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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 in better detail in the corresponding references noted in the following descriptions..

An explicit rule says when and how to remake one or more files called the rule’s targets. It lists the other files on which the targets depend, 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 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 (see Variables Make Makefiles Simpler).

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

Reading another makefile (see Including Other Makefiles).

Deciding (based on the values of variables) whether to use or ignore a part of the makefile (see Conditional Parts of Makefiles).

Defining a variable from a verbatim string containing multiple lines (see Defining Variables Verbatim).

A comment in a line of a makefile starts with ‘#’. 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.