扩展文件中的已知环境并保留其他内容 [英] Expanding known env vers in a file and leaving others

问题描述

我敢肯定有一种非常简单的方法可以做到这一点.我正在尝试获取一个包含一些环境变量的文件并将其展开，以便将已知的那些扩展为它们的值，而那些未被单独保留的值.

I'm sure there's a really easy way of doing this. I'm trying to take a file which contains some environment variables and expand it so that those which are known are expanded to their values whereas those which are not are left alone.

例如，如果我的文件包含以下内容:

For example, if my file contained the following:

${I_EXIST}
${I_ALSO_EXIST}
${I_DONT_EXIST}

这将扩展为:

existValue
alsoExistValue
${I_DONT_EXIST}

理想情况下，我想尽可能简单地做到这一点，所以我不想使用sed，awk或perl进行复杂的替换.我正在考虑类似于此处"文件的内容，但是除了我无法正确使用语法之外，它还会清空任何无法扩展的内容.例如:

I ideally want to do this as simply as possible so I don't want a complex substitution using sed, awk or perl. I'm thinking of something similar to a "Here" file, but apart from the fact that I can't get the syntax right, it also blanks out anything which does not expand. E.g:

cat <<EOF
> ${I_EXIST}
> ${I_ALSO_EXIST}
> ${I_DONT_EXIST}
EOF
existValue
alsoExistValue

(即最后一个值扩展为空)

(i.e. the last value expands to nothing)

更新

真的应该明确地表明，我正在考虑每行可能有多个替换.我确实找到了一种方法，如果我们不对文件中以 $ {MYVAR} 出现的变量大惊小怪，但也许 MYVAR 可以做到:

Should really have made clear that I was thinking about potentially more than one substitution per line. One way I did find to do this, if we're not fussed about the variables appearing in the file as ${MYVAR} but maybe MYVAR will do:

m4 $(env | sed's/\([A-Za-z0-9] * \)= \([[//A-Za-z_0-9:|％*.-@]* \)/-D \ 1 = \ 2')myfile

这使用M4预处理器替换环境中的所有对.这里有几个腔室:

This uses the M4 preprocessor to substitute all the pairs in your environment. A couple of caviats here:

1. 对不起reg exp的东西.看起来很讨厌，我敢肯定有更好的表达方式.如果我的环境变量中有空格或集合中没有的任何不寻常字符，就会发现问题.
2. 当然，这是一个钝器替换工具(我试图避免这种情况)，因此当您不希望变量发生时，变量可能会被替换.

推荐答案

#!/bin/bash

while read a;
do
    n=$(eval echo $a)

    if [[ "$n" == "" ]]
    then
        echo $a
    else
        echo $n
    fi

done < input

将此作为输入

${HOME}
${nonexistent}

给予

/home/myuser
${nonexistent}

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