• yy → copy line
• p → paste
• v → visual mode
• o → open new line below and insert
• O → open new line above and insert
• a → append after cursor
• A → append at end of line
• :q! → quit without saving
• :wq or ZZ → save and quit