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

• 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

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

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

File Display Commands

cat → Show entire content
cat -A → Show non-printable characters

more → Paginate output
less → Same as more but allows navigation with arrow keys

head → Show top lines (default: 10)
tail → Show bottom lines (default: 10)

head -2 myfile
tail -2 myfile

Filters / Text Processing Input

• cut → Cut input
• awk → List by columns
• grep & egrep → Search by keyword
• sort → Sort in alphabetical order
• uniq → Remove duplicate lines
• wc → Word count (including lines)

cut Command Examples

• cut -c1 filename → First letter of each line
• cut -c1,2,4 filename → Characters 1, 2, and 4
• cut -c1-3 filename → Range: characters 1 to 3
• cut -c1-3,6-8 filename → Ranges: characters 1-3 and 6-8
• cut -b1-3 filename → Byte range 1 to 3
• cut -d: -f6 /etc/passwd → Field 6 using : delimiter
• cut -d: -f6-7 → Fields 6 and 7
• ls -l | cut -c2-4 → Extract character range 2 to 4 from ls output

awk Command Examples

• awk '{print $1}' file → Print 1st column of file
• ls -l | awk '{print $1, $3}' → Print 1st and 3rd columns from ls -l
• ls -l | awk '{print $NF}' → Print last column
• awk '/jerry/ {print}' file → Search for "jerry" and print matching lines
• awk -F: '{print $1}' /etc/passwd → Use : as delimiter, print 1st field
• echo "hello tom" | awk '{$2="adam"; print}' → Replace column 2 with "adam"
• awk 'length($0) > 15' file → Print lines longer than 15 characters

grep Command Examples

• grep keyword file → Search for keyword
• grep -c keyword file → Count matching lines
• grep -i keyword file → Ignore case
• grep -n keyword file → Show line numbers
• grep -v keyword file → Invert match (everything except keyword)
• grep keyword file | awk '{print $1}' → Pipe grep to awk to extract 1st column
• ls -l | grep keyword → Filter ls -l output by keyword
• egrep -i "keyword1|keyword2" file → Search for multiple keywords (case-insensitive)

sort Command Examples

• sort file → Sort alphabetically
• sort -r file → Sort in reverse order
• sort -u file → Sort and remove duplicates
• sort file | uniq → Same as above with explicit uniq
• sort file | uniq -d → Show only duplicate lines
• sort file | uniq -c → Count duplicates
• sort -k4 -n file → Sort by 4th field (numeric)
• ls -l | sort -k4 -n → Sort by size from ls -l

wc Command Examples

• wc -c filename → Byte count
• wc -w filename → Word count
• wc -l filename → Line count
• ls -l | wc -l → Count lines from ls -l
• wc -l filename → Count number of lines in file

cmp / diff

• diff → Compare files line by line
• cmp → Compare files byte by byte

tar / gzip

• tar cvf export.tar somedir → Compress directory
• tar xvf export.tar → Extract tar archive
• gzip export.tar → Compress with gzip
• gzip -d export.tar.gz → Decompress gzip archive

truncate Command

• truncate -s10 filename → Chop file to 10 bytes
• truncate -s60 filename → Extend file to 60 bytes

Combining & Splitting Files

• cat file1 file2 file3 > fileN → Combine multiple files into one