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[#]: subject: "How to kill a zombie process on Linux"
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[#]: via: "https://opensource.com/article/21/10/linux-zombie-process"
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[#]: author: "Anderson Silva https://opensource.com/users/ansilva"
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[#]: collector: "lujun9972"
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[#]: translator: "wxy"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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How to kill a zombie process on Linux
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======
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To kill a zombie process, you must remove its name from the process
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list.
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![Full moon on a hazy night][1]
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Happy Halloween Open SOURCE-rers!
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Here's a tale as old as _epoch_ time. Since there has been C and Unix, and (later on) Linux, we've had _zombies_. Specifically, there are processes that get marked as _a zombie process_. Misunderstood by some, ignored by others, and immune to the efforts of so many of us trying to `kill` these processes without much success. Why is that?
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### What is a process in Linux?
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It all begins when a program in Linux gets executed, and when it does, its running instance is called a process. You can see all processes on your Linux environment with the [`ps` command][2].
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```
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$ ps -ax
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PID TTY STAT TIME COMMAND
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1 ? Ss 0:01 /usr/lib/systemd/systemd rhgb --switched-root --sys
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2 ? S 0:00 [kthreadd]
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3 ? I< 0:00 [rcu_gp]
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4 ? I< 0:00 [rcu_par_gp]
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```
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Sometimes a process starts another process, making the first process the parent of the second. The `pstree` command is a great tool that allows you to see the processes' "genealogy" on your system.
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```
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$ pstree -psn
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systemd(1)─┬─systemd-journal(952)
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├─systemd-udevd(963)
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├─systemd-oomd(1137)
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├─systemd-resolve(1138)
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├─systemd-userdbd(1139)─┬─systemd-userwor(12707)
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│ ├─systemd-userwor(12714)
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│ └─systemd-userwor(12715)
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├─auditd(1140)───{auditd}(1141)
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├─dbus-broker-lau(1164)───dbus-broker(1165)
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├─avahi-daemon(1166)───avahi-daemon(1196)
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├─bluetoothd(1167)
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```
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Every process gets assigned a number in the system. Process ID number 1 gets assigned to the very first process executed during the boot process, and every subsequent process after PID 1 is a descendant of it. The PID 1 process is the _init_, which on most newer versions of Linux is just a symbolic link to the `systemd` program.
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### Ending a process with the kill command
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You can terminate processes in a Linux system with the `kill`* _command_. *Despite the name, the `kill` command and a set of others such as `pkill` and `killall` got written/designed to send SIGNALS to one or more processes. When not specified, the default SIGNAL it sends is the SIGTERM signal to terminate the process.
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When a parent process dies or gets killed, and its child process doesn't follow its parent's demise, we call that process an _orphan process_.
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### How to kill a zombie process
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Zombie processes, on the other hand, cannot be killed! Why might you ask? Well, because they are already dead!
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Every child process, when terminated, becomes a zombie process and then removed by the parent. When the process exits its existence and releases the resources it had used, its name is still on the OS process table. It is then the parent's process job to remove its name from the process table. When that fails, we have the zombie process, which isn't really a process anymore, but just an entry on the process table of the OS.
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This is why trying to do a `kill` command even with the `-9` (SIGKILL) option on a defunct (zombie) process doesn't work, because there is nothing to kill.
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So, to kill a zombie process, as in to remove its name from the process list (the process table), you have to kill its parent. For instance, if PID 5878 is a zombie process, and its parent is PID 4809, then to kill the zombie (5878) you end the parent (4809):
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```
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`$ sudo kill -9 4809 #4809 is the parent, not the zombie`
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```
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My final word of warning about zombies. Be very careful when killing parent processes. If the parent of a process is PID 1 and you kill that, you'll reboot yourself!
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And that will be an even scarier story to tell!
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/10/linux-zombie-process
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作者：[Anderson Silva][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/ansilva
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/ganapathy-kumar-unsplash.jpg?itok=5-DT99NA (Full moon on a hazy night)
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[2]: https://opensource.com/article/21/8/linux-procps-ng
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@ -0,0 +1,91 @@
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[#]: subject: "How to kill a zombie process on Linux"
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[#]: via: "https://opensource.com/article/21/10/linux-zombie-process"
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[#]: author: "Anderson Silva https://opensource.com/users/ansilva"
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[#]: collector: "lujun9972"
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[#]: translator: "wxy"
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[#]: reviewer: "wxy"
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[#]: publisher: " "
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[#]: url: " "
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如何在 Linux 上杀死一个僵尸进程
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======
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> 要杀死一个僵尸进程，你必须从进程列表中删除其名称。
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万圣节快乐！
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这里有一个和 Unix 纪元一样古老的故事。自从有了 C 和 Unix，以及（后来的）Linux，我们就有了“<ruby>僵尸<rt>zombie</rt></ruby>”。具体来说，有一些进程被标记为“<ruby>僵尸进程<rt>zombie process</rt></ruby>”。一些人误解它，另一些人忽视它，而它对我们许多人试图 “杀死” 这些进程的努力免疫，没有什么成效。这是为什么呢？
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### Linux 中的进程是什么？
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这一切始于执行 Linux 中的一个程序时，当它被执行时，它的运行实例被称为进程。你可以用 [ps 命令][2] 查看你的 Linux 环境中的所有进程：
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```
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$ ps -ax
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PID TTY STAT TIME COMMAND
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1 ? Ss 0:01 /usr/lib/systemd/systemd rhgb --switched-root --sys
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2 ? S 0:00 [kthreadd]
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3 ? I< 0:00 [rcu_gp]
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4 ? I< 0:00 [rcu_par_gp]
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```
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有时一个进程启动另一个进程，使第一个进程成为第二个进程的父进程。`pstree` 命令是一个很好的工具，可以让你看到系统中进程的“家谱”：
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```
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$ pstree -psn
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systemd(1)─┬─systemd-journal(952)
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├─systemd-udevd(963)
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├─systemd-oomd(1137)
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├─systemd-resolve(1138)
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├─systemd-userdbd(1139)─┬─systemd-userwor(12707)
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│ ├─systemd-userwor(12714)
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│ └─systemd-userwor(12715)
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├─auditd(1140)───{auditd}(1141)
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├─dbus-broker-lau(1164)───dbus-broker(1165)
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├─avahi-daemon(1166)───avahi-daemon(1196)
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├─bluetoothd(1167)
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```
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每个进程在系统中都会被分配一个编号。进程编号（PID）1 被分配给启动过程中执行的第一个进程，PID 1 之后的每一个进程都是它的子孙。PID 1 进程是初始化进程，在大多数新版本的 Linux 中，它只是一个指向 `systemd` 程序的符号链接。
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### 用 kill 命令结束一个进程
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你可以用 `kill` 命令来终止 Linux 系统中的进程。尽管名字叫 “杀死”，但 `kill` 命令和其他一些命令，如 `pkill` 和 `killall`，都是为向一个或多个进程发送信号而编写/设计的。当没有指定信号时，它发送的默认信号是 `SIGTERM` 信号，以终止进程。
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当一个父进程死亡或被杀死，而其子进程没有跟随其父进程的死亡，我们称该进程为“<ruby>孤儿进程<rt>orphan process</rt></ruby>”。
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### 如何杀死一个僵尸进程
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另一方面，僵尸进程是不能被“杀死”的！你可能会问为什么？因为它们已经死了！
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每一个子进程，当被终止时，都会成为一个僵尸进程，然后被父进程删除。当进程退出并释放它所使用的资源时，它的名字仍然在操作系统的进程表中。这时，父进程的工作就是把它的名字从进程表中删除。如果没有删除，我们就有了僵尸进程，它不再是一个真正的进程，而只是操作系统进程表上的一个条目。
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这就是为什么试图对一个<ruby>失效的<rt>defunct</rt></ruby>（僵尸）进程执行 “杀死” 命令，即使使用 `-9`（`SIGKILL`）选项也不起作用，因为没有什么可以杀死的。
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因此，要杀死一个僵尸进程，比如从进程列表（进程表）中删除其名称，你必须杀死其父进程。例如，如果 PID 5878 是一个僵尸进程，而它的父进程是 PID 4809，那么要杀死僵尸进程（5878），就必须结束父进程（4809）：
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```
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$ sudo kill -9 4809 # 4809 是父进程，而非僵尸
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```
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我对僵尸的最后一句警告：在杀死父进程时要非常小心。如果一个进程的父进程是 PID 1，而你杀了它，你就会重新启动自己！
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而这将是一个更可怕的故事！
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/10/linux-zombie-process
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作者：[Anderson Silva][a]
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选题：[lujun9972][b]
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译者：[wxy](https://github.com/wxy)
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校对：[wxy](https://github.com/wxy)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/ansilva
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/ganapathy-kumar-unsplash.jpg?itok=5-DT99NA (Full moon on a hazy night)
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[2]: https://opensource.com/article/21/8/linux-procps-ng
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