[pm-h] use of $$ versus $ in makefile and bash

[Russell L. Harris]

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, and I have searched with Google, but I have
found no answer.

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

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

    -(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 'miscellany' -prune  -o \
        -type d -print);                  \
    do mkdir --parents ../stageweb/$$d;   \
    done;)

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

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

    # Why was "bn" chosen as the name of the loop variable?

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

My interest is fueled by the need to create a makefile which automates
the following commands for any number of files, which necessitates a
pair of for-loops:
    
    hevea -fix -I jan article.hva jan/jan-01.tex
    hevea -fix -I jan article.hva jan/jan-02.tex
    ...
    hevea -fix -I feb article.hva feb/feb-01.tex
    hevea -fix -I feb article.hva feb/feb-02.tex
    ...
    hevea -fix -I mar article.hva mar/mar-02.tex
    hevea -fix -I mar article.hva mar/mar-02.tex
    ...

    # (The hevea command creates files named "jan-01.html", etc.)
    
    cp jan/jan-*.html ../stageweb/jan/
    cp jan/feb-*.html ../stageweb/feb/
    cp jan/mar-*.html ../stageweb/mar/
    ...

Looking at other code in the makefile which I am using for a go-by, I
expect to start out with variables such as:

    JAN := $(wildcard jan/jan*.tex)
    FEB := $(wildcard feb/feb*.tex)
    MAR := $(wildcard mar/mar*.tex)
    ...

    DOCUMENTS := $(JAN) $(FEB) $(MAR) ...

or else:

    MONTHS := jan feb mar ...

RLH