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.
See 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. See Using 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
as a list of all object files (see Variables Make Makefiles Simpler).
- A directive is a command for
make to do something special while
reading the makefile. These include:
- ‘#’ 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.
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
make comments or as command script text, depending on the
context in which the variable is evaluated.