Linux Documentation

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# View file contents
cat filename
less filename        # View file with pagination
head filename        # View first 10 lines
tail filename        # View last 10 lines
tail -f filename     # Follow file as it grows

# Create empty file
touch filename

# View file type
file filename

# Find files
find /path -name filename
find /path -type f -name "*.txt"    # Find all .txt files

# Search within files
grep "pattern" filename
grep -r "pattern" /path/to/directory    # Recursive search

# Count lines, words, characters
wc filename

# Sort file contents
sort filename

# Remove duplicate lines
uniq filename

# Compare files
diff file1 file2
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# View file permissions
ls -l filename

# Change file permissions
chmod permissions filename

# Symbolic notation
chmod u+x filename        # Add execute permission for user
chmod g-w filename        # Remove write permission for group
chmod o=r filename        # Set read-only for others
chmod a+r filename        # Add read permission for all

# Octal notation
chmod 755 filename        # rwxr-xr-x
chmod 644 filename        # rw-r--r--
chmod 600 filename        # rw-------

# Change file ownership
chown user:group filename
chown user filename       # Change only owner
chown :group filename     # Change only group

# Change group ownership
chgrp group filename
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Tip Permission numbers: 4=read, 2=write, 1=execute. Add them up for combinations (e.g., 7=4+2+1=rwx).

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# Create hard link
ln source_file link_name

# Create symbolic link
ln -s source_file link_name

# View link information
ls -l link_name

# Find broken symbolic links
find /path -type l -exec test ! -e {} \; -print
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Note Hard links point to the same inode on disk, while symbolic links point to the path of another file.

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# Create new group
groupadd groupname

# Modify group
groupmod -n newname oldname    # Change group name

# Delete group
groupdel groupname

# Add user to group
usermod -aG groupname username    # Add to additional group
gpasswd -a username groupname     # Alternative method

# Remove user from group
gpasswd -d username groupname

# List all groups
cat /etc/group

# View groups for a user
groups username
id username
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# Switch to root user
su -

# Execute command as root
sudo command

# Edit sudoers file (use visudo, not direct editing)
visudo

# Common sudoers entries
username ALL=(ALL) ALL                    # Full sudo access
%groupname ALL=(ALL) ALL                  # Full sudo access for group
username ALL=(ALL) NOPASSWD: /usr/bin/apt  # No password for specific command

# View sudo privileges
sudo -l
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Tip Always use visudo to edit the sudoers file, as it validates syntax before saving.

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# Set special permissions
chmod 4755 filename    # Set SUID (Set User ID)
chmod 2755 filename    # Set SGID (Set Group ID)
chmod 1755 directory   # Set sticky bit

# View special permissions
ls -l filename

# Set default ACL (Access Control List)
setfacl -m u:username:rw filename
setfacl -m g:groupname:rw filename
setfacl -m o::r filename

# View ACLs
getfacl filename

# Remove ACL
setfacl -b filename    # Remove all ACLs
setfacl -x u:username filename    # Remove specific ACL
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Note SUID allows a program to run with the permissions of its owner, SGID with its group, and sticky bit prevents users from deleting files they don't own in a shared directory.

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# Search for packages
apt search keyword

# Show package details
apt show package_name

# List installed packages
apt list --installed

# List upgradable packages
apt list --upgradable

# Check which package provides a file
apt-file search filename

# Show files installed by a package
dpkg -L package_name

# Check if a package is installed
dpkg -l | grep package_name
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# Add repository
sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu/ focal main"

# Add PPA (Personal Package Archive)
sudo add-apt-repository ppa:user/ppa-name

# Remove repository
sudo add-apt-repository --remove "deb http://archive.ubuntu.com/ubuntu/ focal main"

# Add repository key
wget -qO - https://example.com/archive.key | sudo apt-key add -

# List repositories
apt-cache policy

# Edit sources file
sudo nano /etc/apt/sources.list
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# Fix broken dependencies
sudo apt install -f

# Reconfigure packages
sudo dpkg --configure -a

# Clear package cache
sudo apt-get clean
sudo apt-get autoclean

# Fix held packages
sudo apt-mark hold package_name
sudo apt-mark unhold package_name

# Check for broken packages
sudo apt-get check
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# Run process in background
command &

# Bring background process to foreground
fg

# List background jobs
jobs

# Send job to background
bg %job_number

# Terminate process
kill PID

# Forcefully terminate process
kill -9 PID

# Kill process by name
pkill process_name

# Kill all processes owned by user
pkill -u username

# Suspend process
kill -STOP PID

# Resume suspended process
kill -CONT PID
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# View process priority
ps -o pid,ni,comm

# Change process priority (nice value)
renice priority PID

# Run command with specific priority
nice -n priority command

# View I/O priority
ionice -p PID

# Change I/O priority
ionice -c class -n priority PID
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Note Nice values range from -20 (highest priority) to 19 (lowest priority). Only root can set negative nice values.

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

# Check service status
systemctl status service_name

# Start service
sudo systemctl start service_name

# Stop service
sudo systemctl stop service_name

# Restart service
sudo systemctl restart service_name

# Reload service configuration
sudo systemctl reload service_name

# Enable service to start at boot
sudo systemctl enable service_name

# Disable service from starting at boot
sudo systemctl disable service_name

# View service logs
journalctl -u service_name

# Follow service logs in real-time
journalctl -u service_name -f
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# View system logs (systemd systems)
journalctl

# View logs for specific service
journalctl -u service_name

# View logs from specific time
journalctl --since "2023-01-01" --until "2023-01-02"

# Follow logs in real-time
journalctl -f

# View kernel messages
dmesg

# View authentication logs
sudo cat /var/log/auth.log

# View system logs (traditional systems)
cat /var/log/syslog
tail -f /var/log/syslog

# View application logs
tail -f /var/log/application.log
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# View system performance statistics
vmstat 1

# View I/O statistics
iostat 1

# View network statistics
sar -n DEV 1

# View process tree with resource usage
pstree -p

# View detailed process information
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem

# Find processes using high CPU
ps aux --sort=-%cpu | head

# Find processes using high memory
ps aux --sort=-%mem | head

# Monitor system activity
atop
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# View logrotate configuration
cat /etc/logrotate.conf

# View specific logrotate configuration
cat /etc/logrotate.d/application

# Force log rotation
sudo logrotate -f /etc/logrotate.conf

# Test logrotate configuration
sudo logrotate -d /etc/logrotate.conf

# View logrotate status
cat /var/lib/logrotate/status
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# Test network connectivity
ping hostname_or_ip

# Trace route to host
traceroute hostname_or_ip

# DNS lookup
nslookup hostname
dig hostname    # More detailed

# View network connections
netstat -tuln
ss -tuln    # Modern alternative

# View active connections
netstat -tupan
ss -tupan    # Modern alternative

# Transfer files over network
scp file user@host:/path/to/destination
rsync -avz source/ user@host:/path/to/destination

# Secure shell
ssh user@host

# Download files from web
wget url
curl -O url
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# View firewall status (ufw)
sudo ufw status

# Enable firewall
sudo ufw enable

# Disable firewall
sudo ufw disable

# Allow incoming connections
sudo ufw allow 22/tcp    # SSH
sudo ufw allow 80/tcp    # HTTP
sudo ufw allow 443/tcp   # HTTPS

# Deny incoming connections
sudo ufw deny 23/tcp    # Telnet

# View firewall rules (iptables)
sudo iptables -L

# Add iptables rule
sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Save iptables rules
sudo iptables-save > /etc/iptables/rules.v4
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# Check if port is open
telnet hostname port
nc -zv hostname port    # Modern alternative

# View network interface statistics
cat /proc/net/dev

# View ARP table
ip neigh show
arp -a    # Legacy command

# View network sockets
ss -tuln

# Capture network traffic
sudo tcpdump -i eth0

# Monitor network bandwidth
iftop
nethogs    # Show bandwidth by process
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#!/bin/bash

# Command line arguments
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of arguments: $#"

# Check if arguments provided
if [ $# -eq 0 ]; then
    echo "No arguments provided"
    exit 1
fi

# Process all arguments
for arg in "$@"; do
    echo "Processing: $arg"
done

# Read user input
echo "Enter your name:"
read name
echo "Hello, $name!"

# Read with prompt
read -p "Enter your age: " age
echo "You are $age years old"
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#!/bin/bash

# Define function
function greet() {
    local name=$1
    echo "Hello, $name!"
}

# Alternative function definition
say_goodbye() {
    local name=$1
    echo "Goodbye, $name!"
}

# Call functions
greet "John"
say_goodbye "Jane"

# Function with return value
function add() {
    local num1=$1
    local num2=$2
    local result=$((num1 + num2))
    echo $result
}

# Use function return value
sum=$(add 5 3)
echo "5 + 3 = $sum"

# Function with default parameters
function greet_with_default() {
    local name=${1:-"Guest"}
    echo "Hello, $name!"
}

greet_with_default    # Uses default value
greet_with_default "Alice"    # Uses provided value
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#!/bin/bash

# Arrays
fruits=("Apple" "Banana" "Orange")
echo "First fruit: ${fruits[0]}"
echo "All fruits: ${fruits[@]}"

# Add to array
fruits+=("Grape")

# Loop through array
for fruit in "${fruits[@]}"; do
    echo "Fruit: $fruit"
done

# File operations
if [ -f "myfile.txt" ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

# Read file line by line
while IFS= read -r line; do
    echo "Line: $line"
done < myfile.txt

# Error handling
set -e    # Exit on error
set -u    # Exit on undefined variable

# Trap signals
trap 'echo "Script interrupted"; exit 1' INT

# Long-running task with progress bar
for i in {1..100}; do
    sleep 0.1
    printf "\rProgress: [%-50s] %d%%" "$(printf '=%.0s' $(seq 1 $((i/2))))" "$i"
done
echo
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# Update system packages
sudo apt update && sudo apt upgrade

# Remove unnecessary packages
sudo apt autoremove

# Disable unused services
sudo systemctl disable service_name

# View open ports
sudo netstat -tuln
sudo ss -tuln

# Check for listening services
sudo systemctl list-units --type=service --state=running

# Secure SSH
sudo nano /etc/ssh/sshd_config

# Recommended SSH settings:
# PermitRootLogin no
# PasswordAuthentication no
# PubkeyAuthentication yes
# Port 2222    # Change default port

# Restart SSH after changes
sudo systemctl restart sshd

# Enable automatic security updates
sudo apt install unattended-upgrades
sudo dpkg-reconfigure unattended-upgrades
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# Install intrusion detection system
sudo apt install fail2ban

# Configure fail2ban
sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local
sudo nano /etc/fail2ban/jail.local

# Enable fail2ban
sudo systemctl enable fail2ban
sudo systemctl start fail2ban

# Check fail2ban status
sudo fail2ban-client status

# View banned IPs
sudo fail2ban-client status sshd

# Install rootkit detector
sudo apt install rkhunter chkrootkit

# Run rootkit scan
sudo rkhunter --check
sudo chkrootkit

# View system audit logs
sudo ausearch -m AVC -ts recent
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# Check file permissions
ls -la filename

# Find files with world-writable permissions
find / -type f -perm -002 -ls 2>/dev/null

# Find files with SUID bit set
find / -type f -perm -4000 -ls 2>/dev/null

# Find files with SGID bit set
find / -type f -perm -2000 -ls 2>/dev/null

# Calculate file checksum
md5sum filename
sha256sum filename

# Verify file integrity
md5sum -c checksums.md5

# Encrypt file with GPG
gpg -c filename

# Decrypt file with GPG
gpg filename.gpg

# Create encrypted archive
tar -czf - directory/ | gpg -c > archive.tar.gz.gpg
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# Create backup with tar
tar -czf backup-$(date +%Y%m%d).tar.gz /path/to/backup

# Create backup with exclusion
tar -czf backup-$(date +%Y%m%d).tar.gz --exclude=/path/to/exclude /path/to/backup

# Extract backup
tar -xzf backup-20230101.tar.gz

# Create backup with rsync
rsync -avz /source/ /destination/

# Create incremental backup with rsync
rsync -avz --delete /source/ /destination/

# Schedule backup with cron
crontab -e

# Add to crontab for daily backup at 2 AM
0 2 * * * /path/to/backup-script.sh

# Backup MySQL database
mysqldump -u username -p database_name > backup.sql

# Restore MySQL database
mysql -u username -p database_name < backup.sql
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# View system resource usage
top
htop

# View memory usage
free -h

# View disk I/O
iotop

# View network connections
netstat -tuln

# Optimize system parameters
sudo nano /etc/sysctl.conf

# Add to sysctl.conf for performance tuning:
# vm.swappiness=10
# net.core.rmem_max=16777216
# net.core.wmem_max=16777216
# net.ipv4.tcp_rmem=4096 87380 16777216
# net.ipv4.tcp_wmem=4096 65536 16777216

# Apply sysctl changes
sudo sysctl -p

# Monitor system performance
sar -u 1 10    # CPU usage every second for 10 seconds
sar -r 1 10    # Memory usage every second for 10 seconds
sar -n DEV 1 10    # Network usage every second for 10 seconds
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# View system logs
journalctl -f

# View kernel messages
dmesg

# Check disk space
df -h

# Check disk usage
du -sh /path/to/directory

# Find large files
find / -type f -size +100M 2>/dev/null

# Check for zombie processes
ps aux | grep Z

# Check for high CPU usage
ps aux --sort=-%cpu | head

# Check for high memory usage
ps aux --sort=-%mem | head

# Check network connectivity
ping hostname

# Check port availability
telnet hostname port
nc -zv hostname port

# Check service status
systemctl status service_name

# Restart service if needed
sudo systemctl restart service_name
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