Linux Signals

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Sources:

  1. Kill Command in Linux.
  2. Machtelt Garrels. Chapter 12. Catching signals. Bash Guide for Beginners.

Linux Signals

Find available signals:

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man signal

A signal is a kind of (usually software) interrupt, used to announce asynchronous events to a process.

There is a limited list of possible signals; we do not invent our own. (There might be 64 signals, for instance.)

The name of a LINUX signal begins with "SIG". Although signals are numbered, we normally refer to them by their names. For example:

Signal name Signal value Effect
SIGHUP 1 Hangup
SIGINT 2 Interrupt from keyboard
SIGKILL 9 Kill signal
SIGTERM 15 Termination signal
SIGSTOP 17,19,23 Stop the process

  • SIGINT can be generated when a user presses Control-C.

  • SIGSTOP can be generated when a user presses Control-Z.

Usage of signals with kill

The kill command sends a signal to specified processes or process groups, causing them to act according to the signal.

To get a list of all available signals, invoke the command with the -l option:

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kill -l

Usage:

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kill [OPTIONS] [PID]...

The most commonly used signals are:

  • 1 (HUP) - Reload a process.
  • 9 (KILL) - Kill a process.
  • 15 (TERM) - Gracefully stop a process.

When the signal is not specified, it defaults to -15 (-TERM).

Terminating Processes Using the kill Command

To terminate or kill a process with the kill command, first you need to find the process ID number (PID). You can do this using different commands such as top, ps , pidof and pgrep .

Let’s say the Firefox browser has become unresponsive, and you need to kill the Firefox process. To find the browser PIDs use the pidof command:

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pidof firefox

The command will print the IDs of all Firefox processes:

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6263 6199 6142 6076

Once you know the processes numbers, you can kill all of them by sending the TERM signal:

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kill -9 6263 6199 6142 6076

Instead of searching for PIDs and then killing the processes, you can combine the above commands into one:

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kill -9 $(pidof firefox)

Reloading Processes Using the kill Command

Another common use case for kill is to send the HUP signal, which tells the processes to reload its settings.

For example, to reload Nginx , you need to send a signal to the master process. The process ID of the Nginx master process can be found in the nginx.pid file, which is typically is located in the /var/run directory.

Use the cat command to find the master PID:

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cat /var/run/nginx.pid
30251

Once you found the master PID reload the Nginx settings by typing:

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sudo kill -1 30251

The command above must be run as root or user with sudo privileges.