How to grep for the dollar symbol ($)?

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

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

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

To differentiate, I have marked the prompt as %.

• What is the problem here?
• What should the grep string be?开发者_如何学编程

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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 $.

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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When you use double quotes " or none use double\: "\\\$\\\$\\\$"

cat t | grep \\\$\\\$\\\$ 

if you use in single quotes ' you may use:

cat t | grep '\$\$\$'

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$ grep '\$\$\$' temp
$$$ hello1
hello5 $$$

There's a superflous 'cat' in your command.

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Works for me:

user@host:~$ cat temp | grep '\$\$\$'
$$$ hello1
hello5 $$$
user@host:~$ 

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How about ^.*[$]{3}.*$