Using Kill Commands In Linux

Server Management Service

Kill command in Linux (located in /bin/kill), is a built-in command which is used to terminate processes manually.
Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to perform Linux related tasks.
In this context, we shall look into how to simply kill the process when it becomes unresponsive or fails to respond.

What does a Process mean in Linux ?

A process is the working mechanism of a program that is currently in execution. Upon its creation, every process automatically assigns to a unique process identification number, also known as a PID.
When a process dies, its PID returns to the pool of available PIDs, and another process can then reuse it.
We can use multiple commands on a server to find a specific PID of a process. We can use the top command, to find the PID of a process along with other useful system information.

Another way to find the PID is with the ps command:

[root@host ~]# ps faux | grep systemd

Root 1 0.0 0.6 96656 11260 ? Ss Jan24 0:08 /usr/lib/systemd/systemd –switched-root –system –deserialize 18

Root 533 0.0 1.3 125588 22832 ? Ss Jan24 0:25 /usr/lib/systemd/systemd-journald

Root 564 0.0 0.5 107440 9248 ? Ss Jan24 0:00 /usr/lib/systemd/systemd-udevd

Dbus 684 0.0 0.3 73540 5524 ? Ss Jan24 0:06 /usr/bin/dbus-daemon –system –address=systemd: –nofork –nopidfile –systemd-activation –syslog-only

Root 743 0.0 0.4 95736 7760 ? Ss Jan24 0:01 /usr/lib/systemd/systemd-logind

Root 66199 0.0 0.0 12108 1060 pts/0 S+ 08:28 0:00 \_ grep –color=auto systemd

Root 66153 0.0 0.5 93212 9472 ? Ss 08:27 0:00 /usr/lib/systemd/systemd –user


[root@host ~]# ps -eo user,pid,command | grep systemd

[root@host ~]# ps -eo user,pid,command | grep systemd

root 1 /usr/lib/systemd/systemd –switched-root –system –deserialize 18

root 533 /usr/lib/systemd/systemd-journald

root 564 /usr/lib/systemd/systemd-udevd

dbus 684 /usr/bin/dbus-daemon –system –address=systemd: –nofork –nopidfile –systemd-activation –syslog-only

root 743 /usr/lib/systemd/systemd-logind

root 66153 /usr/lib/systemd/systemd –user

root 66213 grep –color=auto systemd

We can also use the pidof process_name, and the pgrep process_name commands to find out the corresponding PIDs of that process:

[root@host ~]# pidof systemd

66156 66153 1


[root@host ~]# pgrep systemd

To terminate a process, we can use the kill commands in Linux. This is a built-in command that sends a signal to a specified process, which terminates the process in question.

What Are Signals in Linux ?

Signals are a form of a dialogue between the processes that can come from other processes, the kernel, or the process itself.
Each process has a current behavior, of which there are five dispositions:
i. Term
ii. Ign
iii. Core
iv. Stop
v. Cont

The Term's default action is to terminate the process, and it is the one primarily used by the kill command.
There are a total of sixty-four (64) signals. The one we send will depend on the result that we want, as different signals have different effects and outcomes.
Out of these sixty-four signals, the first thirty-one are standard signals and the last thirty-three are real-time signals.

We can see a full list of available signals with the command kill -l:

[root@host ~]# 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

For most of these signals, a program may or may not specify a different action. If it defines, it is called catching or handling the signal, but if no action occurs, then the signal will be ignored.

The most commonly used signals are:
Signal Name            Single Value    Effect
SIGHUP                 1               Hangup, reload a process
SIGINT                 2               Interrupt from keyboard
SIGKILL                9               Kill a process
SIGTERM                15              Terminate a process gracefully
SIGSTOP                17, 19, 23      Stop a process

We can use SIGKILL when SIGTERM fails to stop or end the process.

[root@host ~]# ps -eo user,pid,command | grep java

tomcat 59815 /usr/bin/java -Djava.util.logging.config.file=/usr/local/tomcat9/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dorg.apache.catalina.security.SecurityListener.UMASK=0027 -Dignore.endorsed.dirs= -classpath /usr/local/tomcat9/bin/bootstrap.jar:/usr/local/tomcat9/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat9 -Dcatalina.home=/usr/local/tomcat9 -Djava.io.tmpdir=/usr/local/tomcat9/temp org.apache.catalina.startup.Bootstrap start

root 66401 grep –color=auto java

[root@host ~]# kill -15 59815

Unfortunately, the kill -15 (SIGTERM) did not accomplish stopping the Java process.

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Function of Kill Commands In Linux ?

The kill command sends a signal to terminate a process. By default, it will send a TERM signal. It will try to stop the process gracefully. If that fails, we can try terminating the process with another signal.
To kill a process with a different signal, it can be defined using a number (-9), with a SIG prefix (-SIGkill) with or without the SIG prefix (-kill).

[root@host ~]# ps -eo user,pid,command | grep java

tomcat 66469 /usr/bin/java -Djava.util.logging.config.file=/usr/local/tomcat9/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dorg.apache.catalina.security.SecurityListener.UMASK=0027 -Dignore.endorsed.dirs= -classpath /usr/local/tomcat9/bin/bootstrap.jar:/usr/local/tomcat9/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat9 -Dcatalina.home=/usr/local/tomcat9 -Djava.io.tmpdir=/usr/local/tomcat9/temp org.apache.catalina.startup.Bootstrap start

root 66516 grep –color=auto java

[root@host ~]# kill -9 66469

[root@host ~]# ps -eo user,pid,command | grep java

root 66530 grep –color=auto java

As we can see, the kill -9 (SIGKILL) command terminates the Java process.
The kill command can be invoked with the following syntax:

kill [OPTIONS] [PID]

For example, if we find that the process ID for a httpd process is 21567, we can try to kill it gracefully by invoking:

[root@host ~]# kill 21567

This will send the TERM signal. If this does not stop the process, we can then try to kill it with the signal SIGKILL (-15).

We can accomplish it in one of the following ways:

[root@host ~]# kill -9 21567

[root@host ~]# kill -SIGKILL 21567

[root@host ~]# kill -kill 21567

1. Killall
Additionally, we can kill a process by using the unique process ID, by using the kill command, or with the process name, by using the killall command.
The same syntax applies with the killall command, which can invoke with killall [signal] [process_name], for example:

killall -9 httpd

This command will terminate the httpd process without waiting for it to stop gracefully. When determining which process the signal will be sent to, it will match the argument name exactly.
For example:

killall httpd

The command will send a termination signal to all processes that are exactly named “httpd.”
Instead, if we try to run this command:

killall http

It will show an error that there is no process with that name since Apache uses “httpd” and not “http.”

2. Pkill
A command similar to killall is pkill, which also can kill a process by name. This command takes a pattern as an argument, which matches against the names of running processes, so it does not need an exact name.
For example, we can terminate httpd with something like this:

pkill http

Although this is not the exact process name, it will find the processes that contain this pattern and send the terminate signal to httpd.

3. Skill
We can use the skill command to send termination signals, much like kill command. Here it uses the default TERM signal.

skill [signal] [options]

[root@host ~]# ps -eo user,pid,command | grep tomcat

tomcat 66546 /usr/bin/java -Djava.util.logging.config.file=/usr/local/tomcat9/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -Dorg.apache.catalina.security.SecurityListener.UMASK=0027 -Dignore.endorsed.dirs= -classpath /usr/local/tomcat9/bin/bootstrap.jar:/usr/local/tomcat9/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat9 -Dcatalina.home=/usr/local/tomcat9 -Djava.io.tmpdir=/usr/local/tomcat9/temp org.apache.catalina.startup.Bootstrap start

root 66846 grep –color=auto tomcat

[root@host ~]# skill -9 66546

You can use the kill, pkill, or killall commands instead.

4. Mysql_zap
Another command that will end a process that matches a specific pattern is mysql_zap. An identified process will match this pattern if its output line from the ps command contains that pattern.
As with other commands, mysql_zap will send a TERM signal by default. The syntax used for this command is:

[root@host ~]# mysql_zap [signal] [pattern].

We should note that the mysql_zap command depreciates in MariaDB as of version 10.2 in favor of the pkill command.

5. Mk-kill
The mk-kill command is to kill MySQL queries that match a specific criterion.
If a file pass to mk-kill, it will read the queries from that file, which contain the output of SHOW PROCESSLIST. If no file is given as an argument, mk-kill will execute a SHOW PROCESSLIST in MySQL to obtain the queries.

Here are two examples of that command:

[root@host ~]# mk-kill –busy-time 60 –print

[root@host ~]# mk-kill –match-command Sleep –kill –no-only-oldest –interval 10

The first command will kill any queries found to be running longer than sixty seconds. The second command checks for any sleeping processes every ten seconds, and if any are found, the mk-kill command ends those queries.

6. Mkill
Another command similar to mk-kill is the mkill command. This command will kill slow queries.
It will kill these queries based on several factors, such as query time, host, user, database, state, and query content. Note that the mkill command is still in an alpha version status.

7. Tkill/Tgkill
If we want to kill a specific thread, instead of an arbitrary thread within an entire process, we can do so with tkill and tgkill commands.
We should note that the tkill command should avoid as it is the depreciated predecessor to tgkill. The tkill command only allows for the target thread ID to be specified, which could lead to the termination of an incorrect thread if that thread ID is recycled.
On the other hand, tgkill will send a signal to the thread with the thread ID in the thread group tgid.

8. Killpg
It is also worth noting that several commands are to send signals to a process group. One of those commands is killpg.
The killpg command will send a signal to a process group (or pgrp). If the pgrp is 0, then a signal will be sent to the calling process’ process group.
Otherwise, the killpgrp command does not take any other arguments so that it will kill all processes within the same process group.

9. Tcpkill
We can also kill specific TCP connections that are in progress with the tcpkill command. We can specify a network interface to listen on, or we can also determine the degree of force to use in terminating a connection, where the default is 3.
The following command is the syntax of tcpkill.

[root@host ~]# tcpkill [-i interface] [-1…9] expression

[root@host ~]# tcpkill -i eth0 port 21

10. Lxc-kill
If we need to stop a process running inside a virtualized container running on Ubuntu, we can use the lxc-kill command. This command will send a numeric signal to the first process of the container to end a process.
The syntax of the lxc-kill command is as follows:

lxc-kill –name=NAME SIGNUM

11. Arckill
The arckill command is to kill running jobs on an ARC enabled resource. We can either use the jobid, or the jobname, if such an attribute was submitted, to refer to a job.

arckill -j filename.xml (<jobid#>)

12. Pvm_kill
To terminate a specific PVM process, we can use the pvm_kill command. It will send a terminate signal to a PVM process that is identified by the task identifier tid.
We use this command with a larger program:
In C: info = pvm_kill( tid );
In Fortran: CALL PVMFKILL( TID, INFO )

13. Xkill
There is also the xkill command, which is a graphical way to kill an application.
When we enter the command xkill, the mouse cursor will change into a plus sign. It allows us to left-click an unresponsive window to close it. The xkill command instructs the xserver to terminate the window displaying a program.
Its syntax is as follows:

xkill [-display displayname] [-id resource] [-button number] [-frame] [-all]

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General Rules of Kill Commands In Linux ?

It is essential to note that regular users can send signals to their own processes. However not to those that belong to other users.
The root user, on the other hand, can send signals to all other user’s processes.

This also depends on what PID we pass to the command:
i. If the PID is greater than zero, then the signal sends to the process with that PID.
ii. When PID is equal to zero, the signal sends to all processes in the process group (PGID) of the shell that is invoking the kill command.
iii. If PID is equal to -1, the signal sends to all processes with the same user ID as the user who has invoked the kill command.
iv. If PID is less than -1, the signal sends to all processes in the process group with the process group ID that is equal to the absolute value of the PID.