新建一个Makefile，内容为：
ifdef DEFINE_VAR VAR = “Hello World!”elseendififeq ($(OPT),define) VAR ?= “Hello World! First!”endififeq ($(OPT),add) VAR += “John!”endififeq ($(OPT),recover) VAR := “Hello World! Again!”endifall: @echo $(VAR)敲入以下make命令：
#make DEFINE_VAR=true OPT=define #Hello World! #make DEFINE_VAR=true OPT=add #Hello World! John! #make DEFINE_VAR=true OPT=recover #Hello World! Again! #make DEFINE_VAR= OPT=define #Hello World! First! #make DEFINE_VAR= OPT=add #John! #make DEFINE_VAR= OPT=recover #Hello World! Again!
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