🚀 go-pugleaf

>file name as part of what it does.  The macro does not
>work if the file name has an underscore character in it,
>since it interprets the underscore as the subscript
>character and starts inserting $ signs.

>\def\foo#1{ {\tt #1} }
>\foo{file_name.ext}

>I tried defining foo as follows:

>\def\foo#1{ {\catcode`_=12 {\tt #1} } }

This is a very common error: The catcode is already fixed when the
argument is passed!

>{\catcode`_=12 \foo{file_name.ext} }

>Is there a way of avoiding this and doing everything
>inside the macro?

Sure: Use a macro to call \foo:
e.g.: \def \Foo{\bgroup\catcode`\_=12\foo}
and add a \egroup to the end of \foo

Peter