How to grep for the dollar symbol ($)?

Question

% 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?

Answer 1

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: '\$\$\$'.

Answer 2

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

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

if you use in single quotes ' you may use:

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