like other linux server, this note is for fedora38 operation system


how to use ps and kill, to stop a program?


source:
https://www.digitalocean.com/community/tutorials/how-to-use-ps-kill-and-nice-to-manage-processes-in-linux


//simple, quick, read me:
kill -kill PID_of_target_process_you_want_to_kill












All processes in Linux respond to signals. 
Signals are an operating system-level way of telling programs to terminate or modify their behavior.

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, 
you 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”.

Signals are not only used to shut down programs. 
They can also be used to perform other actions.




//what is daemons?
For instance, many processes that are designed to run constantly in the background 
(sometimes called “daemons”) will automatically restart when they are given the HUP, 
or hang-up signal. 
The Apache webserver typically operates this way.






sudo kill -HUP pid_of_apache

The above command will cause Apache to reload its configuration file and resume serving content.

//Note: 
Many background processes like this are managed through system services which provide an additional surface for interacting with them, 
and it is usually preferable to restart the service itself rather than sending a HUP signal directly to one running process. 

If you review the configuration files of various services, 
you may find that the various service restart hooks are designed to do exactly that – send signals to specific processes – while also providing logs and other reporting.



You can list all of the signals that are possible to send with kill with the -l flag:
kill -l

//Output:
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






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
//killall nginx



The above command will send the TERM signal to every instance of firefox running on the computer.