How to kill a Process from CLI in Linux

As we all know that when any program or application is running then it is called a process. When the process is created a Process Control Block (PCB) a data structure also gets created. A PCB keeps all the information about the process and it is maintained by the operating system. The PCB is identified by a process ID (PID).

As a system admin, we know that Linux is a multitasking OS. Which means multiple processes (tasks) are running at the same time. There are many instances when you have to kill the process reason could be using a high amount of resources making other processes starve, Unresponsive processes etc.

Therefore, in this article, we will look at some of the commands which are used to delete the process but first of all we need information about the process which we want to delete. In the following section first of all we will find the process information about the process.

Finding the process

There are many ways to find information regarding the PID of the process here we will discuss only two ways.

Using “pidof” command

It finds the process ID of a running program and prints the PID on the terminal. For more information use the manpage of the command. Use the following command to get the PID.

pidof <program_name>

For example, you are running gedit editor on your Linux and you want to find the PID of the gedit. See the following output in my environment

foofunc@ubuntu:~$ pidof gedit
22531
foofunc@ubuntu:~$ 

Using “ps” command

Command explained in the previous section only shows the PID of the process, you can use the ps command to see the PID and other important information about the process. Refer to manpage for more information.

With the ps command we use grep to capture the required process. In this section, we will discuss two ps commands with grep in the above-mentioned example. Let’s see the example first.

ps -ef|grep <process_name>

An example of the above command is:

foofunc@ubuntu:~$ ps -ef|grep gedit
UID          PID    PPID  C STIME TTY          TIME CMD
foofunc     22531    1611  0 13:19 ?        00:00:06 /usr/bin/gedit --gapplication-service
foofunc     42507   41021  0 21:20 pts/0    00:00:00 grep --color=auto gedit
foofunc@ubuntu:~$ 

ps aux|grep <process_name> 

An example of the above command is:

foofunc@ubuntu:~$ ps aux|grep gedit
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
foofunc     22531  0.0  0.3 862636 91196 ?        Sl   13:19   0:06 /usr/bin/gedit --gapplication-service
foofunc     42521  0.0  0.0  17676  2704 pts/0    S+   21:21   0:00 grep --color=auto gedit
foofunc@ubuntu:~$ 

Options mentioned in the above ps command when using the standard syntax are:

• -e: Select all processes
• -f: Do full-format listing
• -F: Same like Extra full format.
• -l : long format
• -y : Do not show flags.

ps -e
ps -ef
ps -eF
ps -ely

To see every process on the system using BSD syntax. The option mentioned in the command are:

• a and x: list all processes when used together with the x option
• u: Display user oriented format.

ps ax
ps axu

Killing the process

To kill a process we will use kill command. It basically sends the signal to a process. Firstly let us list the different signals by running kill-l command, it list all the signals. Output of the command is:

foofunc@ubuntu:~$ 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
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	
foofunc@ubuntu:~$ 

In the above output we can see that signal 9 is for killing so we will use this signal. See the syntax to kill the command below.

sudo kill -9 pid 

Example of the above command is:

foofunc@ubuntu:~$ sudo kill -9 22531
[sudo] password for foofunc: 
foofunc@ubuntu:~$ 

Other process kill method.

You can use pkill and killall commands to kill the process aswell. They both command use the process name as an argument. Below mentioned command are example of kill gedit process.

pkill command example :

foofunc@ubuntu:~$ sudo pkill -9 gedit
foofunc@ubuntu:~$ 

killall command example:

foofunc@ubuntu:~$ sudo killall gedit
foofunc@ubuntu:~$ 

Why kill command is better

pkill and killall command use argument process name which is easy to use in comparison to kill. But as we mentioned before that Linux OSes are multiuser and for example, there can be instances when two users are running same application with pkill and killall command we kill application for both user which obviously you don’t want. Therefore using kill command is considered good.

Summary

In this article, we discussed about the process, how to get information regarding the process and how to kill a process. We saw three different ways to kill the process.

In case you face any issues and have any doubt don’t hesitate to comment. I will reply as soon as possible.