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3.1 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, called the "prerequisites" of the target, and
may also give commands to use to create or update the targets.
Writing Rules 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. 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 ( Variables Make
Makefiles Simpler Variables Simplify.).
* A "directive" is a command for `make' to do something special while
reading the makefile. These include:
* Reading another makefile ( Including Other Makefiles
Include.).
* Deciding (based on the values of variables) whether to use or
ignore a part of the makefile ( Conditional Parts of
Makefiles Conditionals.).
* Defining a variable from a verbatim string containing
multiple lines ( Defining Variables Verbatim Defining.).
* `#' 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. A line containing just a comment (with perhaps
spaces before it) is effectively blank, and is ignored. If you
want a literal `#', escape it with a backslash (e.g., `\#').
Comments may appear on any line in the makefile, although they are
treated specially in certain situations.
Within a command script (if the line begins with a TAB character)
the entire line is passed to the shell, just as with any other
line that begins with a TAB. The shell decides how to interpret
the text: whether or not this is a comment is up to the shell.
Within a `define' directive, comments are not ignored during the
definition of the variable, but rather kept intact in the value of
the variable. When the variable is expanded they will either be
treated as `make' comments or as command script text, depending on
the context in which the variable is evaluated.
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