How to kill a process in Linux

Managing processes in Linux can be a bit tricky if you are just getting started. In the Linux world, everything is a process, and managing these processes can be daunting. With so many options and commands available, it can be challenging to know where to start.

First, let's start with the basics:

What is a process in Linux?

A process is simply an instance of a running program. So every time you open up a new terminal and run a command or a program, that's a new process. Processes can be either running or sleeping. When a process is sleeping, it is not using any CPU resources.

Use the ps command to see all the processes running on your system. It will show you a list of all the processes, their PIDs (process IDs), and some other information about them.

ps

Returns:
j4y@DESKTOP-71FSHR2:~$ ps
  PID TTY          TIME CMD
   10 tty1     00:00:00 bash
   72 tty1     00:00:00 ps

The ps command is extensive. You can do a lot with it, but that is a story for another time. For now, let's focus on how to kill a process.

As mentioned earlier, processes can be either running or sleeping. Depending on which state they're in, they are killed differently.

To stop a running process, you can use the kill command.
To kill a sleeping process (one that has been started but is not currently running), you can use the killall command.

Using the kill command

To kill a process, you have to know its PID first. By running the command ps, you get a list of all processes with their PID. Once you have the PID, use the kill command to send a signal to the process telling it to stop running. For example, if I wanted to kill the process with PID 1234, I would use the following command:

kill 1234

It sends a SIGTERM signal to the process with PID 1234, telling it to terminate.

There are other signals that you can send to processes as well. For example, SIGKILL is a signal that will kill the process immediately, without giving it a chance to clean up first. Therefore, this command should only be used as a last resort when other signals have failed.

All kill Signals

The kill command may be used with a number of different signals, To check the other signals available, use the kill command with the -l option:

kill -l

Returns:

 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
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

The most commonly used signals are SIGTERM (15) and SIGKILL (9).

We often find ourselves using ctrl + c or ctrl + z, which are also kill signals.

SIGSTOP (19) is triggered every time we press ctrl + z.

SIGCONT (18) - (Sig Continue) triggers when we press ctrl + z the second time.

SIGINT (2) - (Sig interrupt) triggers when we press ctrl + c

You can specify these signals by name or number. For example, to kill a process with a PID of 1234, you would run:

kill 1234

kill -15 1234

kill -SIGTERM 1234

These three commands do the same thing. They send the default "terminate" signal to the process (15), which is usually what you want. If the process refuses to die after that, you can try sending it a SIGKILL signal:

kill -SIGKILL 1234

kill -9 1234

This command will kill the process immediately.

Using the killall command

The killall command kills all processes that are running a specified program. It is useful when a program is frozen or unresponsive, and you want to force it to quit. For example, if you want to kill all processes that are running the Firefox web browser, you would use the following command:

killall firefox

This command will send a SIGTERM signal to all processes associated with firefox.

The killall command can also be used to kill multiple programs at once. To do this, list the names of the programs you want to quit, separated by spaces. For example:

killall firefox chrome

Using pkill command

If you want to get a little more granular with your killing, you can use the pkill command which allows you to kill processes based on their name rather than their PID.

So, for example, if I wanted to kill all processes named "firefox", I would run:

pkill firefox

The pkill command can also kill all instances of a program. For example, having ten instances of firefox running, Using pkill firefox would kill them all.

That is all there is to killing processes in Linux.

Wrapping Up

As you can see, there are various methods for killing processes in Linux. Different signals can also be used depending on what you're trying to accomplish. In most cases, the kill and killall commands will be all you need. However, if you're having trouble with an unresponsive or stuck process, the kill -9 signal may be necessary.

Questions? Feel free to leave a comment below. Alternatively, check out the man pages for the ps and kill commands (man ps and man kill). Finally, if you found the article helpful, consider sharing it.

Thanks for reading!

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