Understanding Linux Processes

Every running program on Linux is a process with its own PID (process ID). Knowing how to inspect and control processes is a fundamental sysadmin skill.

List all running processes with a snapshot view:

ps aux              # all processes, BSD-style output
ps -ef              # all processes, full-format output
ps aux | grep nginx # filter for a specific program

For a live, updating view of resource usage use top or the more user-friendly htop:

top
htop   # install first: sudo apt install htop

Send signals to processes with kill. The default signal is SIGTERM (15), which asks the process to exit gracefully. SIGKILL (9) forces immediate termination:

kill 1234          # send SIGTERM to PID 1234
kill -9 1234       # force-kill PID 1234
killall nginx      # send SIGTERM to all processes named nginx
pkill -f "python3 manage.py"  # match by command line pattern

Run a process in the background with &, bring it back with fg, and suspend it with Ctrl+Z:

long_running_command &
jobs               # list background jobs
fg %1              # bring job 1 to foreground