How can I search for (escape) a dollar sign using ack?

Question

I want to ack for the literal string: "$$" in a code base, but escaping the dollar sign like this:

ack \$\$

doesn't work.

Answer 1

You are getting confused by shell quoting. When you type:

ack "\\\$\\\$\("

the shell interpolates the double quoted string so that \\ is translated to \, \$ is translated to $ and \( is translated to \( and ack gets the string \$\$\( as its argument. It is much simpler to avoid the shell interpolation by using single quotes and invoke:

ack '\$\$\('

Replace ack with echo to explore how the shell is expanding the strings. Note that

ack "\\$\\$\("

will also work, but for slightly different reasons. Here, the first two \ are treated as a single (escaped) \, then the $ is translated as a $ because it is followed by a character that is not a valid character in a variable name. \( expands to \( instead of simply ( because ( is not subject to interpolation and therefore does not need to be escaped. But note that outside of double quotes, \( is converted to (.

Shell quoting rules get confusing sometimes!