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This article will first discuss the different concepts related to Linux processes. After this, we will learn the different ways to terminate a process.

Before going into the kill command, we must understand some preliminary concepts.

Simple procedures and Bash procedures

An active or running program in an operating system is called a process. For example, when we run an editor or a browser, the operating system creates a process for each.

On the other hand, a simple executable file that is run in the system through Bash is called a Bash process. For example, whenever we run any application like calculator through Bash shell, a new Bash process is created.

Process Identifier (PID)

The unique number or identifier assigned to each running process is called PID (Process Identifier). The ps command displays all running processes and their PIDs.

1. The ps command displays the PIDs of all running processes. For example:

   

2. The pidof command also displays the PID of the running process. We can find the PID of a running process using the process name.

   

   The above figure shows that a sleep command is executed in the background, and the pidof command is used to find the PID of the process.
3. The pgrep command also displays the PID of the running process. We can grep the PID by process name using the pgrep command.

   

   The above figure shows that a sleep command is executed in the background, and the pgrep command is used to find the PID of the process.

Terminating a Process in Bash

There are many options in Bash to terminate a running process. Here are some of the options you can use:

Use the Ctrl+C signal

We can interrupt or terminate the running process and send SIGINT using Ctrl+C keys. Ctrl+C sends an interrupt signal and the running program is killed or the running program is interrupted.

The above image shows the sleep 100 command being interrupted using Ctrl+C. When we use Ctrl+C to terminate or interrupt a running program, the exit code is 130 and we can use echo $?the command.

Using the kill command

We can killkill a running process using the kill command. To kill a running process, we need the PID of the process.

Following is killthe syntax of the command:

kill <pid>

The kill command uses the PID of any process to kill it. We can find the PID of any process using ps, pidof or pgrep commands.

The above image shows a process with PID 14857 being killed by the kill command.

We can also kill any process using the process name instead of its PID. pkill or killall command can kill any process with a name.

Following is the syntax of pkill and killall commands:

pkill <name>
killall<name>

All the above kill commands will send SIGKILL to the running process. If any command hangs the running process, we need to specify the signal number or signal.

The above figure shows a kill -9process killed using the command. We can use the kill -SIGKILL <PID>or kill -9 <PIDcommand to send a termination signal number or termination signal, where 9 is the SIGKILL number.

We can also use SIGKILL or 9 with the -h pkillor -h killallcommand.

The above image shows the use of pkill -9the command to kill a sleeping process.