Signals | Linux

The Linux command line interface uses signals as a means of communication between processes. Understanding Linux signals is essential for managing and interacting with running processes.

Understanding Linux Signals:

Linux signals are software interrupts that notify processes about specific events. Some commonly used signals include:

SIGHUP (1) :
- Hangup signal, typically used to restart a process.
SIGINT (2) :
- Signal interrupt — it tells the program to interrupt, usually triggered by pressing Ctrl + C.
SIGQUIT (3) :
- Ctrl + D — directly terminates your program.
SIGKILL (9) :
- Forcefully terminates a process.
- Program assassination — shutdown immediately.
SIGTERM (15) :
- Requests a process to terminate gracefully.
- Signal terminate.
- Typically issued before a system shutdown to close all programs properly.

General Rule: If You Don’t Know How to Exit or Terminate:

First try Ctrl + C.
If that doesn’t work, try Ctrl + D.

Demo Commands:

# Run yes in the background, continuously writing output to /dev/null (/dev/null is where you send anything you don't want to keep)
yes > /dev/null &
# Search for the process with the "yes" keyword (PID)
ps aux | grep yes

# Display the different types of signals
kill -l

# Send SIGTERM to the process with PID 1234
kill -15 1234

# Forcefully terminate the process with PID 5678
kill -9 5678
sudo kill -SIGKILL <process id>

# Restart a process by sending SIGHUP
kill -1 7890

# Interrupt a process (Ctrl + C)

Process Management:

By leveraging Linux signals, users can properly control running processes — performing shutdowns, restarts, and even emergency terminations when necessary. Understanding the differences between signals enables precise process management from the command line.