Thread by @devops_tech

What are different Process states in Linux!

What is a Zombie Process!

How to Clean a Linux Zombie Process!

A Thread 👇

What Is a Zombie Process?

Zombie processes in Linux are sometimes also referred to as defunct or dead processes. They’re processes that have completed their execution, but their entries are not removed from the process table.

What are different Process States?

Linux maintains a process table of all the processes running, along with their states. Let’s briefly overview the various process states:

Running (R): These processes are currently running or runnable.

Waiting (S/D): These are the processes that are waiting for an event or a resource. The wait can either be an interruptible sleep (S) or an uninterruptible sleep (D).

Stopped (T): We can stop a Linux process by sending an appropriate signal.

Zombie (Z): When a process finishes its task, it releases the system resources it was using and cleans up its memory. However, its entry from the process table is not removed, and its status is set as EXIT_ZOMBIE.

How Zombie Processes gets created?

When a process completes its job, the Linux kernel notifies the exiting process’s parent by sending the SIGCHLD signal. The parent then executes the wait() system call to read the status of the child process and reads its exit code.

This also cleans up the entry of the child process from the process table, and hence, the process finishes.

However, if a parent process isn’t programmed to execute the wait() system call on the creation of the child process, proper cleanup doesn’t happen.

In such cases, the parent process cant monitor the state changes of the child processes, and eventually, it ignores the SIGCHLD signal. This causes the zombie state of the finished process to stay in the process table, and hence it appears in the process list as a zombie process.

Another case of interest is when a parent process is unable to handle or receive the SIGCHLD signal from the child process. Such cases also lead to zombie creation.

How to Identify Zombie Processes?

We can identify the list of zombies using the ps command:

As observed from the output, the Z in the STAT column or zombie or <defunct> pattern from the output of the ps command can be used to identify the zombies.

$ ps aux | egrep "Z|defunct"

We can find out Z process state using the awk command as well:

$ ps ux | awk '{if($8=="Z") print}'

How to clean a Zombie Process?

You can’t really kill a zombie process since it’s already dead. However, there are a few workarounds we can use to clean up a zombie process.

Method 1 -
Using SIGCHLD Signal -

We can manually send the SIGCHLD signal to d parent of a zombie process. Consequently, it'll intimate d parent to trigger d wait() system call, which will clean up d defunct child process from d process table.

Let’s find d parent id of our defunct process:
$ ps -A -ostat,pid,ppid | grep -e '[zZ]'

Next, let’s send the SIGCHLD signal to d parent process using d kill cmd:

$ kill -s SIGCHLD 103

However, it isn’t really guaranteed that sending the SIGCHLD signal to the parent will kill a zombie process. It works only in cases where parent processes can handle the SIGCHLD signals.

Method 2 -
Killing d Parent Process -

If d method discussed in d previous section is unable to clear d defunct process, we should consider killing its parent process:

$ kill -9 103

Here, 103 is the parent id of our defunct process with PID 108.

However, killing a parent can affect all its childs. Hence, we shud exercise extra caution & must identify d impact before killing a parent process.

If der are a lot of Z processes,or if d parent of d Z process is d init process (with pid=1),we can also consider a system reboot.

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PC: baeldung