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Re: Missing Something Obvious

From: Marc Smith
Subject: Re: Missing Something Obvious
Date: Mon, 14 Nov 2011 11:39:46 -0500

On Mon, Nov 14, 2011 at 11:26 AM, Paul Smith <address@hidden> wrote:
> On Mon, 2011-11-14 at 11:17 -0500, Marc Smith wrote:
>> Did that behavior change recently in GNU make?
> No.
>> I've been reading the O'Reilly book titled Managing Projects with GNU
>> Make (3rd edition) and on page 70 it gives this example to highlight
>> the difference between simple/recursive variables with the shell
>> function:
>> START_TIME := $(shell date)
>> CURRENT_TIME = $(shell date)
>> "The START_TIME variable causes the date command to execute once when
>> the variable is defined. The CURRENT_TIME variable will reexecute date
>> each time the variable is used in the makefile."
> Sure.  But that's at all the same thing as what you did.  What you did
> is this:
>>> blah = $(shell date)
>>> all:
>>>         @ echo $(blah)
>>>         @ sleep 10
>>>         @ echo $(blah)
> In the O'Reilly book they are using first a simple variable which has
> immediate expansion, so START_TIME is evaluated when the makefile is
> read in, while END_TIME is a recursive variable and not evaluated until
> the variable is used.
> Your example puts the variables in the same recipe twice.  You are
> assuming that make will expand the first line of the recipe, run it,
> expand the second line of the recipe, run that, then expand the third
> line of the recipe and run that.

Gotcha, I'm not trying to be difficult, just wrapping my head around this. =)
So, something like this should yield different values:

blah := $(shell date)
blah2 = $(shell date)

        @ echo $(blah)
        @ sleep 10
        @ echo $(blah2)


Is there another way to use the same variable and have the shell
function execute each time the variable is used in a makefile?

Thanks for your help.

> But that's not how GNU make works, nor has it ever worked that way.
> Instead, make expands the first, second, and third lines at the same
> time and then runs the first one, then the second, then the third in
> order.
> --
> -------------------------------------------------------------------------------
>  Paul D. Smith <address@hidden>          Find some GNU make tips at:
>  "Please remain calm...I may be mad, but I am a professional." --Mad Scientist

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