makefile - 如何在makefile配方中设置环境变量？

Question

这是一个简化的Makefile:

all:
    @for (( i = 0; i < 5; ++i )); do \
         var="$$var $$i"; \
         echo $$var; \
     done
    @echo $$var

我假设“var”的值为“0 1 2 3 4”，但输出为:

0
0 1
0 1 2
0 1 2 3
0 1 2 3 4
               <--- NOTHING!!!

如您所见，最后一个回声是“NOTHING”。怎么了？

Answer 1

从here:

When it is time to execute recipes to update a target, they are executed by invoking a new subshell for each line of the recipe...

Please note: this implies that setting shell variables and invoking shell commands such as cd that set a context local to each process will not affect the following lines in the recipe. If you want to use cd to affect the next statement, put both statements in a single recipe line. Then make will invoke one shell to run the entire line, and the shell will execute the statements in sequence.

请尝试以下方法:

all:
    @for (( i = 0; i < 5; ++i )); do \
         var="$$var $$i"; \
         echo $$var; \
     done; \
    echo $$var