Re: simple question about variables in a rule

[David Boyce]

At 06:51 AM 5/16/2004, Robert P. J. Day wrote:

>   currently, i have a rule that i expect to invoke with a couple variables
> from the command line -- call them A and B:
>
> $ make A=somevalue B=someothervalue target
>
> and the makefile will have the target:
>
> target:
>         @echo "A = $A"
>         @echo "B = $B"
>         ... rest of rule ...
>
> now, for efficiency, i'll always use these variables as "${A}/${B}", so i
> just want to temporarily set that value to, say, C (and *only* within the
> scope of this rule):

Classic make, per se, offers no help with this. By the time you're into the 
command, make has handed off to the shell; you're not even operating in the 
make process anymore and the shell has no access to make's symbol table. 
But this is easily solved by setting up quoting such that A and B are 
expanded by make before handing off to the shell, whereas C is assigned as 
a *shell* variable which is assigned and then expanded by the shell. This 
in turn can be done two ways, either as an environment variable:

target:
        @echo "A = $A"
        @echo "B = $B"
        C=$A/$B rest of rule ...

Or you can set up C as a regular (unexported) shell variable:

target:
        @echo "A = $A"
        @echo "B = $B"
        C=$A/$B;\
        rest of rule ...

I believe that with sufficiently modern versions of GNU make you could also 
make use of "target-specific variables", e.g.

target: C = $A/$B

(see http://www.gnu.org/software/make/manual/html_mono/make.html#SEC77) for 
details. But the traditional solution is probably preferable where applicable.

-David Boyce

PS You are tripping over the #1 obstacle to proper understanding of make: 
by default, each line of the command is executed as a separate script. In 
your example you were successfully setting C in one script, then letting 
that process end and expecting C to be still set in the next one. This is 
easily remedied by judicious use of backslashes as line continuation 
characters, as shown in my second example.