如何在循环结束后将输入通过管道传输到 Bash 循环并保留变量 [英] How to pipe input to a Bash while loop and preserve variables after loop ends

问题描述

Bash 允许使用:cat <(echo "$FILECONTENT")

Bash 也允许使用:while read i;做回声 $i;完成 </etc/passwd

Bash also allow to use: while read i; do echo $i; done </etc/passwd

结合前两个可以使用: echo $FILECONTENT |读我的时候；做回声 $i;完成

to combine previous two this can be used: echo $FILECONTENT | while read i; do echo $i; done

最后一个的问题是它创建了子shell并且在while循环结束后变量i不能再被访问.

The problem with last one is that it creates sub-shell and after the while loop ends variable i cannot be accessed any more.

我的问题是:

如何实现这样的事情:while read i;做回声 $i;done <(echo "$FILECONTENT") 或者换句话说:我如何确保 i 在循环中存活?

How to achieve something like this: while read i; do echo $i; done <(echo "$FILECONTENT") or in other words: How can I be sure that i survives while loop?

请注意，我知道将 while 语句包含在 {} 中，但这并不能解决问题(想象一下您想在函数中使用 while 循环并返回 i> 变量)

Please note that I am aware of enclosing while statement into {} but this does not solves the problem (imagine that you want use the while loop in function and return i variable)

推荐答案

进程替换是:

while read i; do echo $i; done < <(echo "$FILECONTENT")

在循环中分配的 i 的最后一个值然后在循环终止时可用.另一种选择是:

The last value of i assigned in the loop is then available when the loop terminates. An alternative is:

echo $FILECONTENT | 
{
while read i; do echo $i; done
...do other things using $i here...
}

大括号是 I/O 分组操作，本身不会创建子 shell.在这种情况下，它们是管道的一部分，因此作为子 shell 运行，但这是因为 |，而不是 { ... }.你在问题中提到了这一点.AFAIK，您可以在函数内部从这些内部返回.

The braces are an I/O grouping operation and do not themselves create a subshell. In this context, they are part of a pipeline and are therefore run as a subshell, but it is because of the |, not the { ... }. You mention this in the question. AFAIK, you can do a return from within these inside a function.

Bash 还提供了 shopt 内置，其众多选项之一是:

Bash also provides the shopt builtin and one of its many options is:

最后一个管道

如果设置，并且作业控制未处于活动状态，shell 将运行当前 shell 环境中未在后台执行的管道的最后一个命令.

If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment.

因此，在脚本中使用类似这样的东西会使修改后的sum在循环后可用:

Thus, using something like this in a script makes the modfied sum available after the loop:

FILECONTENT="12 Name
13 Number
14 Information"
shopt -s lastpipe   # Comment this out to see the alternative behaviour
sum=0
echo "$FILECONTENT" |
while read number name; do ((sum+=$number)); done
echo $sum

在命令行中执行此操作通常会违反作业控制未激活"(即，在命令行中，作业控制处于活动状态).不使用脚本进行测试失败.

Doing this at the command line usually runs foul of 'job control is not active' (that is, at the command line, job control is active). Testing this without using a script failed.

此外，正如 Gareth Rees 在他的 answer，您有时可以使用 此处字符串:

Also, as noted by Gareth Rees in his answer, you can sometimes use a here string:

while read i; do echo $i; done <<< "$FILECONTENT"

这不需要shopt；您可以使用它来保存进程.

This doesn't require shopt; you may be able to save a process using it.

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