100 Essential Linux Commands

ls -lah          # List including hidden files, human-readable sizes
ls -lt           # Sort by modification time, newest first

cp -r src/ dest/          # Recursively copy a directory
mv old.txt new.txt        # Rename or move
rm -rf dir/               # Force-delete a directory (use with caution)

find /var/log -name "*.log" -mtime -7   # .log files modified within the last 7 days
find . -type f -size +100M              # Files larger than 100MB
find . -name "*.sh" -exec chmod +x {} \;

grep -rn "ERROR" /var/log/   # Recursive search with line numbers
grep -v "DEBUG" app.log      # Exclude DEBUG
grep -E "error|warn" app.log # Regex OR

awk '{print $1, $3}' access.log          # Print 1st and 3rd columns
awk -F: '{print $1}' /etc/passwd         # Use : as the field separator
awk 'NR>5{print}' file.txt               # Print from line 6 onward

sed -i 's/old/new/g' file.txt    # In-place substitution
sed -n '10,20p' file.txt         # Print lines 10–20 only
sed '/^#/d' config.conf          # Delete comment lines

ps aux | grep nginx          # Check processes
top -c                       # Live CPU/memory view (show full command)
htop                         # Interactive viewer
kill -9 PID                  # Force kill
pkill -f "python app.py"     # Kill by name
nohup ./script.sh &          # Run in background; keep running after the terminal closes

chmod 755 script.sh          # rwxr-xr-x
chown user:group file.txt    # Change owner
umask 022                    # Default permission mask
sudo -u www-data bash        # Run a shell as another user

df -h                        # Disk usage
du -sh /var/*                # Size by directory
free -h                      # Memory status
iostat -xz 1                 # I/O statistics (1-second interval)