How to assign the output of a command to a Makefil

Question

I need to execute some make rules conditionally, only if the Python installed is greater than a certain version (say 2.5).

I thought I could do something like executing:

python -c 'import sys; print int(sys.version_info >= (2,5))'

and then using the output ('1' if ok, '0' otherwise) in a ifeq make statement.

In a simple bash shell script it's just:

MY_VAR=`python -c 'import sys; print int(sys.version_info >= (2,5))'`

but that doesn't work in a Makefile.

Any suggestions? I could use any other sensible workaround to achieve this.

Answer 1

Here's a bit more complicated example with piping and variable assignment inside recipe:

getpodname:
    # Getting pod name
    @eval $$(minikube docker-env) ;\
    $(eval PODNAME=$(shell sh -c "kubectl get pods | grep profile-posts-api | grep Running" | awk '{print $$1}'))
    echo $(PODNAME)

Answer 2

Wrapping the assignment in an eval is working for me.

# dependency on .PHONY prevents Make from 
# thinking there's `nothing to be done`
set_opts: .PHONY
  $(eval DOCKER_OPTS = -v $(shell mktemp -d -p /scratch):/output)

Answer 3

I'm writing an answer to increase visibility to the actual syntax that solves the problem. Unfortunately, what someone might see as trivial can become a very significant headache to someone looking for a simple answer to a reasonable question.

Put the following into the file "Makefile".

MY_VAR := $(shell python -c 'import sys; print int(sys.version_info >= (2,5))')

all:
    @echo MY_VAR IS $(MY_VAR)

The behavior you would like to see is the following (assuming you have recent python installed).

make
MY_VAR IS 1

If you copy and paste the above text into the Makefile, will you get this? Probably not. You will probably get an error like what is reported here:

makefile:4: *** missing separator. Stop

Why: Because although I personally used a genuine tab, Stack Overflow (attempting to be helpful) converts my tab into a number of spaces. You, frustrated internet citizen, now copy this, thinking that you now have the same text that I used. The make command, now reads the spaces and finds that the "all" command is incorrectly formatted. So copy the above text, paste it, and then convert the whitespace before "@echo" to a tab, and this example should, at last, hopefully, work for you.

Answer 4

Use the Make shell builtin like in MY_VAR=$(shell echo whatever)

me@Zack:~$make
MY_VAR IS whatever

me@Zack:~$ cat Makefile 
MY_VAR := $(shell echo whatever)

all:
    @echo MY_VAR IS $(MY_VAR)