Linux Fundamentals¶

🎯 Learning Objectives

Master essential Linux commands
Understand file system and permissions
Learn process and system management
Know networking and troubleshooting commands

Linux is the foundation of DevOps. This chapter covers essential Linux commands and concepts you'll need for DevOps interviews and daily work.

Interview Focus

Interviewers often ask you to explain commands, not just list them. Understand what each command does and when to use it.

Essential Commands¶

File Operations¶

# List files with details
ls -lah

# Create directory
mkdir -p /path/to/directory

# Copy files
cp -r source/ destination/

# Move/rename files
mv oldname newname

# Remove files
rm -rf directory/

# Find files
find /path -name "*.log" -type f

# Search in files
grep -r "pattern" /path/to/search

Command Flags

-r: Recursive (for directories)
-f: Force (no confirmation)
-h: Human-readable sizes
-a: All files (including hidden)
-l: Long format

File Permissions¶

# View permissions
ls -l file.txt
# Output: -rw-r--r-- 1 user group 1024 Jan 1 12:00 file.txt

# Change permissions
chmod 755 script.sh    # rwxr-xr-x
chmod +x script.sh     # Add execute permission
chmod -R 644 directory/ # Recursive

# Change ownership
chown user:group file.txt
chown -R user:group directory/

Permission Numbers: - 4 = Read (r) - 2 = Write (w) - 1 = Execute (x) - 755 = rwxr-xr-x (owner: rwx, group: r-x, others: r-x)

Common Interview Question

"Explain what chmod 755 does" - Be ready to explain each digit and what permissions they grant.

Process Management¶

# List processes
ps aux
ps -ef

# Find process by name
ps aux | grep nginx

# Kill process
kill -9 PID
killall nginx

# Background/foreground
command &          # Run in background
fg                 # Bring to foreground
jobs               # List background jobs

Process States

R: Running
S: Sleeping
Z: Zombie
D: Uninterruptible sleep

System Information¶

# System info
uname -a
hostname
uptime

# Disk usage
df -h
du -sh /path
du -h --max-depth=1

# Memory usage
free -h
cat /proc/meminfo

# CPU info
lscpu
cat /proc/cpuinfo

# Load average
top
htop

File System¶

Important Directories¶

/bin          # Essential binaries
/etc          # Configuration files
/var          # Variable data (logs, etc.)
/tmp          # Temporary files
/home         # User home directories
/usr          # User programs
/opt          # Optional software
/root         # Root user home

Interview Question

"Where are log files typically stored?" - Answer: /var/log/

Working with Files¶

# View file content
cat file.txt          # Entire file
head -n 20 file.txt   # First 20 lines
tail -n 20 file.txt   # Last 20 lines
tail -f file.txt      # Follow (watch) file

# Edit files
nano file.txt
vim file.txt

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

Networking¶

Network Commands¶

# Network configuration
ifconfig
ip addr show
ip route show

# Test connectivity
ping google.com
ping -c 4 google.com

# DNS lookup
nslookup domain.com
dig domain.com
host domain.com

# Network connections
netstat -tulpn
ss -tulpn

# Check ports
netstat -an | grep LISTEN
lsof -i :8080

Interview Tip

Know the difference between netstat and ss. ss is faster and more modern.

Firewall¶

# UFW (Ubuntu)
sudo ufw status
sudo ufw allow 22/tcp
sudo ufw enable

# firewalld (RHEL/CentOS)
sudo firewall-cmd --list-all
sudo firewall-cmd --add-port=8080/tcp --permanent
sudo firewall-cmd --reload

Text Processing¶

sed and awk¶

# sed - stream editor
sed 's/old/new/g' file.txt
sed -i 's/old/new/g' file.txt  # In-place
sed -n '10,20p' file.txt        # Lines 10-20

# awk - pattern processing
awk '{print $1}' file.txt       # First column
awk -F: '{print $1}' /etc/passwd # Using : as delimiter
awk '/pattern/ {print $0}' file.txt

Common Use Case

Extract specific columns from log files or CSV files using awk.

Other Text Tools¶

# Sort
sort file.txt
sort -r file.txt        # Reverse
sort -n file.txt        # Numeric

# Unique lines
sort file.txt | uniq
sort file.txt | uniq -c  # Count occurrences

# Cut
cut -d: -f1 /etc/passwd  # First field with : delimiter

# tr (translate)
echo "HELLO" | tr '[:upper:]' '[:lower:]'

System Services¶

systemd¶

# Service management
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx
sudo systemctl reload nginx
sudo systemctl status nginx

# Enable/disable
sudo systemctl enable nginx
sudo systemctl disable nginx

# List services
systemctl list-units --type=service
systemctl list-units --type=service --state=running

Interview Question

"What's the difference between restart and reload?" - Restart stops and starts, reload applies config without downtime.

Package Management¶

apt (Debian/Ubuntu)¶

# Update package list
sudo apt update

# Upgrade packages
sudo apt upgrade

# Install package
sudo apt install nginx

# Remove package
sudo apt remove nginx
sudo apt purge nginx

# Search packages
apt search nginx

# Show package info
apt show nginx

yum/dnf (RHEL/CentOS)¶

# Install
sudo yum install nginx
sudo dnf install nginx

# Update
sudo yum update
sudo dnf update

# Remove
sudo yum remove nginx

Compression and Archives¶

# tar
tar -czf archive.tar.gz directory/
tar -xzf archive.tar.gz
tar -tzf archive.tar.gz  # List contents

# zip
zip -r archive.zip directory/
unzip archive.zip

# gzip
gzip file.txt
gunzip file.txt.gz

Environment Variables¶

# View
echo $PATH
env
printenv

# Set (current session)
export VAR=value
export PATH=$PATH:/new/path

# Permanent (add to ~/.bashrc or ~/.bash_profile)
echo 'export VAR=value' >> ~/.bashrc
source ~/.bashrc

Comprehensive Interview Questions¶

Q1: Explain the difference between `kill`, `killall`, and `pkill`¶

Answer: - kill: Requires a Process ID (PID), sends signal to specific process

kill -9 1234  # Kill process with PID 1234

- killall: Uses process name, kills all matching processes

killall nginx  # Kill all nginx processes

- pkill: Pattern-based, more flexible matching

pkill -f "python script.py"  # Kill by pattern

When to use: - Use kill when you know the exact PID - Use killall for processes with known names - Use pkill for pattern matching

Q2: What does `grep -r` do and what are other useful grep options?¶

Answer: - -r: Recursive search in directories and subdirectories

grep -r "error" /var/log/

- Other useful options: - -i: Case-insensitive search - -n: Show line numbers - -v: Invert match (show non-matching lines) - -l: Show only filenames - -c: Count matches

grep -rin "error" /var/log/  # Case-insensitive, recursive, with line numbers

Q3: How do you find large files and directories?¶

Answer:

Find files larger than 100MB:

find / -type f -size +100M 2>/dev/null
find / -type f -size +100M -exec ls -lh {} \;

Find top 10 largest files:

find / -type f -exec du -h {} + 2>/dev/null | sort -h | tail -10

Find top 10 largest directories:

du -h /path | sort -h | tail -10
du -sh /path/* | sort -h | tail -10

Using ncdu (interactive):

ncdu /path

Q4: Explain file permissions 755 in detail¶

Answer: - First digit (7): Owner permissions - 4 (read) + 2 (write) + 1 (execute) = 7 - Owner can read, write, and execute - Second digit (5): Group permissions - 4 (read) + 1 (execute) = 5 - Group can read and execute, but not write - Third digit (5): Others permissions - 4 (read) + 1 (execute) = 5 - Others can read and execute, but not write

Common permission patterns: - 755: Executable files (scripts, binaries) - 644: Regular files (readable by all, writable by owner) - 600: Private files (readable/writable by owner only) - 777: Full permissions (not recommended for security)

Q5: What is the difference between `restart` and `reload` for services?¶

Answer: - restart: Stops the service completely, then starts it again - Causes brief downtime - All connections are dropped - Use when configuration changes require full restart

systemctl restart nginx

- reload: Applies new configuration without stopping the service - No downtime - Existing connections are maintained - Use when service supports graceful reload

systemctl reload nginx

When to use: - Use reload when possible to avoid service interruption - Use restart if service doesn't support reload or after major changes

Q6: Explain the difference between `ps aux` and `ps -ef`¶

Answer: - ps aux: BSD-style syntax - Shows: USER, PID, %CPU, %MEM, VSZ, RSS, TTY, STAT, START, TIME, COMMAND - More detailed memory and CPU information - ps -ef: Unix-style syntax - Shows: UID, PID, PPID, C, STIME, TTY, TIME, CMD - Shows parent process ID (PPID) - More compact output

When to use: - Use ps aux for detailed resource usage - Use ps -ef to see process hierarchy (parent-child relationships)

Q7: How do you monitor system resources in real-time?¶

Answer:

Using top:

top  # Interactive process viewer

Using htop (more user-friendly):

htop  # Enhanced version of top

Using vmstat:

vmstat 1  # Update every 1 second

Using iostat (for I/O):

iostat -x 1  # Extended I/O statistics

Using free (for memory):

free -h  # Human-readable memory info
free -h -s 1  # Update every second

Q8: How do you find and kill a process using a specific port?¶

Answer:

Find process using port:

# Method 1: Using lsof
lsof -i :8080

# Method 2: Using netstat
netstat -tulpn | grep :8080

# Method 3: Using ss (modern)
ss -tulpn | grep :8080

Kill process using port:

# Get PID and kill
lsof -ti :8080 | xargs kill -9

# Or in one command
kill -9 $(lsof -ti :8080)

Q9: Explain the difference between `>` and `>>` in redirection¶

Answer: - >: Overwrites the file (truncates if exists)

echo "Hello" > file.txt  # Creates or overwrites file.txt

- >>: Appends to the file (creates if doesn't exist)

echo "World" >> file.txt  # Appends to file.txt

Examples:

# Overwrite
ls > files.txt

# Append
ls >> files.txt

# Redirect both stdout and stderr
command > output.txt 2>&1
command &> output.txt  # Shorthand

Q10: How do you check disk space and inode usage?¶

Answer:

Check disk space:

df -h  # Human-readable format
df -h /  # Specific filesystem

Check inode usage:

df -i  # Show inode usage
df -ih /  # Human-readable inode usage

Find directories using most space:

du -h --max-depth=1 / | sort -h
du -sh /path/* | sort -h | tail -10

Q11: What is the difference between `su` and `sudo`?¶

Answer: - su: Switch user (requires target user's password)

su - username  # Switch to username (needs their password)
su -  # Switch to root (needs root password)

- sudo: Execute command as another user (uses your password)

sudo command  # Execute as root (uses your password)
sudo -u username command  # Execute as specific user

Best practices: - Use sudo instead of su for better audit trails - Configure sudoers file for granular permissions - Never share root password, use sudo instead

Q12: How do you find files modified in the last 24 hours?¶

Answer:

# Files modified in last 24 hours
find /path -type f -mtime -1

# Files modified in last 7 days
find /path -type f -mtime -7

# Files modified in last hour (using -mmin)
find /path -type f -mmin -60

Time modifiers: - -mtime -1: Modified less than 1 day ago - -mtime +7: Modified more than 7 days ago - -mmin -60: Modified less than 60 minutes ago

Q13: Explain the difference between `systemctl` and `service` commands¶

Answer: - systemctl: Modern systemd command (preferred)

systemctl start nginx
systemctl status nginx
systemctl enable nginx

- service: Legacy SysV init command (still works for compatibility)

service nginx start
service nginx status

When to use: - Always use systemctl on systemd-based systems (most modern Linux) - service is a wrapper that calls systemctl on systemd systems - Use systemctl for more features and better control

Practice Exercises¶

Find all .log files older than 7 days and delete them

find /var/log -name "*.log" -mtime +7 -delete

Count number of processes running as root
```
ps aux | grep "^root" | wc -l
```

Find top 10 largest files

find / -type f -exec du -h {} + 2>/dev/null | sort -h | tail -10

Monitor disk I/O
```
iostat -x 1
```

Next Steps¶

Master these commands through practice. Next, we'll learn shell scripting to automate these operations.

Key Takeaways: - Master file operations, permissions, and process management - Understand system information commands - Know networking and troubleshooting tools - Practice explaining commands, not just memorizing - Combine commands with pipes (|) for powerful operations

Previous: Introduction | Next: Shell Scripting

Linux Fundamentals¶

🎯 Learning Objectives

Essential Commands¶

File Operations¶

File Permissions¶

Process Management¶

System Information¶

File System¶

Important Directories¶

Working with Files¶

Networking¶

Network Commands¶

Firewall¶

Text Processing¶

sed and awk¶

Other Text Tools¶

System Services¶

systemd¶

Package Management¶

apt (Debian/Ubuntu)¶

yum/dnf (RHEL/CentOS)¶

Compression and Archives¶

Environment Variables¶

Comprehensive Interview Questions¶

Q1: Explain the difference between kill, killall, and pkill¶

Q2: What does grep -r do and what are other useful grep options?¶

Q3: How do you find large files and directories?¶

Q4: Explain file permissions 755 in detail¶

Q5: What is the difference between restart and reload for services?¶

Q6: Explain the difference between ps aux and ps -ef¶

Q7: How do you monitor system resources in real-time?¶

Q8: How do you find and kill a process using a specific port?¶

Q9: Explain the difference between > and >> in redirection¶

Q10: How do you check disk space and inode usage?¶

Q11: What is the difference between su and sudo?¶

Q12: How do you find files modified in the last 24 hours?¶

Q13: Explain the difference between systemctl and service commands¶

Practice Exercises¶

Next Steps¶

Q1: Explain the difference between `kill`, `killall`, and `pkill`¶

Q2: What does `grep -r` do and what are other useful grep options?¶

Q5: What is the difference between `restart` and `reload` for services?¶

Q6: Explain the difference between `ps aux` and `ps -ef`¶

Q9: Explain the difference between `>` and `>>` in redirection¶

Q11: What is the difference between `su` and `sudo`?¶

Q13: Explain the difference between `systemctl` and `service` commands¶