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 $(shell ...).
See below as well.
> I also just found that this works as well:
>
> --snip--
> [marc.smith@catskill testing]$ cat Makefile
> blah = $$(date)
> all:
> @ echo $(blah)
> @ sleep 10
> @ echo $(blah)
> --snip--
>
> --snip--
> [marc.smith@catskill testing]$ make
> Mon Nov 14 14:41:47 EST 2011
> Mon Nov 14 14:41:57 EST 2011
> --snip--
Just so you're clear, by doing this you're actually following Philip's
suggestion.
The "$$(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.
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