pass env variables in make command in makefile - shell

Question

I am trying to pass a shell variable from one makefile command to another, but so far have not been successful.
target1:
curl ... ${myvar} ## using myvar from target2
target2:
export myvar=$(shell curl .....);
echo $myvar
In the above case, I am not even getting the output on echo $myvar. Is there something I'm doing wrong?

Answer 1

In a Makefile, every line of a target is run in a separate shell. Additionally, a command can only change the environment for itself and its children. So when you have:
target2:
export myvar=$(shell curl .....);
echo $myvar
And you run make target2, the following happens:
Make starts a shell that runs export myvar=...some value...
The shell exits.
Make runs another shell that runs echo $myvar
That shell exits.
First, there's a syntax problem here: when you write $myvar, this
will be interpreted by make as a request for the value $m followed
by the text yvar. To access shell variables, you need to escape the
$, like this:
echo $$myvar
But that won't solve this problem, because as we see from the above
sequence, the export command happens in a shell process which
immediately exits, so it's effectively invisible to anything else.
This target would work the way you expect if you were to write:
target2:
export myvar=$(shell curl .....); \
echo $$myvar
Here, because we're using the \ to escape the end-of-line, this is
all one long "virtual" line and executes in a single shell process, so
the echo statement will see the variable value set in the previous
statement.
But nothing will make an environment variable set in a shell process
in one target visible in another target, because there's no way to get
these to execute in the same process.
If you need to set variables in your Makefile that are visible
across all targets, set make variables:
myvar = $(shell curl ...)
target1:
curl ... $(myvar)
A workaround, as you have discovered, is to re-execute make as a
child process from within the process that set the environment
variable as in:
target2:
export myvar=$(shell curl .....); \
echo $$myvar; \
$(MAKE) myvar=$$myvar
But often this sort of recursive call to make results in a more
complicated Makefile.

Answer 2

Found the answer. Posting if anyone comes across this. I needed to use $$ instead of $
target1:
curl ... ${myvar} ## using myvar from target2
target2:
export myvar=$(shell curl .....);
echo $$myvar
$(MAKE) myvar=$${myvar} target1