( How Make Works

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 2.3 How `make' Processes a Makefile
 By default, `make' starts with the first target (not targets whose
 names start with `.').  This is called the "default goal".  ("Goals"
 are the targets that `make' strives ultimately to update.    You can
 override this behavior using the command line ( Arguments to
 Specify the Goals Goals.) or with the `.DEFAULT_GOAL' special variable
 ( Other Special Variables Special Variables.).  
    In the simple example of the previous section, the default goal is to
 update the executable program `edit'; therefore, we put that rule first.
    Thus, when you give the command:
 `make' reads the makefile in the current directory and begins by
 processing the first rule.  In the example, this rule is for relinking
 `edit'; but before `make' can fully process this rule, it must process
 the rules for the files that `edit' depends on, which in this case are
 the object files.  Each of these files is processed according to its
 own rule.  These rules say to update each `.o' file by compiling its
 source file.  The recompilation must be done if the source file, or any
 of the header files named as prerequisites, is more recent than the
 object file, or if the object file does not exist.
    The other rules are processed because their targets appear as
 prerequisites of the goal.  If some other rule is not depended on by the
 goal (or anything it depends on, etc.), that rule is not processed,
 unless you tell `make' to do so (with a command such as `make clean').
    Before recompiling an object file, `make' considers updating its
 prerequisites, the source file and header files.  This makefile does not
 specify anything to be done for them--the `.c' and `.h' files are not
 the targets of any rules--so `make' does nothing for these files.  But
 `make' would update automatically generated C programs, such as those
 made by Bison or Yacc, by their own rules at this time.
    After recompiling whichever object files need it, `make' decides
 whether to relink `edit'.  This must be done if the file `edit' does
 not exist, or if any of the object files are newer than it.  If an
 object file was just recompiled, it is now newer than `edit', so `edit'
 is relinked.  
    Thus, if we change the file `insert.c' and run `make', `make' will
 compile that file to update `insert.o', and then link `edit'.  If we
 change the file `command.h' and run `make', `make' will recompile the
 object files `kbd.o', `command.o' and `files.o' and then link the file
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