What is zombie process in Linux Ubuntu? Learn how to check running process in Ubuntu and how to force kill a zombie process in Ubuntu Linux. A zombie is a process that has been completed, but its entry still remains in the process table.
Zombie Process
On Unix and Unix-like computer operating systems, a zombie process or defunct 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 the 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.
The term zombie process derives from the common definition of zombie — an undead person. In the term’s metaphor, the child process has “died” but has not yet been “reaped”. Also, unlike normal processes, the kill command has no effect on a zombie process.
Zombie processes should not be confused with orphan processes: an orphan process is a process that is still executing, but whose parent has died. When the parent dies, the orphaned child process is adopted by init (process ID 1). When orphan processes die, they do not remain as zombie processes; instead, they are waited on by init. The result is that a process that is both a zombie and an orphan will be reaped automatically.
Kill or Remove a Zombie Process
To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. If the parent process still refuses to reap the zombie, and if it would be fine to terminate the parent process, the next step can be to remove the parent process. When a process loses its parent, init becomes its new parent. init periodically executes the wait system call to reap any zombies with init as parent.
Click on the link below to learn how to kill process in Linux using PID or Process Name:
How To Kill Process in Linux Ubuntu By PID Or Name
How to See Process in Linux Ubuntu
Users can use ps command to see the list of processes in Linux Ubuntu. The ps command report a snapshot of the current processes. ps displays information about a selection of the active processes. If you want a repetitive update of the selection and the displayed information, use top(1) instead.
By default, ps selects all processes with the same effective user ID (euid=EUID) as the current user and associated with the same terminal as the invoker.
It displays the process ID (pid=PID), the terminal associated with the process (tname=TTY), the cumulated CPU time in [DD-]hh:mm:ss format (time=TIME), and the executable name (ucmd=CMD). Output is unsorted by default.
Command to See Process in Linux Ubuntu
1. To see every process on the system using standard syntax:
ps -e
ps -ef
ps -eF
ps -ely
2. To see every process on the system using BSD syntax:
ps ax
ps axu
3. To print a process tree:
ps -ejH
ps axjf
4. To get info about threads:
ps -eLf
ps axms
5. To get security info:
ps -eo euser,ruser,suser,fuser,f,comm,label
ps axZ
ps -eM
6. To see every process running as root (real & effective ID) in user format:
ps -U root -u root u
7. To see every process with a user-defined format:
ps -eo pid,tid,class,rtprio,ni,pri,psr,pcpu,stat,wchan:14,comm
ps axo stat,euid,ruid,tty,tpgid,sess,pgrp,ppid,pid,pcpu,comm
ps -Ao pid,tt,user,fname,tmout,f,wchan
8. Print only the process IDs of syslogd:
ps -C syslogd -o pid=
9. Print only the name of PID 42:
ps -q 42 -o comm=