One file can be the target of several rules. All the prerequisites mentioned in all the rules are merged into one list of prerequisites for the target. If the target is older than any prerequisite from any rule, the commands are executed.
There can only be one set of commands to be executed for a file. If
more than one rule gives commands for the same file, make uses
the last set given and prints an error message. (As a special case,
if the file's name begins with a dot, no error message is printed.
This odd behavior is only for compatibility with other implementations
of make... you should avoid using it). Occasionally it is
useful to have the same target invoke multiple commands which are
defined in different parts of your makefile; you can use
double-colon rules (see Double-Colon) for this.
An extra rule with just prerequisites can be used to give a few extra
prerequisites to many files at once. For example, makefiles often
have a variable, such as objects, containing a list of all the
compiler output files in the system being made. An easy way to say
that all of them must be recompiled if config.h changes is to
write the following:
objects = foo.o bar.o
foo.o : defs.h
bar.o : defs.h test.h
$(objects) : config.h
This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional prerequisite intermittently.
Another wrinkle is that the additional prerequisites could be specified with
a variable that you set with a command argument to make
(see Overriding Variables). For example,
extradeps=
$(objects) : $(extradeps)
means that the command ‘make extradeps=foo.h’ will consider foo.h as a prerequisite of each object file, but plain ‘make’ will not.
If none of the explicit rules for a target has commands, then make
searches for an applicable implicit rule to find some commands
see Using Implicit Rules).