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# Print current working directory
pwd

# Change directory
cd /path/to/directory  # Absolute path
cd ..                  # Go up one directory
cd ~                   # Go to home directory
cd -                   # Go to previous directory

# List directory contents
ls                     # Basic listing
ls -l                  # Detailed list view
ls -a                  # Show hidden files
ls -lh                 # Human-readable file sizes

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# Create directory
mkdir new_directory
mkdir -p /path/to/nested/directories  # Create parent directories if they don't exist

# Remove directory
rmdir empty_directory  # Only removes empty directories
rm -r directory        # Recursively remove directory and its contents

# Copy files and directories
cp file1 destination/
cp -r directory destination/  # Recursive copy

# Move/Rename files and directories
mv oldname newname     # Rename
mv file destination/   # Move file

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# Permission modes
chmod 755 file         # rwxr-xr-x (Owner: read/write/execute, Group/Others: read/execute)
chmod u+x file         # Add execute permission for user
chmod go-w file        # Remove write permission for group and others

# Common permission values
# 4 = read, 2 = write, 1 = execute
# 7 (4+2+1): read, write, execute
# 6 (4+2): read, write
# 5 (4+1): read, execute
# 3 (2+1): write, execute

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# Change file owner
chown user:group file

# Change group ownership
chgrp group file

# Recursively change ownership
chown -R user:group directory

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# View entire file
cat file.txt

# View file with pagination
less file.txt
more file.txt

# View first/last lines
head -n 10 file.txt    # First 10 lines
tail -n 5 file.txt     # Last 5 lines
tail -f logfile.log    # Follow log file in real-time

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# Nano (beginner-friendly)
nano file.txt          # Open/create file
# Shortcuts: Ctrl+X (exit), Ctrl+S (save)

# Vim (powerful, steep learning curve)
vim file.txt           # Open/create file
# Modes:
# - Normal mode (Esc)
# - Insert mode (i)
# - Command mode (:)
# Commands:
#   :w (save)
#   :q (quit)
#   :wq (save and quit)

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# Find by name
find /path -name "filename"
find . -type f -name "*.txt"  # Find .txt files in current directory

# Find by size
find /path -size +10M   # Files larger than 10MB

# Find by modification time
find /path -mtime -7    # Modified in last 7 days

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# Search in files
grep "pattern" file.txt
grep -r "pattern" directory/  # Recursive search
grep -i "pattern" file.txt   # Case-insensitive
grep -n "pattern" file.txt   # Show line numbers

# Complex pattern matching
egrep "(pattern1|pattern2)" file.txt  # Regex OR

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ps aux                 # List all processes
top                    # Real-time process viewer
htop                   # Interactive process viewer

# Find specific process
ps aux | grep process_name

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# Terminate processes
kill PID               # Terminate process by ID
killall process_name   # Terminate all processes with name

# Background/Foreground
bg                     # Send stopped job to background
fg                     # Bring background job to foreground
jobs                   # List background jobs

# Run process immune to hangup
nohup command &

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uname -a               # System information
whoami                 # Current user
id                     # User and group IDs
uptime                 # System uptime

# Disk and Memory
df -h                  # Disk space usage
du -sh directory       # Directory size
free -h                # Memory usage
lscpu                  # CPU information
lsblk                  # Block devices

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ping google.com        # Test network connectivity
wget url               # Download files
curl url               # Transfer data

# SSH and File Transfer
ssh user@host          # Remote login
scp file user@host:path # Secure copy
rsync -avz source destination  # Efficient file synchronization

# Network Statistics
netstat -tuln          # Listening ports
ss -tuln               # Socket statistics

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# Tar Archives
tar -cvf archive.tar files/     # Create tar archive
tar -xvf archive.tar            # Extract tar archive

# Gzip Compression
gzip file              # Compress
gunzip file.gz         # Decompress

# Zip
zip archive.zip files/
unzip archive.zip

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sort file.txt          # Sort lines
uniq file.txt          # Remove duplicate lines
wc -l file.txt         # Count lines
cut -d: -f1 file       # Cut first field using : delimiter
tr 'A-Z' 'a-z' < file  # Convert to lowercase

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# Redirection
command > output.txt   # Overwrite file
command >> output.txt  # Append to file
command < input.txt    # Input from file

# Pipes
command1 | command2    # Output of command1 as input to command2
command | tee file.txt # Display and save output

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# View/Set Variables
echo $PATH
export VARNAME=value

# Configuration Files
~/.bashrc             # User-specific bash configuration
~/.bash_profile       # Login shell configuration

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apt update             # Update package list
apt upgrade            # Upgrade packages
apt install package    # Install package
apt remove package     # Remove package

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yum update             # Update packages
yum install package    # Install package
yum remove package     # Remove package

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# Systemd
systemctl start service
systemctl stop service
systemctl restart service
systemctl status service
systemctl enable service   # Start on boot

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# Edit crontab
crontab -e

# Example: Run script every day at 2:30 AM
30 2 * * * /path/to/script.sh

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# User Management
useradd username
usermod -aG groupname username
userdel username

# Switch Users
su - username          # Switch user
sudo command           # Run command with elevated privileges

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dmesg                  # Kernel messages
journalctl -xe         # System logs
strace command         # Trace system calls

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Ctrl+C                 # Terminate current process
Ctrl+Z                 # Suspend current process
Ctrl+R                 # Reverse history search
!!                     # Repeat last command

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#!/bin/bash           # Shebang line
variable="value"       # Variable assignment
echo $variable         # Print variable
if [ condition ]; then # Conditional
    # commands
fi