在 unix 或 unix-like 的系統中，當一個子進程退出后，它就會變成一個僵尸進程，如果父進程沒有通過 wait 系統調用來讀取這個子進程的退出狀態的話，這個子進程就會一直維持僵尸進程狀態。

Zombie process - Wikipedia 中是這樣描述的：

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