Remove then add string from variable of Makefile

alex_mianmian

于 2020-10-06 13:21:53 发布

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分类专栏： Makefile

本文链接：https://blog.youkuaiyun.com/alex_mianmian/article/details/108938257

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本文介绍了在Makefile中如何使用shell命令来修改变量的值，通过示例展示了如何使用`$(shell ...)`来执行`sed`命令，从而改变`LDFLAGS`变量中的字符串，并确保在Makefile的其他地方正确地引用修改后的变量。

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As we know, use sed command can change string.

When we want to use it in Makefile, how to use it?

Let's write a Makefile as following:

LDFLAGS = -Txxhello.lds -Lhello
IOT_LDFLAGS = $(LDFLAGS) | sed 's/-T.*lds/-Talex.lds/g'

all:
        @echo $(LDFLAGS)
        @echo $(IOT_LDFLAGS)

now let's execute make:

Looks good, we got what we wanted.

However, when you plan to use $(IOT_LDFLAGS), you will find something different.

let's use echo instead of @echo in Makefile,here is the output:

Here we can see:

read line marks the $(LDFLAGS)

blue line marks the $(IOT_LDFLAGS)

@echo just show the result after echo executed. so $(IOT_LDFLAGS) is not '-Talex.lds -Lhello'.

The correct way is:

LDFLAGS = -Txxhello.lds -Lhello
#IOT_LDFLAGS = $(LDFLAGS) | sed 's/-T.*lds/-Talex.lds/g'
IOT_LDFLAGS = $(shell echo $(LDFLAGS) | sed 's/-T.*lds/-Talex.lds/g')
all:
        echo $(LDFLAGS)
        echo $(IOT_LDFLAGS)

use $(shell xxxxx) to execute sed and return the result to $(IOT_LDFLAGS).

here is make result:

Now the $(IOT_LDFLAGS) is '-Talex.lds -Lhello'