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Although Linux is considered a robust operating system with very few issues with applications, programs sometimes become unresponsive. When this happens, they can consume plenty of system resources or take down the entire system. Usually, these applications cannot be restarted automatically. 

If the process or application is still running and will not get shut down completely, you need to use a command to terminate the process in Linux. It is necessary to either restart the whole system or end the specific application process when this happens. Since restarting the entire system takes time and can create significant inconvenience for the clients, it is much easier to kill a process in Linux.

What is a Process?

A process is the working mechanism of a program that is currently being executed. Upon its creation, every process is automatically assigned a unique process identification number (PID). When a process dies, its PID gets returned to an available pool, and another process can then reuse it.

We can use multiple commands on a server to find a specific PID. For example, we can use the top command, which will give us a table of all the running processes. With this, we can find the PID of a process along with other useful system information.

Another way to find the PID is with the ps command. Below, we see a few different ways to use the ps command to find the PID.

[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 ~]#
[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
[root@host ~]#

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 ~]#
[root@host ~]# pgrep systemd
1
533
564
743
66153
[root@host ~]# 

To terminate a process in Linux, we can use the kill command. It is a built-in command that sends a signal to a specified process. The Linux operating system will stop the process in question.

What Are Signals?

Signals are a form of a dialogue between processes from other processes, the kernel, or the specific process itself. Each process has a current behavior, or disposition, of which there are five types:

  • Term
  • Ign
  • Core
  • Stop
  • Cont

These attributes determine how the process will behave when a signal is delivered to it. The Term’s default action is to terminate the process, which is primarily used by the kill command.

There are a total of 64 signals used with the kill command. The one we send will depend on the desired result, as different signals have different effects and outcomes. Out of these 64 signals, the first 31 are standard signals. The rest are real-time signals.

The main difference between the two types of signals is that standard signals cannot be queued, while real-time signals can. What this means is that the information associated with the first instance of a standard signal is received by a process. Real-time signals will queue the information associated with it, allowing reception of multiple signals. 

We can see a complete list of available signals with the command kill -l or in the signal manual page by entering man 7 signal.

[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
[root@host ~]# 

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

The most commonly used signals are as follows.

Signal NameSingle ValueEffect
SIGHUP1Hangup, reload a process
SIGINT2Interrupt from keyboard
SIGKILL9Kill a process
SIGTERM15Terminate a process gracefully
SIGSTOP17, 19, 23Stop a process

The SIGKILL and SIGSTOP signals cannot be caught, blocked, or ignored, while the SIGTERM signal can be caught or ignored. That is why we can use SIGKILL in Linux when SIGTERM fails to stop or end the process.

In the below example, the kill -15 (SIGTERM) does not stop the Java process. Therefore, in cases like these, Linux must force kill the processes.

[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
[root@host ~]#

General Rules

There are other general rules when it comes to how to stop a process in Linux. It is essential to note that regular users can send signals to their own processes but not to those that belong to other users. The root user, on the other hand, can send signals to all other user’s processes. Here are some additional general rules:

  • Signals sent depend on what PID we pass to the command. 
  • If the PID is greater than zero, then the signal is sent to the process with that PID.
  • If the PID is equal to zero, the signal is sent to all processes in the process group (PGID) of the shell involved in the kill command. 
  • If PID is equal to -1, the signal is sent to all processes with the same user ID as the user who has invoked the kill command. 
  • If PID is less than -1, the signal is sent to all processes in the process group with the process group ID that is equal to the absolute value of the PID.

Using The Kill Commands

As noted above, the kill command sends a signal to terminate a process in Linux. By default, it will send a TERM signal, if no other signal is defined. That signal will try to stop the process gracefully in the Linux operating system.

If that fails, try terminating the process with another signal because the signal may have been caught or ignored. You kill a process in Linux with a different signal by defining it using a number (kill -9), with a SIG prefix (kill -SIGkill), or without the SIG prefix (kill -kill).

As you can see, the kill -9 (SIGKILL) command terminated the Java process.

[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 ~]# 
[root@host ~]# ps -eo user,pid,command | grep java
root 66530 grep --color=auto java
[root@host ~]#

Invoke the kill command with the following syntax.

kill [OPTIONS] [PID]

For example, if we find that the process ID for an httpd process is 21567, we can try to kill it gracefully by invoking the following command. It will send the TERM signal.

[root@host ~]# kill 21567 

If this does not stop the process, we can then try to kill it with the signal SIGKILL, which will kill it without first waiting for the process to be properly closed. To do this, you use one of the following commands.

[root@host ~]# kill -9 21567
[root@host ~]# kill -SIGKILL 21567
[root@host ~]# kill -kill 21567

Killall

Additionally, we can kill a process in Linux by using the unique PID, the kill command, or with the process name and the killall command. Apply the same syntax with the killall command, invoke killall [signal] [process_name], as in this example.

killall -9 httpd 

This command will terminate the httpd process in Linux without waiting for it to stop gracefully. 

When determining which process the signal is sent to, it will match the argument name exactly, as in this example.

killall httpd 

The command will send a termination signal to all processes that are exactly named httpd.

If you run the same command with a service that does not exist, it will show an error that there is no process with that name. Since Apache uses httpd as the process name instead of http, the following command would yield such an error.

killall http

Pkill

A command similar to killall is pkill, which also can kill a process in Linux by name. It takes a pattern as an argument and matches it against running processes’ names, so it doesn’t 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.

Skill

The skill command uses the same syntax as the kill command to send termination signals. The default TERM signal is primarily used here.

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

It is recommended to use the kill, pkill, or killall commands instead of skill.

Mysql_zap

Another command that will stop a process in Linux 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 unless another signal is specified. The syntax used for this command is as follows.

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

The mysql_zap command was depreciated in MariaDB as of version 10.2. The alternative is the pkill command.

Mk-kill

Use the mk-kill command to kill MySQL queries that match a specified criterion. If a file is passed to mk-kill, it will read the queries from that file, containing the output of SHOW PROCESSLIST. If there is no file 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 MySQL queries found to be running longer than sixty seconds. The second command checks for any sleeping process (a process that needs resources that are not currently available) every ten seconds and ends any that are found. 

Mkill

Another command related to mk-kill is the mkill command. This command will kill slow queries where found.

It will kill these queries based on the following factors:

  • Query time
  • Host
  • User
  • Database
  • State
  • Query content

The mkill command is still in an alpha version status. 

Tkill and Tgkill

If we would like to kill a specific thread instead of an arbitrary one within an entire process, we can do so with tkill and tgkill commands. The tkill command is the depreciated predecessor to tgkill.

The tkill command only allows for the target thread ID to be specified, leading to the termination of an incorrect thread if that thread ID is recycled (used for another thread. On the other hand, tgkill will signal the thread with the thread ID in the thread group ID (tgid).

Killpg

Another command used to send signals to a process group is killpg. The killpg command will send a signal to a process group (pgrp). If the pgrp is 0, a signal is sent to the calling process group. Otherwise, the killpgrp command does not take any other arguments and kills all processes within the same process group.

Tcpkill

We can also kill specific transmission control protocol (TCP) connections that are in progress with the tcpkill command. Using this command, we can specify a network interface to listen on (as an option) or determine the degree of force (using a number from 1-9) to use in terminating a connection. The default degree of force 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

Lxc-kill

If we need to stop a process in Linux running inside a virtualized container on Ubuntu (e.g., if using Kubernetes or Docker), we can use the lxc-kill command. The 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

Arckill

The arckill command is used to kill running jobs on an ARC-enabled resource. Advanced resource connector (ARC) is a grid computing middleware that provides a common interface for submitting computational tasks to distributed systems. 

You can either use the jobid or the jobname to refer to a job, if the attribute was submitted.

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

Pvm_kill

To terminate a specified PVM process (a message-passing system), we can use the pvm_kill command. It will send a termination signal to a PVM process that is identified by the task identifier (tid). This command is primarily used with a larger program  like so:

  • In C: info = pvm_kill( tid );
  • In Fortran: CALL PVMFKILL( TID, INFO )

Xkill

The xkill command is a graphical way to kill an application. When you enter the command xkill in the terminal on a system with a graphical user interface (GUI), the mouse cursor will change into a plus sign (depending on your icon set). You then left-click an unresponsive window to close it.

The xkill command instructs the xserver to terminate the program in the selected window. Its syntax is as follows.

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

Conclusion

As we can see, there are several ways to terminate a process in Linux. However, one should exercise caution when using these commands to prevent incorrectly terminating a needed process that can kill an application that should not be killed. It could also cause an interruption to an essential service, which could lead to many issues.

With that in mind, always consult the appropriate man pages in the Linux terminal for each command. If you are not completely sure, get in touch with your web hosting support team or system administrator for assistance.

As we continue to move forward into the future, your knowledge should grow to keep up with your competitors. Learn how one of our clients has capitalized on this idea by reading the Rapid Crush Case Study, which details their journey!

Avatar for Ronald Caldwell

About the Author: Ronald Caldwell

Ron is a Technical Writer with Liquid Web working with the Marketing team. He has 8+ years of experience in Technology. He obtained an Associate of Science in Computer Science from Prairie State College in 2015. He is happily married to his high school sweetheart and lives in Michigan with her and their children.

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