GNU Make Manual

Values

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Variables can get values in several different ways:

You can specify an overriding value when you run make. Overriding Variables.
You can specify a value in the makefile, either with an assignment (Setting Variables) or with a verbatim definition (Defining Variables Verbatim).
Variables in the environment become make variables. Variables from the Environment.
Several automatic variables are given new values for each rule. Each of these has a single conventional use. Automatic Variables.
Several variables have constant initial values. Variables Used by Implicit Rules.