All processes in Linux respond to signals. Signals are an os-level way of telling programs to terminate or modify their behavior.
How To Send Processes Signals by PID
The most common way of passing signals to a program is with the kill command.
As you might expect, the default functionality of this utility is to attempt to kill a process:
kill PID_of_target_process
This sends the TERM signal to the process. The TERM signal tells the process to please terminate. This allows the program to perform clean-up operations and exit smoothly.
If the program is misbehaving and does not exit when given the TERM signal, we can escalate the signal by passing the KILL signal:
kill -KILL PID_of_target_process
This is a special signal that is not sent to the program.
Instead, it is given to the operating system kernel, which shuts down the process. This is used to bypass programs that ignore the signals sent to them.
Each signal has an associated number that can be passed instead of the name. For instance, You can pass “-15” instead of “-TERM”, and “-9” instead of “-KILL”.
How To Use Signals For Other Purposes
Signals are not only used to shut down programs. They can also be used to perform other actions.
For instance, many daemons will restart when they are given the HUP, or hang-up signal. Apache is one program that operates like this.
sudo kill -HUP pid_of_apache
The above command will cause Apache to reload its configuration file and resume serving content.
You can list all of the signals that are possible to send with kill by typing:
kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
. . .
How To Send Processes Signals by Name
Although the conventional way of sending signals is through the use of PIDs, there are also methods of doing this with regular process names.
The pkill command works in almost exactly the same way as kill, but it operates on a process name instead:
pkill -9 ping
The above command is the equivalent of:
kill -9 `pgrep ping`
If you would like to send a signal to every instance of a certain process, you can use the killall command:
killall firefox
The above command will send the TERM signal to every instance of firefox running on the computer.