如何grep美元符号（$）？ [英] How to grep for the dollar symbol ($)?

问题描述

% cat temp
$$$ hello1
$$  hello2
    hello3
##  hello4
    hello5 $$$
% cat temp | grep "$$$"
Illegal variable name.
% cat temp | grep "\$\$\$"
Variable name must contain alphanumeric characters.
%

我想为 $$$ ，我预计结果是



I want to grep for $$$ and I expect the result to be

% cat temp | grep <what should go here?>
$$$ hello1
    hello5 $$$
%

为了区分，我标记了提示符为％。

To differentiate, I have marked the prompt as %.

• 什么是问题这里<？li>
  
• grep string 应该是什么？

• What is the problem here?
• What should the grep string be?

推荐答案

问题在于shell在双引号字符串内展开变量名称。因此，对于$$$，它会尝试从第一个 $ 开始读取变量名称。

The problem is that the shell expands variable names inside double-quoted strings. So for "$$$" it tries to read a variable name starting with the first $.

另一方面，在单引号中，变量不会被扩展。因此，'$$$' 会工作 - 如果不是因为 $ 是表示行尾的正则表达式中的特殊字符。所以它需要被转义：'\ $ \ $ \ $'。

In single quotes, on the other hand, variables are not expanded. Therefore, '$$$' would work – if it were not for the fact that $ is a special character in regular expressions denoting the line ending. So it needs to be escaped: '\$\$\$'.

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