Linux Command Line Basics

user@hostname:~$ 

# Print working directory - shows where you are
pwd

# List directory contents
ls

# List with details
ls -l

# List all files (including hidden)
ls -la

# Change directory
cd /path/to/directory

# Go to home directory
cd ~

# Go to previous directory
cd -

# Go up one directory
cd ..

# Absolute paths (start from root)
cd /home/user/documents
cd /var/log
cd /usr/bin

# Relative paths (from current location)
cd documents
cd ../pictures
cd ./scripts

# Create empty files
touch newfile.txt
touch file1.txt file2.txt file3.txt

# Create directories
mkdir new_directory
mkdir project1 project2 project3

# Create nested directories
mkdir -p projects/web/frontend
mkdir -p data/2023/january

# Copy files
cp source.txt destination.txt
cp file.txt backup/file.txt

# Copy multiple files
cp *.txt backup/
cp file1.txt file2.txt file3.txt destination/

# Copy directories
cp -r source_directory destination_directory
cp -r /path/to/source /path/to/destination

# Move files
mv file.txt new_location/
mv file.txt new_name.txt

# Move directories
mv old_directory new_directory

# Rename files (same as move)
mv old_name.txt new_name.txt

# Remove files
rm file.txt
rm file1.txt file2.txt

# Remove directories (must be empty)
rmdir empty_directory

# Remove directories and contents
rm -r directory_name
rm -rf directory_name  # Force remove without prompts

# Display entire file
cat filename.txt

# Display with line numbers
cat -n filename.txt

# View file page by page
less filename.txt
more filename.txt

# View first lines
head filename.txt
head -n 20 filename.txt  # First 20 lines

# View last lines
tail filename.txt
tail -n 10 filename.txt  # Last 10 lines

# View file in real-time (useful for logs)
tail -f logfile.txt

# Nano editor (beginner-friendly)
nano filename.txt

# Vim editor (powerful but steep learning curve)
vim filename.txt

# Emacs editor
emacs filename.txt

# Search for text in files
grep "search_term" filename.txt

# Search case-insensitive
grep -i "search_term" filename.txt

# Search recursively in directories
grep -r "search_term" /path/to/directory

# Show line numbers
grep -n "search_term" filename.txt

# Count occurrences
grep -c "search_term" filename.txt

# View file permissions
ls -l

# Example output:
# -rw-r--r-- 1 user group 1024 Jan 1 12:00 file.txt
# ^^^ ^^^ ^^^
# |   |   |
# |   |   +-- Others permissions
# |   +------ Group permissions
# +---------- Owner permissions

# Symbolic method
chmod u+x file.txt      # Add execute for owner
chmod g-w file.txt      # Remove write for group
chmod o+r file.txt      # Add read for others
chmod a+r file.txt      # Add read for all

# Numeric method
chmod 755 script.sh     # rwxr-xr-x
chmod 644 file.txt      # rw-r--r--
chmod 600 private.txt   # rw-------

# Change ownership
sudo chown user:group file.txt
sudo chown user file.txt
sudo chown :group file.txt

# System information
uname -a                # All system info
uname -r               # Kernel version
lsb_release -a         # Linux distribution info

# Hardware information
lscpu                  # CPU info
free -h                # Memory usage
df -h                  # Disk usage
lsblk                  # Block devices

# Process information
ps aux                 # All running processes
top                    # Interactive process viewer
htop                   # Better process viewer (if installed)

# Resource usage
uptime                 # System uptime and load
iostat                 # I/O statistics
vmstat                 # Virtual memory statistics

# Network information
ip addr                # IP addresses
netstat -tuln          # Network connections
ss -tuln               # Socket statistics (modern)

# Update package lists
sudo apt update

# Upgrade packages
sudo apt upgrade

# Install packages
sudo apt install package_name
sudo apt install vim git curl

# Remove packages
sudo apt remove package_name
sudo apt purge package_name  # Remove with config files

# Search for packages
apt search keyword

# Show package information
apt show package_name

# Update package lists
sudo dnf update

# Install packages
sudo dnf install package_name

# Remove packages
sudo dnf remove package_name

# Search for packages
sudo dnf search keyword

# Update system
sudo pacman -Syu

# Install packages
sudo pacman -S package_name

# Remove packages
sudo pacman -R package_name

# Search for packages
pacman -Ss keyword

# List processes
ps aux
ps -ef

# Find specific process
ps aux | grep firefox

# Kill processes
kill PID
kill -9 PID  # Force kill

# Kill by name
pkill firefox
killall firefox

# Run processes in background
command &

# Bring background process to foreground
fg

# List background jobs
jobs

# Systemd (modern Linux)
sudo systemctl status service_name
sudo systemctl start service_name
sudo systemctl stop service_name
sudo systemctl restart service_name
sudo systemctl enable service_name  # Start on boot
sudo systemctl disable service_name

# Service examples
sudo systemctl status nginx
sudo systemctl restart sshd

# Test connectivity
ping google.com
ping 8.8.8.8

# Download files
wget https://example.com/file.zip
curl -O https://example.com/file.zip

# Network configuration
ip addr show
ip route show

# DNS lookup
nslookup google.com
dig google.com

# Network connections
netstat -tuln
ss -tuln

# Connect to remote server
ssh user@hostname
ssh -p 2222 user@hostname  # Different port

# Copy files remotely
scp file.txt user@hostname:/path/to/destination/
scp -r directory user@hostname:/path/to/destination/

# Secure copy with options
scp -P 2222 file.txt user@hostname:~/file.txt

# Sort lines
sort filename.txt
sort -r filename.txt  # Reverse sort
sort -n filename.txt  # Numeric sort

# Remove duplicate lines
sort filename.txt | uniq
uniq filename.txt

# Count lines, words, characters
wc filename.txt
wc -l filename.txt  # Lines only
wc -w filename.txt  # Words only
wc -c filename.txt  # Characters only

# Extract columns
cut -d',' -f1,3 filename.txt  # Fields 1 and 3, comma delimiter
cut -d' ' -f2 filename.txt    # Field 2, space delimiter

# Find and replace text
sed 's/old_text/new_text/g' filename.txt

# Delete lines containing pattern
sed '/pattern/d' filename.txt

# Print specific lines
sed -n '10,20p' filename.txt  # Lines 10-20

# AWK for complex text processing
awk '{print $1}' filename.txt  # Print first column
awk -F',' '{print $2}' filename.txt  # Second column, comma delimiter

#!/bin/bash

# This is a comment
echo "Hello, World!"

# Variables
NAME="John"
echo "Hello, $NAME!"

# Command substitution
CURRENT_DATE=$(date)
echo "Today is: $CURRENT_DATE"

# Conditional statement
if [ "$NAME" = "John" ]; then
    echo "Welcome, John!"
else
    echo "Welcome, guest!"
fi

# Create script file
nano myscript.sh

# Make executable
chmod +x myscript.sh

# Run script
./myscript.sh

# View command history
history

# Search command history
Ctrl + r  # Then type to search

# Run previous command
!!

# Run command with sudo
sudo !!

# Run nth command from history
!50

# Use Tab key to autocomplete
cd /ho[Tab]  # Becomes cd /home
ls file[Tab] # Shows matching files

# Create temporary alias
alias ll='ls -la'
alias la='ls -A'
alias grep='grep --color=auto'

# Make aliases permanent (add to ~/.bashrc)
echo "alias ll='ls -la'" >> ~/.bashrc
source ~/.bashrc

# Pipe output to another command
ls -la | grep ".txt"
ps aux | grep firefox

# Redirect output to file
ls -la > file_list.txt
echo "Hello" >> logfile.txt  # Append

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

# Always be careful with rm -rf
# Double-check before running destructive commands

# Use interactive mode for dangerous operations
rm -i filename.txt  # Ask before removing
cp -i source.txt dest.txt  # Ask before overwriting

# Test commands with echo first
echo rm *.txt  # See what would be deleted

# Create backups before major changes
cp important.txt important.txt.backup

# Use rsync for backups
rsync -av source/ backup/

# Create compressed backups
tar -czf backup.tar.gz directory/

# Manual pages
man command_name
man ls
man grep

# Help flag
command --help
ls --help
grep --help

# Info pages (more detailed)
info command_name

# Find command location
which ls
whereis python

# Search for commands
apropos keyword
man -k keyword