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Signals: Communicating With Processes

Signals communicate with processes. Stop them. Kill them. Reload them.

Here's the thing: Signals are how you control processes. Learn them. Use them.

What Signals Are​

Signals are:

  • Messages to processes
  • Ways to control processes
  • Interrupts and commands
  • Process communication

My take: Signals are like commands to processes. Send a signal. Process reacts.

Common Signals​

SIGTERM (15): Terminate Gracefully​

kill PID                           # Send SIGTERM (default)
kill -15 PID                       # Explicit SIGTERM

My take: SIGTERM asks process to stop. Graceful. Use it first.

SIGKILL (9): Force Kill​

kill -9 PID                        # Force kill
kill -KILL PID                     # Same thing

My take: SIGKILL forces stop. Can't be ignored. Last resort.

SIGHUP (1): Hang Up​

kill -1 PID                        # Send SIGHUP
kill -HUP PID                      # Same thing

My take: SIGHUP often reloads config. Use it for reloading.

SIGINT (2): Interrupt​

kill -2 PID                        # Send SIGINT
# Or Ctrl+C

My take: SIGINT is like Ctrl+C. Interrupts process.

Signal Numbers​

SignalNumberMeaning
SIGHUP1Hang up (reload)
SIGINT2Interrupt (Ctrl+C)
SIGTERM15Terminate gracefully
SIGKILL9Force kill

My take: Learn these. They're what you'll use.

Using Signals​

Send Signal​

kill -SIGNAL PID
kill -9 1234                       # Force kill
kill -15 1234                      # Graceful kill

My take: kill -signal PID sends signals. Use it.

Send to Process Name​

killall -9 process-name            # Force kill all
pkill -9 process-name              # Force kill by pattern

My take: killall and pkill kill by name. Useful.

Common Patterns​

Graceful Stop Then Force​

kill PID                           # Try graceful
sleep 5
kill -9 PID                        # Force if still running

My take: Try graceful first. Force if needed.

Reload Configuration​

kill -HUP PID                      # Reload config

My take: SIGHUP reloads. Use it for services.

Common Mistakes (I've Made These)​

  1. Using SIGKILL first: Try SIGTERM first. SIGKILL is last resort.

  2. Wrong signal number: Learn signal numbers. Use the right one.

  3. Killing wrong process: Check PID. Don't kill wrong thing.

  4. Not waiting: Give process time to stop. Don't force immediately.

  5. Not understanding signals: Signals have meanings. Understand them.

Real-World Examples​

Stop Service Gracefully​

kill 1234                          # SIGTERM
# Wait
kill -9 1234                       # SIGKILL if needed

Reload Service​

kill -HUP 1234                     # Reload config

What's Next?​

Now that you understand signals, let's talk about Process Monitoring to watch processes.

Personal note: I used to use kill -9 for everything. Then I learned signals. Now I try graceful first. Processes should have a chance to clean up. Be nice to your processes.