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What Makefiles Contain

   Makefiles contain five kinds of things: "explicit rules", "implicit
rules", "variable definitions", "directives", and "comments".  Rules,
variables, and directives are described at length in later chapters.

   * An "explicit rule" says when and how to remake one or more files,
     called the rule's targets.  It lists the other files that the
     targets "depend on", and may also give commands to use to create
     or update the targets.  Note: Writing Rules.

   * An "implicit rule" says when and how to remake a class of files
     based on their names.  It describes how a target may depend on a
     file with a name similar to the target and gives commands to
     create or update such a target.  *Note Using Implicit Rules:
     Implicit Rules.

   * A "variable definition" is a line that specifies a text string
     value for a variable that can be substituted into the text later.
     The simple makefile example shows a variable definition for
     `objects' as a list of all object files (Note: Variables Make
     Makefiles Simpler.).

   * A "directive" is a command for `make' to do something special while
     reading the makefile.  These include:

        * Reading another makefile (*note Including Other Makefiles:

        * Deciding (based on the values of variables) whether to use or
          ignore a part of the makefile (Note: Conditional Parts of

        * Defining a variable from a verbatim string containing
          multiple lines (Note: Defining Variables Verbatim.).

   * `#' in a line of a makefile starts a "comment".  It and the rest of
     the line are ignored, except that a trailing backslash not escaped
     by another backslash will continue the comment across multiple
     lines.  Comments may appear on any of the lines in the makefile,
     except within a `define' directive, and perhaps within commands
     (where the shell decides what is a comment).  A line containing
     just a comment (with perhaps spaces before it) is effectively
     blank, and is ignored.

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