Re: gmake: testing if a var is [un]defined

[John Graham-Cumming]

Ernest wrote:
> Consider a variable called FOO...is there a more efficient/quick/simple way 
> to test if this variable is defined (vs. undefined) at *runtime* than as 
> follows?  Note that "defined" *can* mean an empty value ("FOO=").
>
> # echo "no" if FOO is undefined
> #  or "yes" if FOO is defined
> foo:
>    echo $(if $(findstring undefined,$(origin FOO)),no,yes)

That looks like a perfectly good way to do it.   You could use filter 
instead of findstring.  And you could even defined a function so that 
it's clear what's going on:

    defined = $(if $(filter undefined,$(origin $1)),no,yes)

    foo: ; @echo $(call defined,FOO)

You might find it's better to have 'defined' return an empty string for 
'no', and a non-empty string for 'yes' if you plan to use the result of 
'defined' in a $(if):

    defined = $(filter-out undefined,$(origin $1))

    foo: ; @echo $(if $(call defined,FOO),foo is defined,foo is undefined)

John.
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John Graham-Cumming
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