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Re: simple question about variables in a rule

From: David Boyce
Subject: Re: simple question about variables in a rule
Date: Sun, 16 May 2004 07:47:29 -0400

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:

        @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:

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

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

        @echo "A = $A"
        @echo "B = $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 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.

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