Linux¶

Overview¶

Linux is the backbone of modern DevOps infrastructure, providing a stable, open-source operating system for servers, containers, and cloud platforms.

Core Linux Concepts¶

File System Hierarchy¶

/
├── /bin          → Essential executables
├── /etc          → Configuration files
├── /home         → User home directories
├── /var          → Variable data (logs, cache)
├── /tmp          → Temporary files
├── /usr          → User programs and libraries
├── /opt          → Optional software packages
└── /root         → Root user home directory

File Permissions¶

# Symbolic representation: rwxrwxrwx (owner-group-others)
# Numeric: 755 = rwxr-xr-x

chmod 755 script.sh          # Make executable
chmod 644 config.conf        # Read/write for owner, read for others
chown user:group file.txt    # Change ownership

Essential Linux Commands¶

Category	Command	Purpose	Example
Navigation	`cd`	Change directory	`cd /home/user/projects`
Navigation	`pwd`	Print working directory	`pwd`
File Listing	`ls -la`	List detailed files	`ls -la /etc`
File Creation	`touch`	Create empty file	`touch new_file.txt`
File Content	`cat`	Display file content	`cat config.yml`
File Search	`find`	Search files	`find / -name "*.log"`
Text Search	`grep`	Search text patterns	`grep -r "error" /var/log`
File Copy	`cp`	Copy files/directories	`cp -r src/ dest/`
File Move	`mv`	Move/rename files	`mv old.txt new.txt`
File Delete	`rm`	Remove files	`rm -rf /tmp/old_data`
Disk Usage	`du -sh`	Directory size	`du -sh /var/log`
Disk Space	`df -h`	Filesystem usage	`df -h`

User & Group Management¶

User Management¶

# Create new user
sudo useradd -m -s /bin/bash john

# Add user to group
sudo usermod -aG docker john

# Set user password
sudo passwd john

# Delete user
sudo userdel -r john

# List all users
cat /etc/passwd

Group Management¶

# Create group
sudo groupadd developers

# Add user to group
sudo usermod -aG developers john

# List groups
cat /etc/group

# Groups for current user
groups

System Administration¶

Service Management¶

# Start service
sudo systemctl start nginx

# Stop service
sudo systemctl stop nginx

# Restart service
sudo systemctl restart nginx

# Enable service on boot
sudo systemctl enable nginx

# Check service status
sudo systemctl status nginx

# List all services
systemctl list-units --type=service

Package Management¶

Debian/Ubuntu (apt)¶

# Update package list
sudo apt update

# Install package
sudo apt install git curl wget

# Remove package
sudo apt remove git

# Upgrade all packages
sudo apt upgrade

# Search for package
apt search nginx

Red Hat/CentOS (yum/dnf)¶

# Install package
sudo yum install git

# Remove package
sudo yum remove git

# List installed packages
yum list installed

# Update packages
sudo yum update

Process Management¶

# List running processes
ps aux

# Display processes in tree format
pstree

# Kill process by PID
kill -9 1234

# Kill process by name
pkill -f nginx

# Display top processes
top

# Send signal to process
kill -SIGTERM 1234

Network Administration¶

Network Configuration¶

# Display network interfaces
ip addr show

# Check IP configuration
ifconfig

# Display routing table
ip route show
route -n

# Test connectivity
ping 8.8.8.8

# DNS lookup
nslookup google.com
dig google.com

# Check open ports
sudo netstat -tlnp
ss -tlnp

Firewall Management¶

# Check UFW status
sudo ufw status

# Enable UFW
sudo ufw enable

# Allow SSH
sudo ufw allow 22/tcp

# Allow HTTP/HTTPS
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

# Deny service
sudo ufw deny 8080/tcp

# List rules
sudo ufw show added

File and Text Processing¶

Text Editing¶

# Edit file with nano
nano config.txt

# Edit file with vim
vim config.txt

# Quick edit with sed
sed -i 's/old_text/new_text/g' file.txt

# View file with less
less large_file.log

# Display first 10 lines
head -10 file.txt

# Display last 10 lines
tail -10 file.txt

# Follow file in real-time
tail -f /var/log/syslog

Text Processing¶

# Count lines, words, characters
wc -l file.txt

# Sort lines
sort file.txt

# Remove duplicates
sort file.txt | uniq

# Print specific columns
awk '{print $1, $3}' file.txt

# Replace text
sed 's/pattern/replacement/g' file.txt

Permissions & Ownership¶

Permission	Number	Meaning
Read (r)	4	View file contents
Write (w)	2	Modify file contents
Execute (x)	1	Run file/access directory

Permission Examples¶

# Owner: rwx (7), Group: rx (5), Others: rx (5)
chmod 755 script.sh

# Owner: rw (6), Group: rw (6), Others: (0)
chmod 660 config.txt

# Add execute for group
chmod g+x script.sh

# Remove write for others
chmod o-w file.txt

# Recursive permission change
chmod -R 755 /var/www/html

Shell Scripting Basics¶

Simple Script¶

#!/bin/bash

# Define variables
NAME="DevOps Team"
VERSION="1.0"

# Display message
echo "Welcome to $NAME"

# Conditional statement
if [ -f "config.txt" ]; then
    echo "Config file exists"
else
    echo "Config file not found"
fi

# Loop through files
for file in *.log; do
    echo "Processing $file"
done

# Function definition
backup_data() {
    echo "Backing up data..."
    tar -czf backup.tar.gz /data
}

backup_data

Script Execution¶

# Make script executable
chmod +x script.sh

# Run script
./script.sh

# Run with bash explicitly
bash script.sh

# Run in background
./script.sh &

# Run with nohup (ignore hangup)
nohup ./script.sh &

System Monitoring¶

# Display system uptime
uptime

# View system info
uname -a

# Check CPU info
lscpu

# Check memory usage
free -h

# Monitor real-time system stats
top

# View system logs
journalctl -xe

# Check disk I/O
iostat

# Monitor network traffic
iftop
nethogs

Security Best Practices¶

Practice	Command	Purpose
SSH Key Auth	`ssh-keygen -t rsa -b 4096`	Secure authentication
Disable Root Login	`PermitRootLogin no` in sshd_config	Reduce attack surface
Fail2Ban	`sudo apt install fail2ban`	Prevent brute force
SELinux/AppArmor	Enable security module	Mandatory access control
Regular Updates	`sudo apt update && apt upgrade`	Security patches
Audit Logging	`auditctl -w /etc/passwd`	Monitor changes

Cron Jobs¶

Schedule Automated Tasks¶

# Edit crontab
crontab -e

# List user's cron jobs
crontab -l

# Remove all cron jobs
crontab -r

# Cron syntax
# MIN HOUR DAY MONTH WEEKDAY COMMAND
# 0   0    1   1     *       /backup.sh

Common Cron Examples¶

Schedule	Cron Expression	Purpose
Every minute	`* * * * * command`	Frequent checks
Every hour	`0 * * * * command`	Hourly tasks
Daily at 2 AM	`0 2 * * * command`	Daily backup
Weekly (Sunday)	`0 0 * * 0 command`	Weekly maintenance
Monthly (1st)	`0 0 1 * * command`	Monthly report

Advanced Topics¶

System Tuning¶

# Display kernel parameters
sysctl -a

# Set kernel parameter
sudo sysctl -w net.ipv4.ip_forward=1

# Make persistent
echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf
sysctl -p

Log Management¶

# View system logs
journalctl

# Follow logs in real-time
journalctl -f

# Filter by priority
journalctl -p err

# Export logs
journalctl --since "2 days ago" > export.log

Systemd Services¶

# Create custom service
sudo nano /etc/systemd/system/myapp.service

# Reload systemd
sudo systemctl daemon-reload

# Enable service
sudo systemctl enable myapp

# Start service
sudo systemctl start myapp

Summary Table: Essential Linux Skills¶

Skill	Importance	Tools
File Navigation	Critical	cd, ls, pwd
User Management	Critical	useradd, usermod, passwd
Package Management	Critical	apt, yum
Service Management	High	systemctl, service
Network Config	High	ip, ifconfig, netstat
Process Management	High	ps, kill, top
Shell Scripting	High	bash, sh
Log Analysis	Medium	grep, tail, journalctl
Security	High	ssh, chmod, firewall

Linux¶

Overview¶

Core Linux Concepts¶

File System Hierarchy¶

File Permissions¶

Essential Linux Commands¶

User & Group Management¶

User Management¶

Group Management¶

System Administration¶

Service Management¶

Package Management¶

Debian/Ubuntu (apt)¶

Red Hat/CentOS (yum/dnf)¶

Process Management¶

Network Administration¶

Network Configuration¶

Firewall Management¶

File and Text Processing¶

Text Editing¶

Text Processing¶

Permissions & Ownership¶

Permission Examples¶

Shell Scripting Basics¶

Simple Script¶

Script Execution¶

System Monitoring¶

Security Best Practices¶

Cron Jobs¶

Schedule Automated Tasks¶

Common Cron Examples¶

Advanced Topics¶

System Tuning¶

Log Management¶

Systemd Services¶

Summary Table: Essential Linux Skills¶

Resources¶