One file can be the target of several rules. All the dependencies mentioned in all the rules are merged into one list of dependencies for the target. If the target is older than any dependency 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.)
There is no reason to
write your makefiles this way; that is why make gives you
an error message.
An extra rule with just dependencies can be used to give a few extra
dependencies to many files at once. For example, one usually has a
variable named 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 dependency intermittently.
Another wrinkle is that the additional dependencies could be specified with
a variable that you set with a command argument to make
(see section Overriding Variables). For example,
extradeps= $(objects) : $(extradeps)
means that the command `make extradeps=foo.h' will consider `foo.h' as a dependency 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 section Using Implicit Rules).