Re: $(eval ) and $(shell )

[CHEN Cheng]

On Fri, Jul 03, 2009 at 02:55:53PM +1000, address@hidden wrote:
> Could someone explain me the actual evaluation order of variables and
> how $, $$, $(value ), $(eval ), $(call ) and $(shell ) operate?
> 
> Reading the info page of $(eval ) gave me the impression that it gets its
> argument, evaluates it once then parses the resulting text as makfile
> source (and interprets it). Obviously that's a rather naive assumption and
> make is doing something more sophisticated. I'd appreciate if someone
> could tell me what or at least directed me towards some docs (other than
> the source of make).
> 

IMHO, there are three passes in the whole make process:

1. eval pass, in which evals are expanded.
    In the first case, your makefile is expanded as:
        all: foo

        foo: Makefile
            @echo "start"
            @echo "shell is <" ">"
        # error occurred above, because expansion of $(VAR) calls $(shell ...),
        # which is an no-existent-command
            @echo "end"

    In the second case, your makefile is expanded as:
        all: foo

        foo: Makefile
            @echo "start"
            @echo "shell is <" $(shell no-existent-command) ">"
            @echo "end"

2. substitution pass, in which variables (and ifdef, etc.) are expanded.
    In the first case, your makefile is expanded as:
        all: foo

        foo: Makefile
            @echo "start"
            @echo "shell is <" ">"
            @echo "end"

    In the second case, your makefile is expanded as:
        all: foo

        foo: Makefile
            @echo "start"
            @echo "shell is <" ">"
        #error generated in the above command while expanding $(shell ...)
            @echo "end"

3. real make pass, in which targets are built consequently.

However, the above description is kind of oversimplified, e.g. target-
specific variables are not discussed.


Hope it helps.

-Cheng