殭屍進程參照百度百科，及其部分論壇內容。
In UNIX System terminology, a process that has terminated,but whose parent has not yet waited for it, is called a zombie. 在UNIX 系統中，一個進程結束了，但是他的父進程沒有等待(調用wait
/ waitpid)他， 那麼他將變成一個殭屍進程。
如果該進程的父進程已經先結束，那麼該進程就不會變成殭屍進程， 因爲系統進程Init 會自動接管他，成爲他的父進程。
殭屍進程（Zombie）可以認爲只是一種數據結構，包括進程號the
process ID，退出狀態the termination status of the process，運行時間the amount of CPU time taken by the process等。進程結束時，內核釋放該進程佔用所有的資源，包括打開的文件，佔用的內存等，但是唯獨沒有釋放標識進程自身狀態信息的結構（PCB），即殭屍進程。
殭屍進程產生的原因：
UNⅨ提供了一種機制可以保證只要父進程想知道子進程結束時的狀態信息，
就可以得到。這種機制就是： 在每個進程退出的時候，內核釋放該進程所有的資源，包括打開的文件，佔用的內存等。但是仍然爲其保留一定的信息（包括進程號the
process ID，退出狀態the termination status of the process，運行時間the amount of CPU time taken by the process等）。直到父進程通過wait
/ waitpid來取時才釋放. 但這樣就導致了問題，如果進程不調用wait / waitpid的話，那麼保留的那段信息就不會釋放，其進程號就會一直被佔用，從而產生殭屍進程。
系統所能使用的進程號是有限的，如果大量的產生僵死進程，將因爲沒有可用的進程號而導致系統不能產生新的進程.
此即爲殭屍進程的危害，應當避免。
註釋：
SIGCHLD屬於unix以及類unix系統的一種信號。
在一個進程終止或者停止時，將SIGCHLD信號發送給其父進程。按系統默認將忽略此信號。如果父進程希望被告知其子系統的這種狀態，則應捕捉此信號。信號的捕捉函數中通常調用wait函數以取得進程ID和其終止狀態。
備註：
kill command
has no effect on a zombie process.
When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. The parent can read the child's exit status by executing the wait system
call, whereupon the zombie is removed. The wait call may be executed in sequential code, but it is commonly executed in a handler for
the SIGCHLD signal,
which the parent receives whenever a child has died.
After the zombie is removed, its process identifier (PID) and entry in the process table can
then be reused. However, if a parent fails to call wait, the zombie will be left in the process table. In some situations this may be desirable, for example if the parent
creates another child process it ensures that it will not be allocated the same PID. On modern UNIX-like systems (that comply with SUSv3 specification
in this respect), the following special case applies: if the parent explicitly ignores SIGCHLD by setting its handler to SIG_IGN (rather than simply ignoring the
signal by default) or has the SA_NOCLDWAIT flag set, all child exit status information will be discarded and no zombie processes will be left.
A zombie process is not the same as an orphan process. An orphan process is a process that is still
executing, but whose parent has died. They do not become zombie processes; instead, they are adopted by init (process
ID 1), which waits on its children.
Zombies can be identified in the output from the Unix ps command by
the presence of a "Z" in the "STAT" column.[1] Zombies
that exist for more than a short period of time typically indicate a bug in the parent program, or just an uncommon decision to reap children (see example). If the parent program is no longer running, zombie processes typically indicate a bug in the operating
system. As with other leaks, the presence of a few zombies is not worrisome in itself, but may indicate a
problem that would grow serious under heavier loads. Since there is no memory allocated to zombie processes except for the process table entry itself, the primary concern with many zombies is not running out of memory, but rather running out of process ID
numbers.
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, the next step would 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.