使用输入/输出重定向在后台运行进程 [英] Running a process in the background with input/output redirection

问题描述

我很想知道'&'在哪里是否有所作为进程具有输入/输出重定向以在后台运行进程时，在代码中使用运算符

I'm curious to know if it makes a difference where the '&' operator is used in code when a process has input/output redirection to run a process in the background

就在后台运行进程而言，这些代码行之间有什么区别/是否存在任何区别.如果有的话，我该如何确定会有什么不同?

What are the differences/are there any differences between these lines of code in terms of running the process in the background. If there are, how can I determine what the differences are going to be?

setsid python script.py < /dev/zero &> log.txt &

setsid python script.py < /dev/zero & > log.txt &

setsid python script.py < /dev/zero > log.txt &

setsid python script.py & < /dev/zero > log.txt

推荐答案

控制操作符

这里&有两种用途.一种是所谓的控制操作员.每个命令都由诸如&，;或<newline>的控制运算符终止.它们之间的区别在于;和<newline>在前台运行命令，而&在后台运行.

Control operator

There are two uses of & here. One is as a so-called control operator. Every command is terminated by a control operator such as &, ; or <newline> . The difference between them is that ; and <newline> run the command in the foreground and & does it in the background.

setsid python script.py < /dev/zero & > log.txt &
setsid python script.py & < /dev/zero > log.txt

因此，这两行实际上分别执行两个命令.第一个等效于两个命令:

These two lines, therefore, actually execute two commands each. The first is equivalent to the two commands:

setsid python script.py < /dev/zero &
> log.txt &

第二个等效于:

setsid python script.py &
< /dev/zero > log.txt

如果您想知道，是的，> log.txt和< /dev/zero > log.txt都是合法命令.缺少命令名称，它们仅处理重定向:每个命令都会创建一个名为log.txt的空文件.

If you're wondering, yes, > log.txt and < /dev/zero > log.txt are both legal commands. Lacking a command name, they simply process the redirections: each one creates an empty file called log.txt.

setsid python script.py < /dev/zero &> log.txt &

此版本的&>与& >的版本不同. &>没有空格是bash中特殊的重定向操作符，它可以同时重定向stdout和stderr.

This version with &> is different from the one with & >. &> without a space is a special redirection operator in bash that redirects both stdout and stderr.

setsid python script.py < /dev/zero > log.txt &

此最终版本与先前版本相似，只是它仅将stdout重定向到log.txt. stderr继续前往终端.

This final version is similar to the previous one except it only redirects stdout to log.txt. stderr continues to go to the terminal.

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