Which Process Has PID 0 ? | Baeldung on Linux

1. Overview

As Linux users, we’re familiar with process identifiers (PID). PID is the operating system’s unique identifier for active programs that are running. A simple command to view the running processes shows that the init process is the owner of PID 1. So, the burning question is: Which process has PID 0? In this tutorial, we’ll review the Linux command to check running processes, what the PID 0 is, and the actual process that has PID 0.

2. How to Check Running Processes in Linux

There are a number of commands to check the running process in Linux. One of these is the ps command :

$ ps -eaf
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 Feb25 ?        00:00:05 /sbin/init splash
root           2       0  0 Feb25 ?        00:00:00 [kthreadd]
root           3       2  0 Feb25 ?        00:00:00 [rcu_gp]
root           4       2  0 Feb25 ?        00:00:00 [rcu_par_gp]
root           9       2  0 Feb25 ?        00:00:00 [mm_percpu_wq]
root          10       2  0 Feb25 ?        00:00:00 [rcu_tasks_rude_]
The ps command displays the current active running processes. From the output above, we can see that the /sbin/init splash process has PID 1, the kthreadd process has PID 2, and so on.
We pass the -eaf options into the ps command to display all the processes on the system as a list. The ea option displays all processes while the f stands for full listing. The ps man page contains other possible arguments that we can use with the ps command.
2.1. What Is PPID 0?
We observe from the output above that each PID has a corresponding PPID. For instance, /sbin/init splash has PPID 0. PPID stands for Parent Process ID. In Linux systems, A parent process ID is always assigned to every process ID. It tells us which process started a particular process. Therefore, the PPID value of 0 for the init process indicates that the init process has no parent.

2.2. What Is PID 0? 
Sometimes, in system design, a zero input to the actuator corresponds to an off state or position. However, when we insert a 0 PID  into a PID controller, it corresponds to doing nothing. No error signal would cause any correction. However, zero is not very useful as a PID in a controller, but technically, it’s valid. Hence, the PID 0 is often referred to as an idle process.
2.3. What Process Has PID 0? 
The process with PID 0 is responsible for paging, and this process is always referred to as the swapper or sched process. This process is a part of the kernel and is not a regular user-mode process. The init process owns PID 1 and is solely responsible for starting and shutting down the system. 
Originally, process ID 1 was not specifically reserved for init by any technical measures: It simply had this ID as a natural consequence of being the first process that the kernel invoked. Furthermore, more recent Unix systems typically have additional kernel components visible as “processes”. In these systems, PID 1 is actively reserved for the init process to maintain consistency with older systems.
3. Conclusion
In this tutorial, we’ve established that the process that has PID 0 is not a user process but the process that the kernel uses for paging. In other words, it’s the swapper or sched process, which helps the operating system store and retrieve data from a secondary storage device.