When the same sequence of commands is useful in making various
targets, you can define it as a canned sequence with the
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 $@ endef
run-yacc is the name of the variable being defined;
endef marks the end of the definition; the lines in between are the
define directive does not expand variable references
and function calls in the canned sequence; the ‘$’ characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.
See Defining Multi-Line Variables,
for a complete explanation of
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
(see Basics of Variable References).
Because variables defined by
define are recursively expanded
variables, all the variable references you wrote inside the
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
make has an implicit rule to figure out these
commands based on the file names involved
(see Using 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
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.
See Writing Recipes in Rules.
For example, using this canned sequence:
define frobnicate = @echo "frobnicating target $@" frob-step-1 $< -o $@-step-1 frob-step-2 $@-step-1 -o $@ endef
make will not echo the first line, the
But it will echo the following two recipe lines.
On the other hand, prefix characters on the recipe line that refers to a canned sequence apply to every line in the sequence. So the rule:
frob.out: frob.in @$(frobnicate)
does not echo any recipe lines. (See Recipe Echoing, for a full explanation of ‘@’.)