Linux Command Line Basics

# Basic SSH connection (replace with your server's IP)
ssh root@YOUR_SERVER_IP

# Connect with a specific SSH key
ssh -i ~/.ssh/my_key root@YOUR_SERVER_IP

# Connect on a non-standard port
ssh -p 2222 root@YOUR_SERVER_IP

# Print your current directory
pwd

# List files in the current directory
ls

# List with details (permissions, size, date)
ls -la

# List with human-readable file sizes
ls -lah

# Change directory
cd /var/www              # Go to an absolute path
cd html                  # Go to a relative subdirectory
cd ..                    # Go up one level
cd ~                     # Go to your home directory
cd -                     # Go back to the previous directory

# Create an empty file
touch myfile.txt

# Create a directory
mkdir mydir

# Create nested directories (parents included)
mkdir -p /var/www/mysite/html

# Copy a file
cp file.txt /tmp/file.txt

# Copy a directory recursively
cp -r /var/www/mysite /var/www/mysite-backup

# Move or rename a file
mv oldname.txt newname.txt

# Move a file to another directory
mv file.txt /tmp/

# Remove a file
rm file.txt

# Remove a directory and all its contents
rm -r mydir

# Remove without confirmation prompts (use with caution!)
rm -rf mydir

# Display entire file contents
cat /etc/hostname

# View a long file page by page (press q to quit)
less /var/log/syslog

# Show the first 20 lines
head -n 20 /var/log/syslog

# Show the last 20 lines
tail -n 20 /var/log/syslog

# Follow a log file in real time (Ctrl+C to stop)
tail -f /var/log/syslog

# Open a file for editing
nano /etc/ssh/sshd_config

# Keyboard shortcuts inside nano:
# Ctrl+O    Save the file
# Ctrl+X    Exit nano
# Ctrl+W    Search for text
# Ctrl+K    Cut the current line
# Ctrl+U    Paste the cut line

-rw-r--r-- 1 www-data www-data  1234 Jun 15 10:30 index.html
drwxr-xr-x 2 root     root      4096 Jun 15 10:30 config/

# Change file permissions using numeric mode
chmod 644 file.txt        # Owner: rw, Group: r, Others: r
chmod 755 script.sh       # Owner: rwx, Group: rx, Others: rx
chmod 600 secrets.key     # Owner: rw, no access for others

# Change file owner
chown deploy:deploy file.txt

# Change owner recursively for a directory
chown -R www-data:www-data /var/www/mysite

# Change only the group
chgrp www-data file.txt

# See who you're logged in as
whoami

# Switch to another user
su - deploy

# Run a single command as root
sudo systemctl restart nginx

# Open a root shell (use sparingly)
sudo -i

# Create a new user
sudo adduser newuser

# Add a user to the sudo group (Ubuntu/Debian)
sudo usermod -aG sudo newuser

# Add a user to the wheel group (AlmaLinux/Rocky)
sudo usermod -aG wheel newuser

# Update the package index
sudo apt update

# Upgrade all installed packages
sudo apt upgrade -y

# Install a package
sudo apt install nginx -y

# Remove a package
sudo apt remove nginx

# Remove a package and its config files
sudo apt purge nginx

# Search for a package
apt search redis

# Show package details
apt show nginx

# Update all packages
sudo dnf update -y

# Install a package
sudo dnf install nginx -y

# Remove a package
sudo dnf remove nginx

# Search for a package
dnf search redis

# Show package details
dnf info nginx

# List installed packages
dnf list installed

# Check if a service is running
sudo systemctl status nginx

# Start a service
sudo systemctl start nginx

# Stop a service
sudo systemctl stop nginx

# Restart a service (stop + start)
sudo systemctl restart nginx

# Reload config without stopping (if supported)
sudo systemctl reload nginx

# Enable a service to start on boot
sudo systemctl enable nginx

# Disable a service from starting on boot
sudo systemctl disable nginx

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

# Show running processes (snapshot)
ps aux

# Interactive process monitor (press q to quit)
top

# Better alternative to top (install with: apt install htop)
htop

# Show memory usage
free -h

# Show disk usage for all mounted filesystems
df -h

# Show disk usage of the current directory
du -sh *

# Show system uptime and load average
uptime

# Find a process by name
ps aux | grep nginx

# Kill a process by PID
kill 1234

# Force-kill an unresponsive process
kill -9 1234

# Kill all processes matching a name
killall nginx

# Show your server's IP addresses
ip addr show

# Test connectivity to a host
ping -c 4 google.com

# Check which ports are listening
ss -tulnp

# Download a file from the web
wget https://example.com/file.tar.gz

# Download with curl
curl -O https://example.com/file.tar.gz

# Make an HTTP request and show headers
curl -I https://example.com

# DNS lookup
dig example.com

# Trace network route
traceroute example.com

# Pipe: search for "error" in a log file
cat /var/log/syslog | grep "error"

# Simpler way to do the same thing
grep "error" /var/log/syslog

# Count the number of matching lines
grep -c "error" /var/log/syslog

# Chain multiple commands
ps aux | grep nginx | grep -v grep

# Redirect output to a file (overwrites)
echo "Hello" > output.txt

# Append output to a file
echo "World" >> output.txt

# Redirect errors to a file
command 2> errors.log

# Redirect both output and errors
command > output.log 2>&1

# Find files by name
find /var/www -name "*.php"

# Find files modified in the last 24 hours
find /var/log -mtime -1

# Find files larger than 100MB
find / -size +100M

# Search inside files for a pattern
grep -r "database" /etc/nginx/

# Search inside files (case-insensitive)
grep -ri "error" /var/log/

# Find which package provides a command
# Ubuntu/Debian:
dpkg -S /usr/bin/curl
# AlmaLinux/Rocky:
rpm -qf /usr/bin/curl

# Find where a command is located
which nginx
whereis nginx

# Create a compressed archive (.tar.gz)
tar -czf backup.tar.gz /var/www/mysite

# Extract a .tar.gz archive
tar -xzf backup.tar.gz

# Extract to a specific directory
tar -xzf backup.tar.gz -C /tmp/

# List contents of an archive without extracting
tar -tzf backup.tar.gz

# Create a zip file
zip -r backup.zip /var/www/mysite

# Extract a zip file
unzip backup.zip