Re: Missing Something Obvious

[Marc Smith]

On Mon, Nov 14, 2011 at 2:51 PM, Paul Smith <address@hidden> wrote:
> On Mon, 2011-11-14 at 14:43 -0500, Marc Smith wrote:
>> I was just using 'date' as an example; my real need involves grep'ing
>> files that are generated at the top of a recipe. Yes, I realize I
>> could just create another recipe and make it a prerequisite.
>
> I'm not sure why that makes a difference.  You're invoking a shell from
> the $(shell ...) function... but you're already in a shell, in your
> recipe line.  Why do you need to use make's $(shell ...) function in
> this case if you're already in a shell?
>
> If you use := then that's a good reason, but if you use "=" then you
> might as well just invoke the command in the recipe shell rather than
> using $(shell ...).

I guess for maintenance reasons -- storing a shell command in a
variable makes it easier to take care of if that shell command ever
changes; that way I won't have to update the same shell command in X
places. I can just modify it in one spot.

Using ":=" wouldn't work in this case since the same grep command is
called multiple times in the recipe and the list it produces
should/will be different each time.


--Marc

>
> See below as well.
>
>> I also just found that this works as well:
>>
>> --snip--
>> address@hidden testing]$ cat Makefile
>> blah = $$(date)
>> all:
>>         @ echo $(blah)
>>         @ sleep 10
>>         @ echo $(blah)
>> --snip--
>>
>> --snip--
>> address@hidden testing]$ make
>> Mon Nov 14 14:41:47 EST 2011
>> Mon Nov 14 14:41:57 EST 2011
>> --snip--
>
> Just so you're clear, by doing this you're actually following Philip's
> suggestion.
>
> The "$$(date)" is expanded to the string "$(date)" which is then passed
> to the shell.
>
> POSIX shells support a syntax "$(command ...)" which is equivalent to
> backticks (`command ...`).  So it's the shell that's running date, not
> make, and that's why you are getting the behavior you expect.
>
> The makefile above is EXACTLY equivalent to this:
>
>        blah = `date`
>        all:
>                @ echo $(blah)
>                @ sleep 10
>                @ echo $(blah)
>
> which is identical to:
>
>        all:
>                @ echo `date`
>                @ sleep 10
>                @ echo `date`
>
> which is exactly what Philip was suggesting: not using make's
> $(shell ...) function at all and instead letting the shell invoke the
> command for you.
>
>