Kill or Terminate a Process in Linux

The kill command sends the specified signal such as kill process to the specified process or process groups. If no signal is specified, the TERM signal is sent. Please note that kill command can be internal as part of modern shells built-in function or external located at /bin/kill. Usage and syntax remain similar regardless internal or external kill command.

A list of common Term signals

Linux and Unix-like operating system supports the standard terminate signals listed below:

• SIGHUP (1) – Hangup detected on controlling terminal or death of controlling process. Use SIGHUP to reload configuration files and open/close log files.
• SIGKILL (9) – Kill signal. Use SIGKILL as a last resort to kill process. This will not save data or cleaning kill the process.
• SIGTERM (15) – Termination signal. This is the default and safest way to kill process.

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 nginx

OR use the combination of ps command and grep command:

ps aux | grep httpd
ps aux | grep apache2
ps aux | grep  nginx

kill command syntax

The syntax is:

kill [signal] PID
kill -15 PID
kill -9 PID
kill -SIGTERM PID
kill [options] -SIGTERM PID

What Linux or Unix permissions do I need to kill a process?

Rules are simple:

• You can kill all your own process.
• Only root user can kill system level process.
• Only root user can kill process started by other users.

kill command examples

In this example, I am going to kill lighttpd server. Step #1: Find out the PID (process id)

Use the ps or pidof command to find out PID for any program. For example, if process name is lighttpd, you can use any one of the following command to obtain process ID:

pidof lighttpd

Sample outputs:

3486

OR

ps aux | grep lighttpd

Sample outputs:

lighttpd  3486  0.0  0.1   4248  1432 ?        S    Jul31   0:00 /usr/sbin/lighttpd -f /etc/lighttpd/lighttpd.conf
lighttpd  3492  0.0  0.5  13752  3936 ?        Ss   Jul31   0:00 /usr/bin/php5-cg

Step #2: kill the process using a PID

The PID # 3486 is assigned to the lighttpd process. To kill the lighttpd server, you need to pass a PID as follows:

# kill 3486

OR

$ sudo kill 3486

This will terminate a process with a PID of 3486. How do I verify that the process is gone / killed?

Use the ps or pidof command:

$ ps aux | grep lighttpd
$ pidof lighttpd

A note about sending stronger signal # 9 (SIGKILL)

If no signal specified in the kill command, signal # 15 (SIGTERM), is sent by default. So the kill 3486 command is same as the following command:

# kill -15 3486
# kill -SIGTERM 3486

OR

$ sudo kill -15 3486
$ sudo kill -SIGTERM 3486

Sometime signal # 15 is not sufficient. For example, lighttpd may not be killed by signal #15 due to open sockets. In that case process (PID) # 3486 would be killed with the powerful signal # 9:

# kill -9 3486
# kill -SIGKILL 3486

OR

$ sudo kill -9 3486
$ sudo kill -SIGKILL 3486

Where,

-9 or -SIGKILL – A special kill signal that nearly guarantee to kill the process with the iron fist.

How can I kill two or more PIDs?

The syntax is as follows to kill two or more PIDs as required can be used in a single command:

kill  pid1 pid2 pid3
kill -15  pid1 pid2 pid3
kill -9  pid1 pid2 pid3
kill  -9 3546 5557 4242

The killall command

This is a Linux only command. to kill processes by name. So no need to find the PIDs using the ‘pidof process’ or ‘ps aux | grep process’ command. Do not use killall command on Unix operating systems. This is a Linux specific command. The syntax is

killall Process-Name-Here

To kill the lighttpd server, enter:

# killall -15 lighttpd

OR

# killall -9 lighttpd

To kill the Firefox web-browser process, enter:

# killall -9 firefox-bin

The killall command on UNIX-like system does something else. It kills all process and not just specific process. Do not use killall on UNIX system.