Linux tip: make the process more reliable running in the background are several ways
We often encounter this problem, use telnet / ssh remote login Linux server, running a number of time-consuming tasks, the result is due to the unstable network led mission aborted. How close the terminal window without the interference of local / disconnected from the network after the command to submit it? The following give some examples, you can choose different ways for different scenarios to deal with this problem.
nohup/setsid/&
Scenes:
If you only have a temporary command needs to run for a long time, what can be the easiest method to ensure that it is stable it runs in the background?
Solution:
We know that when the user logs off (logout) or disconnected from the network, the terminal will receive HUP (hangup) signal to close all of its child processes. Therefore, our solution there are two ways: either allow the process to ignore HUP signal, or let the process run in the new session in order to become the child does not belong to this terminal.
1. nohup
nohup is undoubtedly the way we first thought. As the name suggests, it is to make use nohup command to submit to ignore hangup signals. Let's first look at nohup help information:
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NOHUP(1) User Commands NOHUP(1)
NAME
nohup - run a command immune to hangups, with output to a non-tty
SYNOPSIS
nohup COMMAND [ARG]...
nohup OPTION
DESCRIPTION
Run COMMAND, ignoring hangup signals.
--help display this help and exit
--version
output version information and exit
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Visible, use nohup is very convenient, just add nohup to standard output and standard error will be redirected to the default nohup.out file before the command to be processed. Generally, we can add at the end of "&" to be placed in command while running in the background can also be used to change the default file name redirection.">filename 2>&1"
nohup example
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[root@pvcent107 ~]# nohup ping www.ibm.com &
[1] 3059
nohup: appending output to `nohup.out'
[root@pvcent107 ~]# ps -ef |grep 3059
root 3059 984 0 21:06 pts/3 00:00:00 ping www.ibm.com
root 3067 984 0 21:06 pts/3 00:00:00 grep 3059
[root@pvcent107 ~]#
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2。setsid
nohup certainly can ignore HUP signal to make our way to avoid the process is interrupted, but if we think about another angle, if the process does not belong to our child process HUP signal receiving terminal, then naturally, will not be affected by HUP signal a. setsid can help us do that. Let's first look at setsid help information:
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SETSID(8) Linux Programmer’s Manual SETSID(8)
NAME
setsid - run a program in a new session
SYNOPSIS
setsid program [ arg ... ]
DESCRIPTION
setsid runs a program in a new session.
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Setsid visible use is also very convenient, you can also just add setsid before the command to be processed.
setsid example
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[root@pvcent107 ~]# setsid ping www.ibm.com
[root@pvcent107 ~]# ps -ef |grep www.ibm.com
root 31094 1 0 07:28 ? 00:00:00 ping www.ibm.com
root 31102 29217 0 07:29 pts/4 00:00:00 grep www.ibm.com
[root@pvcent107 ~]#
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It is noteworthy that, in the example above our process ID (PID) is 31094, and its parent ID (PPID) to 1 (that is, the init process ID), not the process ID of the current terminal. Use this example in the nohup example the parent ID comparison.
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。
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。
subshell 示例
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[root@pvcent107 ~]# (ping www.ibm.com &)
[root@pvcent107 ~]# ps -ef |grep www.ibm.com
root 16270 1 0 14:13 pts/4 00:00:00 ping www.ibm.com
root 16278 15362 0 14:13 pts/4 00:00:00 grep www.ibm.com
[root@pvcent107 ~]#
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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。
disown
场景：
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？
解决方法：
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：
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disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of
active jobs. If the -h option is given, each jobspec is not
removed from the table, but is marked so that SIGHUP is not
sent to the job if the shell receives a SIGHUP. If no jobspec
is present, and neither the -a nor the -r option is supplied,
the current job is used. If no jobspec is supplied, the -a
option means to remove or mark all jobs; the -r option without
a jobspec argument restricts operation to running jobs. The
return value is 0 unless a jobspec does not specify a valid
job.
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可以看出，我们可以用如下方式来达成我们的目的。
- 用
disown -h jobspec来使某个作业忽略HUP信号。 - 用
disown -ah来使所有的作业都忽略HUP信号。 - 用
disown -rh来使正在运行的作业忽略HUP信号。
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。
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！
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。
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”）
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[root@pvcent107 build]# cp -r testLargeFile largeFile &
[1] 4825
[root@pvcent107 build]# jobs
[1]+ Running cp -i -r testLargeFile largeFile &
[root@pvcent107 build]# disown -h %1
[root@pvcent107 build]# ps -ef |grep largeFile
root 4825 968 1 09:46 pts/4 00:00:00 cp -i -r testLargeFile largeFile
root 4853 968 0 09:46 pts/4 00:00:00 grep largeFile
[root@pvcent107 build]# logout
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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”）
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[root@pvcent107 build]# cp -r testLargeFile largeFile2
[1]+ Stopped cp -i -r testLargeFile largeFile2
[root@pvcent107 build]# bg %1
[1]+ cp -i -r testLargeFile largeFile2 &
[root@pvcent107 build]# jobs
[1]+ Running cp -i -r testLargeFile largeFile2 &
[root@pvcent107 build]# disown -h %1
[root@pvcent107 build]# ps -ef |grep largeFile2
root 5790 5577 1 10:04 pts/3 00:00:00 cp -i -r testLargeFile largeFile2
root 5824 5577 0 10:05 pts/3 00:00:00 grep largeFile2
[root@pvcent107 build]#
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screen
场景：
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？
解决方法：
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：
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SCREEN(1) SCREEN(1)
NAME
screen - screen manager with VT100/ANSI terminal emulation
SYNOPSIS
screen [ -options ] [ cmd [ args ] ]
screen -r [[pid.]tty[.host]]
screen -r sessionowner/[[pid.]tty[.host]]
DESCRIPTION
Screen is a full-screen window manager that multiplexes a physical
terminal between several processes (typically interactive shells).
Each virtual terminal provides the functions of a DEC VT100 terminal
and, in addition, several control functions from the ISO 6429 (ECMA
48, ANSI X3.64) and ISO 2022 standards (e.g. insert/delete line and
support for multiple character sets). There is a scrollback history
buffer for each virtual terminal and a copy-and-paste mechanism that
allows moving text regions between windows.
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使用 screen 很方便，有以下几个常用选项：
- 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）。 - 用
screen -list来列出所有会话。 - 用
screen -r session name来重新连接指定会话。 - 用快捷键
CTRL-a d来暂时断开当前会话。
screen 示例
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[root@pvcent107 ~]# screen -dmS Urumchi
[root@pvcent107 ~]# screen -list
There is a screen on:
12842.Urumchi (Detached)
1 Socket in /tmp/screens/S-root.
[root@pvcent107 ~]# screen -r Urumchi
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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。
1. 未使用 screen 时新进程的进程树
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[root@pvcent107 ~]# ping www.google.com &
[1] 9499
[root@pvcent107 ~]# pstree -H 9499
init─┬─Xvnc
├─acpid
├─atd
├─2*[sendmail]
├─sshd─┬─sshd───bash───pstree
│ └─sshd───bash───ping
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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）。
2. 使用了 screen 后新进程的进程树
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[root@pvcent107 ~]# screen -r Urumchi
[root@pvcent107 ~]# ping www.ibm.com &
[1] 9488
[root@pvcent107 ~]# pstree -H 9488
init─┬─Xvnc
├─acpid
├─atd
├─screen───bash───ping
├─2*[sendmail]
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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。
总结
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。