man ls  # List manual

ls    # List directory
ls -l # Shows more details
ls -a # Shows hidden 
ls -h # Shows human readable info

cd        # Change directory
cd ..     # Move to parent directory
cd ~      # Move to home
cd <tab>  # View suggested inputs
pwd       # Print working directory full path

tree        # Show all files and folders in the current directory
tree /path  # Show the structure of a specific directory.
tree -L 2   # Limit the depth of displayed subdirectories to 2 levels.
tree -d     # Show only directories
tree -f     # Show the full path 

touch file.txt  # Create file
mkdir dir       # Make directory
rm file.txt     # Remove file
rmdir           # Remove directory
cp test.txt ~/Downloads # Copy file
mv test.txt ~/Downloads # Move file/rename file

cat -n file.txt   # Concatenate and display file content.

which # Locate a command.
which command # Find the path of an executable
which ls # Check if a command is available in the system

history # Show the list of recent commands
history | grep "pattern" # Search for commands containing specific text

alias # Create shortcuts for commands.
alias ll='ls -l' # Create an alias for a command
alias # List all defined aliases

grep: Search for patterns in files.

grep "word" file.txt          # Find word in a file
grep -r "pattern" /path/to/   # Search recursively in directories for a specific pattern
grep -c "pattern" file1 file2 # Count occurrences of a pattern in multiple files
grep -v "pattern" file.txt    # Display lines that do not match a pattern

tty # Show the current terminal name.
who # List logged-in users and their terminals.
echo "Hello" > /dev/tty1 # Send text to a specific terminal.

# Change mode

chmod 755 file.txt  # rwxr-xr-x (Owner: read/write/execute, Group & Others: read/execute)
chmod +x script.sh  # Allow everyone to execute (same as above)
chmod u+x file.sh   # Add execute permission to owner
chmod g-w file.txt  # Remove write permission for group
chmod o= file.txt   # Remove all permissions for others (anyone who is NOT the part of the group assigned to the file)

# Change owner

chown nut17 file.txt          # Change owner to nut17
chown nut17:groupname file.txt # Change owner to a group
sudo chown -R nut17 /var/www    # Recursively change owner of all files in directory

# User File Creation Mask

umask 022  # Default permissions: rw-r--r-- for files, rwxr-xr-x for directories
umask 077  # Private files and directories: rw-------, rwx------
umask      # Display current umask

ps              # Display currently running processes.
ps aux          # List all processes
ps -u username  # Show processes of a specific user
pgrep pname     # Find a specific process by name

kill PID        # Terminate processes by ID or name
kill -9 PID     # Force kill

<command> &     # '&' sends to the background

bg              # Resume a stopped process in the background
fg              # Resume a stopped process in the foreground

nice PID        # Affects priority (-20 highest, 19 lowest)
renice -10 PID  # Increase priority

#!/bin/bash

# Variable & Output
name="Alice"
echo "Hello, $name!"

# Input
read -p "Enter your name: " user
echo "Welcome, $user!"

export MY_VAR="Hello"  # Creates a global variable

# Operators
num=10
if [ $num -gt 5 ]; then
  echo "Number is greater than 5"
else
  echo "Number is 5 or less"
fi

# -eq (equal), -ne (not equal), -gt (greater), -lt (less)
# -f file (file exists), -d dir (directory exists)

# Case
read -p "Enter choice (start/stop): " action
case $action in
  start) echo "Starting..." ;;
  stop) echo "Stopping..." ;;
  *) echo "Invalid option" ;;
esac

# For Loop
for i in {1..5}; do
  echo "Iteration $i"
done

# While Loop
count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

# Always Loop
x=1
until [ $x -gt 3 ]; do
  echo "Value: $x"
  ((x++))
done

# IO Redirection

echo "Hello" > file.txt  # Write to file
echo "More text" >> file.txt  # Append to file
cat < file.txt  # Read from file

# Piping (chain commands by passing output as input)

ls -l | grep "txt"  # Find only .txt files
cat file.txt | sort | uniq  # Sort and remove duplicates

# > → Redirect output (overwrite)
# >> → Append output
# < → Input from file
# 2> → Redirect errors
# &> → Redirect both stdout & stderr

chmod +x script.sh  # Give execute permission
./script.sh  # Run script

tar -czvf archive.tar.gz directory  # Create a compressed archive
tar -xzvf archive.tar.gz -C /path/to/destination # Extract an archive to a specific directory

gzip # Compress files.
gzip file.txt # Compress a file
gzip file1 file2 # Compress multiple files
gzip *.txt # Compress multiple files using wildcards

gunzip *.gz # Decompress multiple files with a wildcard

find / -mtime -1 # Find files modified within the last 24 hours
find / -user username # Search for files owned by a specific user

lsof # List open files and processes.
lsof /path/to/file # List processes using a specific file
lsof -i # Show network-related processes

tail # Display the last part of a file.
tail file.txt # View the last 10 lines of a file
tail -f log.txt # Continuously display new lines appended to a file

head # Display the beginning of a file.
head -n 15 file.txt # Display a specific number of lines from the start

wc # Display line, word, and character count for a file.
wc -l file.txt # Count lines in a file
wc -w file.txt # Count words in a file

uniq # Report or omit repeated lines in a file.
sort file.txt | uniq # Show unique lines in a sorted file
sort file.txt | uniq -c # Display count of occurrences for unique lines

sort # Sort lines of text files.
sort file.txt # Sort lines in a file
sort -n file.txt # Sort lines numerically

diff # Compare files line by line.
diff file1.txt file2.txt # Compare two files and display differences
diff -r dir1/ dir2/ # Show differences between two directories

sudo mount /dev/sdX /mnt
sudo umount /mnt

df -h                     # Show available disk space
df -T /dev/sda1           # Display space usage of a specific filesystem
du -sh /path/to/directory # Size of a directory
du -h --max-depth=1 /path # Sizes of all subdirectories

ping example.com    # Check if host is reachable
curl example.com    # Get content of a website
curl -I example.com # Check response headers
curl -X POST -d "data" URL # Send data via POST request

wget https://example.com/file.zip # Download files from the web
wget -O myfile.zip https://example.com/file.zip # Rename

ifconfig    # Display network interface information.
ifconfig -a # List all network interfaces

ss -tulnp # Show open network connections
ss -tnlp  # Check which processes are using ports

traceroute example.com          # Trace the path of network packets
nslookup domain.com # Query DNS servers. Alias: dig

ssh -p 2222 user@remote-server  # Connect to a remote server at port 2222
ssh -i /path/to/private_key user@host # Connect to a server using a specific private key
ssh -L 8080:localhost:80 user@host # Establish an SSH tunnel for port forwarding

scp file.txt user@remote-server:/path/to/destination # Copy a file to a remote server securely
scp -r mydir user@remote-server:/path/to/destination # Copy a directory recursively
scp file.txt user@remote:/path # Copy a file from local to remote

rsync -avz /local/path user@remote-server:/remote/path # Sync a local directory with a remote one
rsync -av --delete /local/path user@remote-server:/remote/path # Delete extra files at the destination

sudo ifconfig eth0 192.168.1.2 netmask 255.255.255.0 # Set an IP address for a specific interface

netstat # Display network connections
netstat -tulnp # Show all listening ports
netstat -s # Display statistics for a specific network protocol

sudo route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.0.1 # Add a new route to the routing table
sudo route del -net 192.168.1.0 netmask 255.255.255.0 # Delete a specific route

firewall-cmd/ufw # Configure firewall rules.
sudo ufw allow from 192.168.1.100 # Allow all traffic from a specific IP address
sudo ufw default deny incoming # Deny all incoming traffic by default

tcpdump # Network packet analyzer.
sudo tcpdump -i eth0 # Capture packets on a specific interface
sudo tcpdump -i eth0 port 80 # Display HTTP traffic

netcat (nc) # Network utility for reading and writing data across network connections.
nc -l -p 1234 # Create a simple TCP server
nc -l -p 1234 < file.txt # Transfer files between systems

ssh-keygen # Create SSH key pairs for secure authentication.
ssh-keygen -t rsa # Generate an RSA key pair
ssh-keygen -t rsa -f /path/to/key # Create a key pair with a specific filename

ssh-copy-id # Copy SSH keys to a remote server.
ssh-copy-id user@host # Copy the public key to a remote host
ssh-copy-id -p port user@host # Use a specific port for copying the key

printenv # Print all environment variables
echo 'export MY_VAR="Hello, Linux!"' >> ~/.bashrc # Add environment variable
export MY_VAR="Hello, Linux!" # Temporary add environment variable
source ~/.bashrc              # Reload configuration

echo "alias ll='ls -lah'" >> ~/.bashrc # Make Aliases Permanent

top             # Real-time process monitoring
htop            # Real-time process monitoring
vmstat          # Provides information about system performance, including CPU, memory, swap, disk I/O, and system processes
iostat          # Reports CPU usage and disk I/O performance.

tail -n 10 /var/log/syslog  # View Last Few Lines of a Log File
tail -f /var/log/syslog     # Follow log updates in real-time
less /var/log/syslog        # Scroll Through Large Log Files
grep "failed" /var/log/auth.log # Search for Specific Entries in Logs

# Building with CMake
mkdir build && cd build
cmake ..                # Generate Makefiles
make                    # Compile

# Create a virtual environment named "myenv"
python -m venv myenv

# Activate on Linux/macOS
source myenv/bin/activate

# Activate on Windows
myenv\Scripts\activate

deactivate

# Check available Node.js versions
nvm ls-remote 

# Install a specific version
nvm install 18

# Switch Node.js version
nvm use 18

# Set a default version
nvm alias default 18

hostname # Display or set system hostname.
sudo hostname temporary_name # Change the hostname temporarily

whoami # Display current username.
if [ "$(whoami)" != "root" ]; then echo "Please run as root"; fi

# Create a new group
sudo groupadd groupname

# Add a user to a group
sudo usermod -aG groupname username

# List a user's groups
groups username

# View all groups
cat /etc/group

name -r   # Check kernel version
uname -m  # Show system architecture
uname -s  # Show only the kernel name

date -u # Display date and time in UTC
date +"%Y-%m-%d %H:%M:%S" # Display date and time in a specific format

uptime # Show how long the system has been running.
uptime # Display system uptime
uptime -p # Show load average for the last 1, 5, and 15 minutes

systemctl # Control system services.
sudo systemctl enable servicename # Enable a service to start at boot
systemctl status ssh # Check the status of a service

journalctl # Query and display system logs.
journalctl -u servicename # Filter logs for a specific service
journalctl --since "2023-12-01 00:00:00" # Display logs since a specific time