Linux中进程在后台可靠运行的方法总结
<p>这篇文章主要给大家在讲述了在Linux中让一个进程在后台稳定运行的技巧和方法，总结如下：</p>
<p>
我们经常会碰到这样的问题，用 telnet/ssh 登录了远程的 Linux 服务器，运行了一些耗时较长的任务， 结果却由于网络的不稳定导致任务中途失败。如何让命令提交后不受本地关闭终端窗口/网络断开连接的干扰呢？下面举了一些例子， 您可以针对不同的场景选择不同的方式来处理这个问题。</p>
<p>
nohup/setsid/&</p>
<p>
<strong>场景：</strong></p>
<p>
如果只是临时有一个命令需要长时间运行，什么方法能最简便的保证它在后台稳定运行呢？</p>
<p>
<strong>解决方法：</strong></p>
<p>
我们知道，当用户注销（logout）或者网络断开时，终端会收到 HUP（hangup）信号从而关闭其所有子进程。因此，我们的解决办法就有两种途径：要么让进程忽略 HUP 信号，要么让进程运行在新的会话里从而成为不属于此终端的子进程。</p>
<p>
<strong>hangup 名称的来由</strong></p>
<p>
<strong>在 Unix 的早期版本中，每个终端都会通过 modem 和系统通讯。当用户 logout 时，modem 就会挂断（hang up）电话。 同理，当 modem 断开连接时，就会给终端发送 hangup 信号来通知其关闭所有子进程。</strong></p>
<p>
<span><strong>1. nohup</strong></span></p>
<p>
nohup 无疑是我们首先想到的办法。顾名思义，nohup 的用途就是让提交的命令忽略 hangup 信号。让我们先来看一下 nohup 的帮助信息：</p>
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<code class="xhtml plain">NOHUP(1) User Commands NOHUP(1)</code>
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<code class="xhtml plain">NAME</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">nohup - run a command immune to hangups, with output to a non-tty</code>
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<code class="xhtml plain">SYNOPSIS</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">nohup COMMAND ...</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">nohup OPTION</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml plain">DESCRIPTION</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">Run COMMAND, ignoring hangup signals.</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">--help display this help and exit</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">--version</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">output version information and exit</code>
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<p>
可见，nohup 的使用是十分方便的，只需在要处理的命令前加上 nohup 即可，标准输出和标准错误缺省会被重定向到 nohup.out 文件中。一般我们可在结尾加上"&"来将命令同时放入后台运行，也可用">filename 2>&1"来更改缺省的重定向文件名。</p>
<p>
nohup 示例</p>
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<code class="xhtml plain"># nohup ping www.ibm.com &</code>
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<code class="xhtml plain"> 3059</code>
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<code class="xhtml plain">nohup: appending output to `nohup.out'</code>
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<code class="xhtml plain"># ps -ef |grep 3059</code>
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<code class="xhtml plain">root 3059 984 0 21:06 pts/3 00:00:00 ping www.ibm.com</code>
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<code class="xhtml plain">root 3067 984 0 21:06 pts/3 00:00:00 grep 3059</code>
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<code class="xhtml plain">#</code>
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<p>
<span><strong>2、setsid</strong></span></p>
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nohup 无疑能通过忽略 HUP 信号来使我们的进程避免中途被中断，但如果我们换个角度思考，如果我们的进程不属于接受 HUP 信号的终端的子进程，那么自然也就不会受到 HUP 信号的影响了。setsid 就能帮助我们做到这一点。让我们先来看一下 setsid 的帮助信息：</p>
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<code class="xhtml plain">SETSID(8) Linux Programmer's Manual SETSID(8)</code>
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<code class="xhtml plain">NAME</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">setsid - run a program in a new session </code>
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<code class="xhtml plain">SYNOPSIS</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">setsid program [ arg ... ]</code>
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<code class="xhtml plain">DESCRIPTION</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">setsid runs a program in a new session.</code>
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可见 setsid 的使用也是非常方便的，也只需在要处理的命令前加上 setsid 即可。</p>
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setsid 示例</p>
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<code class="xhtml plain"># setsid ping www.ibm.com</code>
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<code class="xhtml plain"># ps -ef |grep www.ibm.com</code>
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<code class="xhtml plain">root 31094 1 0 07:28 ? 00:00:00 ping www.ibm.com</code>
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<code class="xhtml plain">root 31102 29217 0 07:29 pts/4 00:00:00 grep www.ibm.com</code>
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<code class="xhtml plain">#</code>
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值得注意的是，上例中我们的进程 ID(PID)为31094，而它的父 ID（PPID）为1（即为 init 进程 ID），并不是当前终端的进程 ID。请将此例与nohup 例中的父 ID 做比较。</p>
<p>
3、这里还有一个关于 subshell 的小技巧。我们知道，将一个或多个命名包含在“()”中就能让这些命令在子 shell 中运行中，从而扩展出很多有趣的功能，我们现在要讨论的就是其中之一。<br>
当我们将"&"也放入“()”内之后，我们就会发现所提交的作业并不在作业列表中，也就是说，是无法通过jobs来查看的。让我们来看看为什么这样就能躲过 HUP 信号的影响吧。</p>
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subshell 示例</p>
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<code class="xhtml plain"># (ping www.ibm.com &)</code>
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<code class="xhtml plain"># ps -ef |grep www.ibm.com</code>
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<code class="xhtml plain">root 16270 1 0 14:13 pts/4 00:00:00 ping www.ibm.com</code>
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<code class="xhtml plain">root 16278 15362 0 14:13 pts/4 00:00:00 grep www.ibm.com</code>
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<code class="xhtml plain">#</code>
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<p>
从上例中可以看出，新提交的进程的父 ID（PPID）为1（init 进程的 PID），并不是当前终端的进程 ID。因此并不属于当前终端的子进程，从而也就不会受到当前终端的 HUP 信号的影响了。</p>
<p>
disown</p>
<p>
场景：</p>
<p>
我们已经知道，如果事先在命令前加上 nohup 或者 setsid 就可以避免 HUP 信号的影响。但是如果我们未加任何处理就已经提交了命令，该如何补救才能让它避免 HUP 信号的影响呢？</p>
<p>
解决方法：</p>
<p>
这时想加 nohup 或者 setsid 已经为时已晚，只能通过作业调度和 disown 来解决这个问题了。让我们来看一下 disown 的帮助信息：</p>
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<code class="xhtml plain">disown [-ar] [-h] </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">Without options, each jobspec is removed from the table of</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">active jobs. If the -h option is given, each jobspec is not</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">removed from the table, but is marked so that SIGHUP is not</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">sent to the job if the shell receives a SIGHUP. If no jobspec</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">is present, and neither the -a nor the -r option is supplied,</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">the current job is used. If no jobspec is supplied, the -a</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">option means to remove or mark all jobs; the -r option without</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">a jobspec argument restricts operation to running jobs. The</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">return value is 0 unless a jobspec does not specify a valid</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">job.</code>
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<p>
可以看出，我们可以用如下方式来达成我们的目的。</p>
<p>
用disown -h jobspec来使某个作业忽略HUP信号。</p>
<p>
用disown -ah 来使所有的作业都忽略HUP信号。</p>
<p>
用disown -rh 来使正在运行的作业忽略HUP信号。</p>
<p>
<strong>灵活运用 CTRL-z</strong></p>
<p>
<strong>在我们的日常工作中，我们可以用 CTRL-z 来将当前进程挂起到后台暂停运行，执行一些别的操作，然后再用 fg 来将挂起的进程重新放回前台（也可用 bg 来将挂起的进程放在后台）继续运行。这样我们就可以在一个终端内灵活切换运行多个任务，这一点在调试代码时尤为有用。因为将代码编辑器挂起到后台再重新放回时，光标定位仍然停留在上次挂起时的位置，避免了重新定位的麻烦。</strong></p>
<p>
需要注意的是，当使用过 disown 之后，会将把目标作业从作业列表中移除，我们将不能再使用jobs来查看它，但是依然能够用ps -ef查找到它。</p>
<p>
但是还有一个问题，这种方法的操作对象是作业，如果我们在运行命令时在结尾加了"&"来使它成为一个作业并在后台运行，那么就万事大吉了，我们可以通过jobs命令来得到所有作业的列表。但是如果并没有把当前命令作为作业来运行，如何才能得到它的作业号呢？答案就是用 CTRL-z（按住Ctrl键的同时按住z键）了！</p>
<p>
CTRL-z 的用途就是将当前进程挂起（Suspend），然后我们就可以用jobs命令来查询它的作业号，再用bg jobspec来将它放入后台并继续运行。需要注意的是，如果挂起会影响当前进程的运行结果，请慎用此方法。</p>
<p>
disown 示例1（如果提交命令时已经用“&”将命令放入后台运行，则可以直接使用“disown”）</p>
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<code class="xhtml plain"># cp -r testLargeFile largeFile &</code>
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<code class="xhtml plain"> 4825</code>
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<code class="xhtml plain"># jobs</code>
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<code class="xhtml plain">+ Running cp -i -r testLargeFile largeFile &</code>
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<code class="xhtml plain"># disown -h %1</code>
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<code class="xhtml plain"># ps -ef |grep largeFile</code>
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<code class="xhtml plain">root 4825 968 1 09:46 pts/4 00:00:00 cp -i -r testLargeFile largeFile</code>
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<code class="xhtml plain">root 4853 968 0 09:46 pts/4 00:00:00 grep largeFile</code>
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<code class="xhtml plain"># logout</code>
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disown 示例2（如果提交命令时未使用“&”将命令放入后台运行，可使用 CTRL-z 和“bg”将其放入后台，再使用“disown”）</p>
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<code class="xhtml plain"># cp -r testLargeFile largeFile2</code>
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<code class="xhtml plain">+ Stopped cp -i -r testLargeFile largeFile2</code>
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<code class="xhtml plain"># bg %1</code>
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<code class="xhtml plain">+ cp -i -r testLargeFile largeFile2 &</code>
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<code class="xhtml plain"># jobs</code>
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<code class="xhtml plain">+ Running cp -i -r testLargeFile largeFile2 &</code>
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<code class="xhtml plain"># disown -h %1</code>
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<code class="xhtml plain"># ps -ef |grep largeFile2</code>
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<code class="xhtml plain">root 5790 5577 1 10:04 pts/3 00:00:00 cp -i -r testLargeFile largeFile2</code>
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<code class="xhtml plain">root 5824 5577 0 10:05 pts/3 00:00:00 grep largeFile2</code>
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<code class="xhtml plain">#</code>
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screen</p>
<p>
场景：</p>
<p>
我们已经知道了如何让进程免受 HUP 信号的影响，但是如果有大量这种命令需要在稳定的后台里运行，如何避免对每条命令都做这样的操作呢？</p>
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解决方法：</p>
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此时最方便的方法就是 screen 了。简单的说，screen 提供了 ANSI/VT100 的终端模拟器，使它能够在一个真实终端下运行多个全屏的伪终端。screen 的参数很多，具有很强大的功能，我们在此仅介绍其常用功能以及简要分析一下为什么使用 screen 能够避免 HUP 信号的影响。我们先看一下 screen 的帮助信息：</p>
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<code class="xhtml plain">SCREEN(1) SCREEN(1)</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml plain">NAME</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">screen - screen manager with VT100/ANSI terminal emulation</code>
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<code class="xhtml spaces"> </code>
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<code class="xhtml plain">SYNOPSIS</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">screen [ -options ] [ cmd [ args ] ]</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">screen -r [tty[.host]]</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">screen -r sessionowner/[tty[.host]]</code>
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<code class="xhtml plain">DESCRIPTION</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">Screen is a full-screen window manager that multiplexes a physical</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">terminal between several processes (typically interactive shells).</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">Each virtual terminal provides the functions of a DEC VT100 terminal</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">and, in addition, several control functions from the ISO 6429 (ECMA</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">48, ANSI X3.64) and ISO 2022 standards (e.g. insert/delete line and</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">support for multiple character sets). There is a scrollback history</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">buffer for each virtual terminal and a copy-and-paste mechanism that</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">allows moving text regions between windows.</code>
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使用 screen 很方便，有以下几个常用选项：</p>
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用screen -dmS session name来建立一个处于断开模式下的会话（并指定其会话名）。</p>
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用screen -list 来列出所有会话。</p>
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用screen -r session name来重新连接指定会话。</p>
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用快捷键CTRL-a d 来暂时断开当前会话。</p>
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screen 示例</p>
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<code class="xhtml plain"># screen -dmS Urumchi</code>
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<code class="xhtml plain"># screen -list</code>
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<code class="xhtml plain">There is a screen on:</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">12842.Urumchi (Detached)</code>
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<code class="xhtml plain">1 Socket in /tmp/screens/S-root.</code>
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<code class="xhtml plain"># screen -r Urumchi</code>
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当我们用“-r”连接到 screen 会话后，我们就可以在这个伪终端里面为所欲为，再也不用担心 HUP 信号会对我们的进程造成影响，也不用给每个命令前都加上“nohup”或者“setsid”了。这是为什么呢？让我来看一下下面两个例子吧。</p>
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1. 未使用 screen 时新进程的进程树</p>
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<code class="xhtml plain"> 9499</code>
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<code class="xhtml plain"># pstree -H 9499</code>
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<code class="xhtml plain">init─┬─Xvnc</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─acpid</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─atd</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─2* </code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">│ └─sshd───bash───ping</code>
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我们可以看出，未使用 screen 时我们所处的 bash 是 sshd 的子进程，当 ssh 断开连接时，HUP 信号自然会影响到它下面的所有子进程（包括我们新建立的 ping 进程）。</p>
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2. 使用了 screen 后新进程的进程树</p>
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<code class="xhtml plain"># screen -r Urumchi</code>
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<code class="xhtml plain"># ping www.ibm.com &</code>
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<code class="xhtml plain"> 9488</code>
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<code class="xhtml plain">init─┬─Xvnc</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─acpid</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─atd</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─screen───bash───ping</code>
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<code class="xhtml spaces"> </code><code class="xhtml plain">├─2*</code>
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而使用了 screen 后就不同了，此时 bash 是 screen 的子进程，而 screen 是 init（PID为1）的子进程。那么当 ssh 断开连接时，HUP 信号自然不会影响到 screen 下面的子进程了。</p>
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总结</p>
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现在几种方法已经介绍完毕，我们可以根据不同的场景来选择不同的方案。nohup/setsid 无疑是临时需要时最方便的方法，disown 能帮助我们来事后补救当前已经在运行了的作业，而 screen 则是在大批量操作时不二的选择了。</p>
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