use of $$ versus $ in makefile and bash

["Russell L. Harris" <rlharris@oplink.net>]

I am puzzled concerning the use of the symbols "$" and "$$" in a
makefile.  I do not understand why "$$" is required in some instances,
and "$" in other instances.

I have spent several hours searching through the documentation for
BASH and for GNU Make, but I have found no answer.

Here are three examples, all taken from the same makefile:

EXAMPLE 1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

	-ln -s ../journal.cls ../journal.hlx .
	-(for d in ${SYMLINKDIR};		           \
	 do					           \
	     pushd $$d &&			           \
	       ln -sf $(addprefix ../,${SYMLINKTARGET}) .; \
	     popd;				           \
	 done;)

EXAMPLE 2 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

	(for d in $$(find .                           \
	        \( -type d                            \
	                -name '.svn'     -prune    -o \
	                -name 'hevea'    -prune    -o \
	                -name 'metatags' -prune \) -o \
	           -type d -print);                   \
	do mkdir --parents ../stageweb/$$d;           \
	done;)

EXAMPLE 3 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

	(for bn in ${CLEANDIR};		             \
	 do					     \
	     rm -f $(addprefix $$bn/*.,${CLEANEXT}); \
	 done;)
	rm -f ${CLEANOTHER}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Also, in the third example, does the variable name "bn" have
significance?  Could the name "b" or "c" or "x" be used instead?  The
name "f" commonly is used in a for-loop when working with files, and
the name "d", when working with directories.  The name "bn" appears
nowhere else in the makefile.