01-09-2017
Or to add a little to MadeInGermany's answer -
SIGKILL prevents normal process rundown. It can leave open files in bad condition, for example. You should use it only a last resort when you cannot force a process to exit. In some version of UNIX there are processes you cannot kill.
ps -u in Linux shows a column "stat".
Quote:
Status codes for Linux as of 2.6 kernel AFAIK.
Stat Meaning
D Uninterruptible sleep (usually IO)
R Running or runnable (on run queue)
S Interruptible sleep (waiting for an event to complete)
T Stopped, either by a job control signal or because it is being traced.
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z Defunct (“zombie”) process, terminated but not reaped by its parent
D means the process cannot be killed while in the "D" status.
"Z" gets people upset completely. It means all of the processes' resources except the information kept in the kernel process header are gone. It will NOT do anything except reduce the number of total process slots by one. So if your system is configured for 65535 processes maximum, for example, then you just lost one until the "Z" process goes away. This is the result of poor programming practices - exiting and not waiting for child processes to terminate and be 'reaped'.
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preap(1) User Commands preap(1)
NAME
preap - force a defunct process to be reaped by its parent
SYNOPSIS
preap [-F] pid...
DESCRIPTION
A defunct (or zombie) process is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
happen if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
to PID 1. See init(1M), which periodically reaps such processes.
An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
do consume a small amount of system memory.
preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.
preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:
o The process is a child of init(1M).
o The parent process is stopped and might wait on the child when it is again allowed to run.
o The process has been defunct for less than one minute.
OPTIONS
The following option is supported:
-F Forces the parent to reap the child, overriding safety checks.
OPERANDS
The following operand is supported:
pid Process ID list.
EXIT STATUS
The following exit values are returned by preap, which prints the exit status of each target process reaped:
0 Successfully operation.
non-zero Failure, such as no such process, permission denied, or invalid option.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Availability |SUNWesu (32-bit) |
+-----------------------------+-----------------------------+
| |SUNWesxu (64-bit) |
+-----------------------------+-----------------------------+
SEE ALSO
proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)
WARNINGS
preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes will
not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.
SunOS 5.10 26 Mar 2001 preap(1)