Process management & systemd

Every running program is a process, identified by a PID. ps and top/htop are how you see what's actually running and how much of the machine it's using.

ps aux                    # every process, full detail
ps aux --sort=-%cpu       # sorted by CPU, worst offender first
top                       # live, refreshing view (htop if installed — nicer)
pgrep nginx               # find PIDs by name
kill 4821                 # ask a process to terminate gracefully (SIGTERM)
kill -9 4821              # force-kill, no cleanup (SIGKILL) — last resort

Most services you'll manage don't run as bare processes though — they're wrapped by systemd, the init system on nearly every modern Linux distro. systemd starts services at boot, restarts them if they crash, and manages dependencies between them.

sudo systemctl status nginx
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx
sudo systemctl enable nginx    # start automatically on boot
sudo systemctl disable nginx

journalctl -u nginx -f       # follow that service's logs live