Remove then add string from variable of Makefile

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'