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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
ls -lhS                # Sort by size, largest first
ls --color=auto        # Colorized output

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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
rm -i file             # Interactive (prompt before delete) — safer

# Copy files and directories
cp file1 destination/
cp -r directory destination/  # Recursive copy
cp -a source destination/     # Archive mode (preserves attributes, links, etc.)

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

# Create empty file or update timestamp
touch file.txt

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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
chmod -R 644 directory # Recursively set permissions

# 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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getfacl file           # View ACL
setfacl -m u:alice:rw file   # Grant alice read/write
setfacl -x u:alice file      # Remove alice's ACL entry

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# View entire file
cat file.txt

# View file with pagination
less file.txt          # Preferred (supports search with /)
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
tail -F logfile.log    # Follow even if file is rotated

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# Nano (beginner-friendly)
nano file.txt          # Open/create file
# Shortcuts: Ctrl+X (exit), Ctrl+O (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)
#   :q! (quit without saving)

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# Find by name
find /path -name "filename"
find . -type f -name "*.txt"  # Find .txt files in current directory
find . -type f -iname "*.TXT" # Case-insensitive name match

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

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

# Execute command on results (safer with -exec ... +)
find . -name "*.log" -exec rm {} +

# Faster filesystem-wide search (requires updatedb)
locate filename

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# Search in files
grep "pattern" file.txt
grep -r "pattern" directory/   # Recursive search
grep -R "pattern" directory/   # Recursive, follow symlinks
grep -i "pattern" file.txt     # Case-insensitive
grep -n "pattern" file.txt     # Show line numbers
grep -v "pattern" file.txt     # Invert match
grep -E "(p1|p2)" file.txt     # Extended regex (preferred over egrep)
grep -F "literal" file.txt     # Fixed string (no regex)

# Faster code search (often pre-installed)
rg "pattern"                   # ripgrep — fast, respects .gitignore

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ps aux                 # List all processes (BSD style)
ps -ef                 # List all processes (System V style)
top                    # Real-time process viewer
htop                   # Interactive process viewer
btop                   # Modern interactive resource monitor

# Find specific process
pgrep -af process_name
ps aux | grep process_name

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# Terminate processes
kill PID               # Send SIGTERM (graceful)
kill -9 PID            # Send SIGKILL (forceful)
killall process_name   # Terminate all processes with name
pkill -f pattern       # Kill by name/pattern

# 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 &

# Run command in a persistent session (survives disconnect)
tmux                   # Modern terminal multiplexer
screen                 # Older terminal multiplexer

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uname -a               # System information
hostnamectl            # Hostname and OS details (systemd)
whoami                 # Current user
id                     # User and group IDs
uptime                 # System uptime

# Disk and Memory
df -h                  # Disk space usage
du -sh directory       # Directory size
du -sh * | sort -h     # Sort directories by size
free -h                # Memory usage
lscpu                  # CPU information
lsblk                  # Block devices
lsmem                  # Memory ranges
lsusb                  # USB devices
lspci                  # PCI devices

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ping google.com        # Test network connectivity
wget url               # Download files
curl url               # Transfer data
curl -O url            # Save with remote filename

# SSH and File Transfer
ssh user@host          # Remote login
ssh-keygen -t ed25519  # Generate modern SSH key pair
ssh-copy-id user@host  # Install your public key on a remote host
scp file user@host:path        # Secure copy (legacy; consider rsync/sftp)
rsync -avz source destination  # Efficient file synchronization
rsync -avzP --delete src/ user@host:dst/  # Mirror with progress

# Network Statistics
ss -tuln               # Socket statistics (replacement for netstat)
ss -tulpn              # Include owning process

# Network Configuration (iproute2 replaces ifconfig/route)
ip addr                # Show IP addresses
ip link                # Show network interfaces
ip route               # Show routing table

# DNS
dig example.com
host example.com

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# Tar Archives
tar -cvf archive.tar files/      # Create tar archive
tar -xvf archive.tar             # Extract tar archive
tar -czvf archive.tar.gz files/  # Create gzip-compressed archive
tar -xzvf archive.tar.gz         # Extract gzip-compressed archive
tar -cJvf archive.tar.xz files/  # Create xz-compressed archive (better ratio)

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

# Zstandard (fast, modern compressor)
zstd file
unzstd file.zst

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

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sort file.txt          # Sort lines
sort -u file.txt       # Sort and remove duplicates
uniq file.txt          # Remove adjacent duplicate lines (sort first)
wc -l file.txt         # Count lines
cut -d: -f1 file       # Cut first field using : delimiter
tr 'A-Z' 'a-z' < file  # Convert to lowercase
awk '{print $1}' file  # Print first column
sed 's/old/new/g' file # Stream substitution
jq '.key' data.json    # Parse and query JSON

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# Redirection
command > output.txt   # Overwrite stdout to file
command >> output.txt  # Append stdout to file
command 2> err.txt     # Redirect stderr
command &> all.txt     # Redirect stdout and stderr (bash)
command < input.txt    # Input from file

# Pipes
command1 | command2    # Output of command1 as input to command2
command | tee file.txt # Display and save output
command |& less        # Pipe stdout and stderr (bash)

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# View/Set Variables
echo $PATH
export VARNAME=value
printenv               # List all environment variables

# Configuration Files
~/.bashrc              # User-specific bash configuration
~/.bash_profile        # Login shell configuration
~/.profile             # POSIX login shell configuration
~/.zshrc               # Zsh configuration (default shell on many systems)
/etc/environment       # System-wide environment variables

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sudo apt update             # Update package list
sudo apt upgrade            # Upgrade packages
sudo apt install package    # Install package
sudo apt remove package     # Remove package
sudo apt autoremove         # Remove unused dependencies
apt search keyword          # Search packages
apt show package            # Show package details

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sudo dnf upgrade            # Upgrade packages (replaces 'yum update')
sudo dnf install package    # Install package
sudo dnf remove package     # Remove package
sudo dnf search keyword     # Search packages
# Note: 'yum' is deprecated on modern RHEL-family systems; use 'dnf'

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sudo pacman -Syu            # Sync and upgrade
sudo pacman -S package      # Install package
sudo pacman -R package      # Remove package

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# Flatpak
flatpak install flathub app.id
flatpak run app.id

# Snap
sudo snap install package
snap list

# AppImage — just chmod +x and run
chmod +x app.AppImage && ./app.AppImage

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# Systemd
sudo systemctl start service
sudo systemctl stop service
sudo systemctl restart service
sudo systemctl reload service
systemctl status service
sudo systemctl enable service       # Start on boot
sudo systemctl enable --now service # Enable and start immediately
sudo systemctl disable service
systemctl list-units --type=service
systemctl --user start service      # User-level service

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journalctl -xe                 # Recent logs with explanations
journalctl -u service_name     # Logs for a specific service
journalctl -f                  # Follow logs in real-time
journalctl --since "1 hour ago"
journalctl -p err              # Filter by priority (err, warning, info, etc.)
journalctl --vacuum-time=7d    # Trim journal older than 7 days

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# Edit crontab
crontab -e
crontab -l             # List scheduled jobs

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

# systemd timers (modern alternative to cron)
systemctl list-timers

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# User Management
sudo useradd -m username
sudo usermod -aG groupname username
sudo userdel -r username
sudo passwd username

# Switch Users
su - username          # Switch user
sudo command           # Run command with elevated privileges
sudo -i                # Start a root login shell

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dmesg                  # Kernel messages
dmesg -T               # With human-readable timestamps
journalctl -xe         # System logs
strace -p PID          # Trace system calls of a running process
ltrace command         # Trace library calls
lsof -i :PORT          # Find process using a port
lsof -p PID            # List files opened by a process
watch -n 2 command     # Re-run command every 2 seconds

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Ctrl+C                 # Terminate current process
Ctrl+Z                 # Suspend current process
Ctrl+D                 # Send EOF / log out of shell
Ctrl+R                 # Reverse history search
Ctrl+L                 # Clear the screen
Ctrl+A / Ctrl+E        # Jump to start / end of line
Ctrl+U / Ctrl+K        # Cut from cursor to start / end of line
!!                     # Repeat last command
!$                     # Last argument of previous command
sudo !!                # Re-run last command with sudo

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#!/usr/bin/env bash    # Portable shebang
set -euo pipefail      # Safer defaults: exit on error, undefined vars, pipe failures

variable="value"       # Variable assignment (no spaces around =)
echo "$variable"       # Always quote variables

# Conditional
if [[ -f "$file" ]]; then
    echo "File exists"
fi

# Loops
for f in *.txt; do
    echo "$f"
done

# Functions
greet() {
    local name="$1"
    echo "Hello, $name"
}
greet "world"

# Command substitution (prefer $() over backticks)
today=$(date +%F)

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# Often-recommended replacements
bat        # 'cat' with syntax highlighting
eza        # Modern 'ls' replacement (successor to exa)
fd         # Friendlier 'find'
rg         # Fast 'grep' (ripgrep)
dust       # Visual 'du'
duf        # Visual 'df'
btop       # Modern 'top'/'htop'
fzf        # Fuzzy finder for files, history, etc.
zoxide     # Smarter 'cd' that learns your habits