(make.info.gz) Empty Targets

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 4.8 Empty Target Files to Record Events
 =======================================
 
 The "empty target" is a variant of the phony target; it is used to hold
 recipes for an action that you request explicitly from time to time.
 Unlike a phony target, this target file can really exist; but the file's
 contents do not matter, and usually are empty.
 
    The purpose of the empty target file is to record, with its
 last-modification time, when the rule's recipe was last executed.  It
 does so because one of the commands in the recipe is a `touch' command
 to update the target file.
 
    The empty target file should have some prerequisites (otherwise it
 doesn't make sense).  When you ask to remake the empty target, the
 recipe is executed if any prerequisite is more recent than the target;
 in other words, if a prerequisite has changed since the last time you
 remade the target.  Here is an example:
 
      print: foo.c bar.c
              lpr -p $?
              touch print
    
 With this rule, `make print' will execute the `lpr' command if either
 source file has changed since the last `make print'.  The automatic
 variable `$?' is used to print only those files that have changed
 ( Automatic Variables).
 

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