Variables in GNU Make recipes, is that possible?

Question

Is it possible to have variables in GNU Make recipes?

Something like this doesn't work:

%_t.mkd : %.mkd
    REV=$$(svn info $<|grep 'Last Changed Rev'|cut -f4 -d\ )
    echo $${REV}

Is there some way to make that work at all?

As you can see what I want is to extract the revision that a file was changed and then use that later in the full recipe. Unfortunately I can't use svn:keywords as I need the revision number outside of the document in question.

Answer 1

This doesn't work because the make tool starts a new shell process for each recipe line. And shell variables – even 'exported' environment variables – cannot possibly propagate "upwards"; they're gone as soon as the shell process exits.

• The traditional method is to join the recipe lines using \ in the Makefile:

  foo: bar baz
      line1; \
      line2; \
      line3
  

  (Note that the commands must be separated using ; or &&, because the backslashes are also passed to the shell which does the same line-joining.)

  See also info make "Splitting Lines" and info make "Splitting Recipe Lines" in the GNU Make manual.

• The other method is to tell make to always use one shell process for the entire recipe, using the .ONESHELL directive:

  .ONESHELL:
  
  foo: bar baz
      line1
      line2
      line3
  

  See info make "One Shell".

  (Note that while .ONESHELL is recommended by POSIX, not all make versions support it; e.g. BSD make only has a command-line flag for it. This shouldn't be a problem though.)

Answer 2

Thanks to https://stackoverflow.com/questions/6519234/cant-assign-variable-inside-recipe

This is the solution to change a variable in a recipe:

recipe: 
        $(eval variablename=whatever)

Answer 3

taking what @user3645902 mentioned, here is the solution to the main question:

recipe:
    @$(eval REV=`svn info $<|grep 'Last Changed Rev'|cut -f4 -d`)
    @echo $(REV)

Answer 4

According to Gnu Make 6.5 Setting Variables:

The shell assignment operator != can be used to execute a program and set a variable to its output. This operator first evaluates the right-hand side, then passes that result to the shell for execution. If the result of the execution ends in a newline, that one newline is removed; all other newlines are replaced by spaces. The resulting string is then placed into the named recursively-expanded variable.

So you could try the following (not tested):

REV != $$(svn info $<|grep 'Last Changed Rev'|cut -f4 -d\ ) \
echo $${REV}

Answer 5

this way of setting undef Makefile function parameter in this case $(1) argument works:

# usage:
# install-api-node:
#   $(call install-img,linux_user)

define install-img

    APPUSR=$(eval APPUSR=`echo appusr`)
    APPUSR=$(or $(1),$(APPUSR))
   
    @echo example usage : docker run -it -d --restart=always \
        -v $$(pwd):/opt/project-source-dir \
        -v $$HOME/.aws:/home/$(APPUSR)/.aws \
        --name my-container my-image ;


endef