Terminate a Process in Linux

Terminate a Process in Linux

Linux operating system comes with kill command to end a process. The command makes it possible without the need to restart after a large change / upda

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Linux operating system comes with kill command to end a process. The command makes it possible without the need to restart after a large change / update to continue operation of the server. Here comes the great power of Linux and that is one of the reasons why Linux runs on 90% of the server on the planet.

Linux and Unix-like operating system come with the kill command to terminates stalled or unwanted processes without having to log out or restart the server.

What is a PID?

A Linux or Unix process is running instance of a program. For example, Firefox is a running process if you are browsing the Internet. Each time you start Firefox browser, the system is automatically assigned a unique process identification number (PID). A PID is automatically assigned to each process when it is created on the system. To find out PID of firefox or httpd process use the following command:

pidof httpd
pidof apache2
pidof firefox

For a kill command a Signal Name could be:

Signal Name		Signal Value			Behaviour

SIGHUP			      1				Hangup
SIGKILL			      9				Kill Signal
SIGTERM			      15			Terminate

Clearly from the behaviour above SIGTERM is the default and safest way to kill a process. SIGHUP is less secure way of killing a process as SIGTERM. SIGKILL is the most unsafe way among the above three, to kill a process which terminates a process without saving.

In order to kill a process, we need to know the Process ID of a process. A Process is an instance of a program. Every-time a program starts, automatically an unique PID is generated for that process. Every Process in Linux, have a pid. The first process that starts when Linux System is booted is – init process, hence it is assigned a value of ‘1‘ in most of the cases.

Init is the master process and can not be killed this way, which insures that the master process don’t gets killed accidentally. Init decides and allows itself to be killed, where kill is merely a request for a shutdown.

To know all the processes and correspondingly their assigned pid, run.

# ps -A
Sample Output
PID TTY          TIME CMD
    1 ?        00:00:01 init
    2 ?        00:00:00 kthreadd
    3 ?        00:00:00 migration/0
    4 ?        00:00:00 ksoftirqd/0
    5 ?        00:00:00 migration/0
    6 ?        00:00:00 watchdog/0
    7 ?        00:00:01 events/0
    8 ?        00:00:00 cgroup
    9 ?        00:00:00 khelper
   10 ?        00:00:00 netns
   11 ?        00:00:00 async/mgr
   12 ?        00:00:00 pm
   13 ?        00:00:00 sync_supers
   14 ?        00:00:00 bdi-default
   15 ?        00:00:00 kintegrityd/0
   16 ?        00:00:00 kblockd/0
   17 ?        00:00:00 kacpid
   18 ?        00:00:00 kacpi_notify
   19 ?        00:00:00 kacpi_hotplug
   20 ?        00:00:00 ata/0
   21 ?        00:00:00 ata_aux
   22 ?        00:00:00 ksuspend_usbd

How about Customising the above output using syntax as ‘pidof process‘.

# pidof mysqld
Sample Output
1684

Another way to achieve the above goal is to follow the below syntax.

# ps aux | grep mysqld
Sample Output
root      1582  0.0  0.0   5116  1408 ?        S    09:49   0:00 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql --socket=/var/lib/mysql/mysql.sock --pid-file=/var/run/mysqld/mysqld.pid --basedir=/usr --user=mysql
mysql     1684  0.1  0.5 136884 21844 ?        Sl   09:49   1:09 /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock
root     20844  0.0  0.0   4356   740 pts/0    S+   21:39   0:00 grep mysqld

Before we step ahead and execute a kill command, some important points to be noted:

  1. A user can kill all his process.
  2. A user can not kill another user’s process.
  3. A user can not kill processes System is using.
  4. A root user can kill System-level-process and the process of any user.

Another way to perform the same function is to execute ‘pgrep‘ command.

# pgrep mysq
Sample Output
3139

To kill the above process PID, use the kill command as shown.

kill -9 3139

The above command will kill the process having pid=3139, where PID is a Numerical Value of process.

Another way to perform the same function, can be rewritten as.

# kill -SIGTERM 3139

Similarly ‘kill -9 PID‘ is similar to ‘kill -SIGKILL PID‘ and vice-versa.

How about killing a process using process name

You must be aware of process name, before killing and entering a wrong process name may screw you.

# pkill mysqld

Kill more than one process at a time.

# kill PID1 PID2 PID3

or

# kill -9 PID1 PID2 PID3

or

# kill -SIGKILL PID1 PID2 PID3

What if a process have too many instances and a number of child processes, we have a command ‘killall‘. This is the only command of this family, which takes process name as argument in-place of process number.

Syntax:
# killall [signal or option] Process Name

To kill all mysql instances along with child processes, use the command as follow.

# killall mysqld

You can always verify the status of the process if it is running or not, using any of the below command.

# service mysql status
# pgrep mysql
# ps -aux | grep mysql

That’s all for now, from my side.

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