Variable assignment of Makefile

There are 4 ways to assign variables in makefile:

= ,   := ,  += ,  ?=

　　= : Direct Assignment

Variable = value

　　: = : Position-dependent assignment

If the right value is a value, then it is not the same as =, if the right value is a variable, then the value of the left variable is equal to the value of the current position of the right variable, what does it mean?

A = ABCB = $ (a) a = Jklall: @echo $ (b). Phony:all

Executing this makefile, the value of output B is jkl, and in the sentence B = $ (a) , the value of variable A is deferred, which is determined by the last assignment statement a = JKL of variable A. If B is assigned a value of: =, the situation is different:

A = ABCB: = $ (a) a = Jklall: @echo $ (b). Phony:all

At this point, the value of output B is ABC, which means to get the value of the current position variable a assigned to B.

　　? = : Asks if the left variable is defined, that is, whether an assignment is used, or not, otherwise ignored. Note whether it is defined, not whether it is a null value, and also note that null values are not equal to spaces, tab keys, and so on.

Here is the space, how to spaces grid? or Tab

Spacevalue= Space/tab#

Note that the # specifier is required, at which point the value of Spacevalue is the assignment = after the beginning of the #, there is a trap for the comment, assuming that a variable dir is defined, assigning it a path

Dir =/home/thomas  #xx的路径

This is due to variable assignment, the left and right side of the assignment number of spaces, tab will be ignored, is from the first non-space, non-tab to the last non-space, non-tab end, unless you encounter an annotation, otherwise to the end of the comment.

　　+ = : Append Assignment

　　

　　the final value of a variable is determined by one of the above assignment operations, based on the rules .

　　

Variable assignment of Makefile