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A process who's parent process has exited abnormally (perhaps killed?) in a way that did not allow the child process(es) to exit as well.
Zombie process appear in the list of running processes as "defunct". It should normally disappear after a while but if they didn't, then a reboot may be required.
Zombie process is a process which completes its execution but still remains in the process table.when there parents are killed init process become there parent processes which waits for there finish. they are also called dead processes which do not take resource.
Zombies is orphan process which means process with no parent process "PID without PPID" and the most common problem regarding such processing is they keep consume your server process , to find your process and check if you have zombies or not just execute top command and check if there is flagged with Z letter or not
When a process finishes execution, it will have an exit status to report to its parent process. Because of this last little bit of information, the process will remain in the operating system’s process table as a zombie process, indicating that it is not to be scheduled for further execution, but that it cannot be completely removed (and its process ID cannot be reused) until it has been determined that the exit status is no longer needed.
When a child exits, the parent process will receive a SIGCHLD signal to indicate that one of its children has finished executing; the parent process will typically call the wait() system call at this point. That call will provide the parent with the child’s exit status, and will cause the child to be reaped, or removed from the process table.
the zombi process is the process that was running and create d by parent process and after that the parent process was dead and the child stel alive
A zombie process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the "Terminated state".
This occurs for child processes, where the entry is still needed to allow the parent process to read its child's exit status: once the exit status is read via the wait system call, the zombie's entry is removed from the process table and it is said to be "reaped".
A child process always first becomes a zombie before being removed from the resource table.
In most cases, under normal system operation zombies are immediately waited on by their parent and then reaped by the system – processes that stay zombies for a long time are generally an error and cause a resource leak.
if the excuation of process is done but it is still showing in the process list then such process is called zombie process.
Zombie processes don’t use up any system resources. (Actually, each one uses a very tiny amount of system memory to store its process descriptor.) However, each zombie process retains its process ID (PID). Linux systems have a finite number of process IDs –7 by default on-bit systems. If zombies are accumulating at a very quick rate – for example, if improperly programmed server software is creating zombie processes under load — the entire pool of available PIDs will eventually become assigned to zombie processes, preventing other processes from launching
Keep in mind, you can not kill who is already dead, same with the Zombiee process, it cannot be killed by normal kill command because this process is already dead so you need SILVER or HOLY WATER to kill zombies and so use this command kill -s SIGCHLD pid
hope it is clear now
A child process that has exited normally, but still alive due to the parent not acknowledging it's exit because Linux allows the parent process to read it's exit status.
a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the "Terminated state".
A zombie is already dead, so you cannot kill it. To clean up a zombie,it must be waited on by its parent, so killing the parent should work to eliminate the zombie.
Those processes which are running in background and working unnecessary in the system and using resources of the system.
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