Assign variable once when first used in Makefile [duplicate]












2
















This question already has an answer here:




  • How to get “at most once” semantics in variable assignments?

    2 answers




How do I only invoke a make shell function once in a variable expansion, and only when the variable is first used? I don't want to use := to expand at declaration time (ie: simple expansion) because the expansion is expensive and only some of my targets need the variable.



I tried using conditional variable assignment but it invokes the shell every time, for example, the following invokes shell ls twice:



.PHONY: test



FILES ?= $(warning Invoking the shell)$(shell ls)



test:

    echo $(FILES) one

    echo $(FILES) two





makefile gnu-make







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marked as duplicate by Renaud Pacalet, Community♦ Nov 21 '18 at 6:54


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.











  • 1





    A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

    – MadScientist
    Nov 18 '18 at 14:23
















2
















This question already has an answer here:




  • How to get “at most once” semantics in variable assignments?

    2 answers




How do I only invoke a make shell function once in a variable expansion, and only when the variable is first used? I don't want to use := to expand at declaration time (ie: simple expansion) because the expansion is expensive and only some of my targets need the variable.



I tried using conditional variable assignment but it invokes the shell every time, for example, the following invokes shell ls twice:



.PHONY: test



FILES ?= $(warning Invoking the shell)$(shell ls)



test:

    echo $(FILES) one

    echo $(FILES) two





makefile gnu-make







share|improve this question













marked as duplicate by Renaud Pacalet, Community♦ Nov 21 '18 at 6:54


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.











  • 1





    A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

    – MadScientist
    Nov 18 '18 at 14:23














2












2








2









This question already has an answer here:




  • How to get “at most once” semantics in variable assignments?

    2 answers




How do I only invoke a make shell function once in a variable expansion, and only when the variable is first used? I don't want to use := to expand at declaration time (ie: simple expansion) because the expansion is expensive and only some of my targets need the variable.



I tried using conditional variable assignment but it invokes the shell every time, for example, the following invokes shell ls twice:



.PHONY: test



FILES ?= $(warning Invoking the shell)$(shell ls)



test:

    echo $(FILES) one

    echo $(FILES) two





makefile gnu-make







share|improve this question















This question already has an answer here:




  • How to get “at most once” semantics in variable assignments?

    2 answers




How do I only invoke a make shell function once in a variable expansion, and only when the variable is first used? I don't want to use := to expand at declaration time (ie: simple expansion) because the expansion is expensive and only some of my targets need the variable.



I tried using conditional variable assignment but it invokes the shell every time, for example, the following invokes shell ls twice:



.PHONY: test



FILES ?= $(warning Invoking the shell)$(shell ls)



test:

    echo $(FILES) one

    echo $(FILES) two




This question already has an answer here:




  • How to get “at most once” semantics in variable assignments?

    2 answers







makefile gnu-make




makefile gnu-make






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 18 '18 at 2:38









tekumaratekumara

5,14774058




5,14774058




marked as duplicate by Renaud Pacalet, Community♦ Nov 21 '18 at 6:54


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.






marked as duplicate by Renaud Pacalet, Community♦ Nov 21 '18 at 6:54


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.










  • 1





    A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

    – MadScientist
    Nov 18 '18 at 14:23














  • 1





    A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

    – MadScientist
    Nov 18 '18 at 14:23








1




1





A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

– MadScientist
Nov 18 '18 at 14:23





A longer writeup is here: make.mad-scientist.net/deferred-simple-variable-expansion

– MadScientist
Nov 18 '18 at 14:23












1 Answer
1






active

oldest

votes


















-1














Move the assignment inside the recipe



test:

    FILES=$$(ls) ;

    echo $$FILES one ;

    echo $$FILES two





share|improve this answer






























    1 Answer
    1






    active

    oldest

    votes








    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    -1














    Move the assignment inside the recipe



    test:
    
    FILES=$$(ls) ;
    
    echo $$FILES one ;
    
    echo $$FILES two





    share|improve this answer




























      -1














      Move the assignment inside the recipe



      test:
      
    FILES=$$(ls) ;
      
    echo $$FILES one ;
      
    echo $$FILES two





      share|improve this answer


























        -1












        -1








        -1







        Move the assignment inside the recipe



        test:
        
    FILES=$$(ls) ;
        
    echo $$FILES one ;
        
    echo $$FILES two





        share|improve this answer













        Move the assignment inside the recipe



        test:
        
    FILES=$$(ls) ;
        
    echo $$FILES one ;
        
    echo $$FILES two






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 18 '18 at 7:21









        user657267user657267

        17.3k53160




        17.3k53160















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