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make: Canned Recipes

 
 5.8 Defining Canned Recipes
 ===========================
 
 When the same sequence of commands is useful in making various targets,
 you can define it as a canned sequence with the 'define' directive, and
 refer to the canned sequence from the recipes for those targets.  The
 canned sequence is actually a variable, so the name must not conflict
 with other variable names.
 
    Here is an example of defining a canned recipe:
 
      define run-yacc =
      yacc $(firstword $^)
      mv y.tab.c $@
      endef
 
 Here 'run-yacc' is the name of the variable being defined; 'endef' marks
 the end of the definition; the lines in between are the commands.  The
 'define' directive does not expand variable references and function
 calls in the canned sequence; the '$' characters, parentheses, variable
 names, and so on, all become part of the value of the variable you are
 defining.  Defining Multi-Line Variables Multi-Line, for a
 complete explanation of 'define'.
 
    The first command in this example runs Yacc on the first prerequisite
 of whichever rule uses the canned sequence.  The output file from Yacc
 is always named 'y.tab.c'.  The second command moves the output to the
 rule's target file name.
 
    To use the canned sequence, substitute the variable into the recipe
 of a rule.  You can substitute it like any other variable (Basics
 of Variable References Reference.).  Because variables defined by
 'define' are recursively expanded variables, all the variable references
 you wrote inside the 'define' are expanded now.  For example:
 
      foo.c : foo.y
              $(run-yacc)
 
 'foo.y' will be substituted for the variable '$^' when it occurs in
 'run-yacc''s value, and 'foo.c' for '$@'.
 
    This is a realistic example, but this particular one is not needed in
 practice because 'make' has an implicit rule to figure out these
 commands based on the file names involved (Using Implicit Rules
 Implicit Rules.).
 
    In recipe execution, each line of a canned sequence is treated just
 as if the line appeared on its own in the rule, preceded by a tab.  In
 particular, 'make' invokes a separate sub-shell for each line.  You can
 use the special prefix characters that affect command lines ('@', '-',
 and '+') on each line of a canned sequence.  Writing Recipes in
 Rules Recipes.  For example, using this canned sequence:
 
      define frobnicate =
      @echo "frobnicating target $@"
      frob-step-1 $< -o $@-step-1
      frob-step-2 $@-step-1 -o $@
      endef
 
 'make' will not echo the first line, the 'echo' command.  But it _will_
 echo the following two recipe lines.
 
    On the other hand, prefix characters on the recipe line that refers
 to a canned sequence apply to every line in the sequence.  So the rule:
 
      frob.out: frob.in
              @$(frobnicate)
 
 does not echo _any_ recipe lines.  (Recipe Echoing Echoing, for a
 full explanation of '@'.)
 

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