What is Orphan Process?

It’s a pretty much common question asked in most of the interviews related to Linux, and most of the time people got it confused with Zombie Process. But these two are totally different from each other. An Orphan Process is nearly the same thing which we see in real world. Orphan means someone whose parents are dead. The same way this is a process, whose parents are dead, that means parents are either terminated, killed or exited but the child process is still alive.

In Linux/Unix like operating systems, as soon as parents of any process are dead, re-parenting occurs, automatically. Re-parenting means processes whose parents are dead, means Orphaned processes, are immediately adopted by special process “init”. Thing to notice here is that even after re-parenting, the process still remains Orphan as the parent which created the process is dead,

Reasons for Orphan Processes:

A process can be orphaned either intentionally or unintentionally. Sometime a parent process exits/terminates or crashes leaving the child process still running, and then they become orphans.

Also, a process can be intentionally orphaned just to keep it running. For example when you need to run a job in the background which don’t need any manual intervention and going to take long time, then you detach it from user session and leave it there. Same way, when you need to run a process in the background for infinite time, you need to do the same thing. Processes running in the background like this are known as daemon process.

At the same time, when a client connects to a remote server and initiated a process, and due to some reason the client crashes unexpectedly, the process on the server becomes Orphan.

Finding a Orphan Process:

It’s very easy to spot a Orphan process. Orphan process is a user process, which is having init (process id – 1) as parent. You can use this command in linux to find the Orphan processes.

# ps -elf | head -1; ps -elf | awk '{if ($5 == 1 && $3 != "root") {print $0}}' | head

This will show you all the orphan processes running in your system. The output from this command confirms that they are Orphan processes but doesn’t mean that they are all useless, so confirm from some other source also before killing them.

Killing a Orphan Process:

As orphaned processes waste server resources, so it’s not advised to have lots of orphan processes running into the system. To kill a orphan process is same as killing a normal process.

# kill -15 <PID>

If that don’t work then simply use

# kill -9 <PID>

FAQs:

Q. Is Orphan process different from an Zombie process ?

A. Yes, Orphan process are totally different from Zombie processes. Zombie processes are the ones which are not alive but still have entry in parent table. For more details, please refer to — Zombie Processes.

Q. Are Orphan processes harmful for system ?

A. Yes. Orphan processes take resources while they are in the system, and can potentially leave a server starved for resources. Having too many Orphan processes will overload the init process and can hang-up a Linux system. We can say that a normal user who has access to your Linux server is capable to easily kill your Linux server in a  minute.

References:

  1. http://en.wikipedia.org/wiki/Orphan_process
  2. http://en.wikipedia.org/wiki/Init
  3. http://en.wikipedia.org/wiki/Daemon_(computer_software)
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