僵屍程序概念
僵屍程序(zombie process)通俗來說指那些雖然已經終止的程序，但仍然保留一些資訊，等待其父程序為其收屍.
書面形式一點：一個程序結束了，但是他的父程序沒有等待(調用wait /
waitpid)他，那麼他将變成一個僵屍程序。通過ps指令檢視其帶有defunct的标志。僵屍程序是一個早已死亡的程序，但在程序表
(processs table)中仍占了一個位置(slot)。
但是如果該程序的父程序已經先結束了，那麼該程序就不會變成僵屍程序。因為每個程序結束的時候，系統都會掃描目前系統中所運作的所有程序，看看有沒有哪
個程序是剛剛結束的這個程序的子程序，如果是的話，就由init程序來接管他，成為他的父程序，進而保證每個程序都會有一個父程序。而init程序會自動
wait其子程序，是以被init接管的所有程序都不會變成僵屍程序
與zombie對應的程序狀态還有running（正在運作或等待運作狀态），uninterruptable（不可中斷阻塞狀态），interruptable（可中斷阻塞狀态），stopped（挂起狀态）等。
關于僵屍程序的維基百科介紹：
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 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.
these do not remain as zombie processes; instead, (like all orphaned
processes) they are adopted by init (process id 1), which waits on its
children. the result is that a process that is both a zombie and an
orphan will be reaped automatically.
僵屍程序檢視
檢視系統裡面有那些僵屍程序，有很多方法，例如top指令，ps指令等
另外，使用ps和grep指令結合也能檢視僵屍程序，當然有非常多的形式，如下所。
檢視僵屍程序的個數指令
僵屍程序清除
僵屍程序的清除有時候是一個頭痛的問題，僵屍程序有時候很殺不掉，有時候還不能亂殺。
要殺掉僵屍程序，一般有兩個方法：
1：找到該defunct僵屍程序的父程序，将該程序的父程序殺掉，則此defunct程序将自動消失
2：重新開機伺服器。
檢視僵屍程序并殺掉
一般情況下，不建議莽撞的kill掉這些僵屍程序，還是檢查一下具體原因後，根據具體情況再做清除，如下所示。
檢視僵屍程序的父程序，發現是對應的是oracle裡面pspo程序，關于這個程序，我也沒有把握是否可以kill掉。是以選擇重新開機伺服器比較保險一點。