4.10 Multiple Targets in a Rule

A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same commands apply to all the targets, but their effects may vary because you can substitute the actual target name into the command using ‘$@’. The rule contributes the same prerequisites to all the targets also.

This is useful in two cases.

• You want just prerequisites, no commands. For example:

            kbd.o command.o files.o: command.h
  

  gives an additional prerequisite to each of the three object files mentioned.

• Similar commands work for all the targets. The commands do not need to be absolutely identical, since the automatic variable ‘$@’ can be used to substitute the particular target to be remade into the commands (see Automatic Variables). For example:

            bigoutput littleoutput : text.g
                    generate text.g -$(subst output,,$@) > $@
  

  is equivalent to

            bigoutput : text.g
                    generate text.g -big > bigoutput
            littleoutput : text.g
                    generate text.g -little > littleoutput
  

  Here we assume the hypothetical program generate makes two types of output, one if given ‘-big’ and one if given ‘-little’. See Functions for String Substitution and Analysis, for an explanation of the subst function.

Suppose you would like to vary the prerequisites according to the target, much as the variable ‘$@’ allows you to vary the commands. You cannot do this with multiple targets in an ordinary rule, but you can do it with a static pattern rule. See Static Pattern Rules.