Stage 1 · Lesson 3 of 10
Processes and signals
A is a running program, and each one has a number called a process ID (PID). List processes with ps aux, watch them live with top or htop, and find what is using a port with ss -ltnp. You stop a process by sending it a signal, a short message from the system.
SIGTERM (kill PID) politely asks it to shut down cleanly. SIGKILL (kill -9) ends it at once, with no chance to tidy up. SIGHUP often tells a background service to reload its settings.
This matters well beyond the . systemd, and all stop apps by sending SIGTERM, waiting a grace period, then sending SIGKILL. An app that ignores SIGTERM drops the requests it is handling on every deploy.
Common trapreaching for kill -9 first, which can leave , half-written data and orphaned child processes behind.
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Resources for this stage
Start with· follow one of these
- CourseFreeThe Missing Semester of Your CS Education
MIT · Beginner
Shell, scripting, data wrangling, SSH and Git in a dozen focused lectures
- BookFreeThe Linux Command Line
William Shotts · Beginner
Free book: permissions, processes, packages and longer shell scripts, chapter by chapter