Linux essentials

Permissions

chmod 750 file        # rwx for owner, rx for group, none for others
chown user:group file

Processes

ps aux | grep name
kill -9 PID
top                    # or htop
nohup command &        # survive shell exit

Disk & files

df -h                  # disk usage by filesystem
du -sh *                # size of each item in current dir
find . -name "*.log" -mtime +7 -delete

Networking

ss -tulpn               # listening ports
curl -I https://example.com   # headers only
dig example.com