Kill Commands in Linux is to terminate any unresponsive process.
As part of our Server Management Services, we assist our customers with several Linux queries.
Today, let us focus on how to simply kill the process when it becomes unresponsive or fails to respond.
What Is Process?
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
[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 1 533 564 743 66153
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?
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:
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
[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.
[Want to know more? Contact us now]
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
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
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.
The command will send a termination signal to all processes that are exactly named “
Instead, if we try to run this command:
It will show an error that there is no process with that name since Apache uses “
httpd” and not “
A command similar to
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:
Although this is not the exact process name, it will find the processes that contain this pattern and send the terminate signal to
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
Our Support Techs suggest using the
killall commands instead.
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
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.
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.
If we want to kill a specific thread, instead of an arbitrary thread within an entire process, we can do so with
We should note that the
tkill command should avoid as it is the depreciated predecessor to
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
It is also worth noting that several commands are to send signals to a process group. One of those commands is
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.
killpgrp command does not take any other arguments so that it will kill all processes within the same process group.
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
The following command is the syntax of
[root@host ~]# tcpkill [-i interface] [-1…9] expression [root@host ~]# tcpkill -i eth0 port 21
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
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#>)
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
We use this command with a larger program:
In C: info = pvm_kill( tid ); In Fortran: CALL PVMFKILL( TID, INFO )
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]
[Finding it hard to execute a command? We’d be happy to assist]
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:
- If the PID is greater than zero, then the signal sends to the process with that PID.
- 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.
- 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.
- 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.
[Confused with the rules? We’ll make it easy for you]
To conclude, there are several ways to terminate a process. However, one should exercise caution to prevent incorrectly terminating a process that should not be killed. Get in touch with our Support Engineers who will happily assist you in matters of Kill Commands In Linux.