Export environment variables from child bash script in GNU Make

Question

I have a script "set_env.py" that outputs the following uppon execution:

export MY_VAR_1=some_value
export MY_VAR_2=some_other_value

I cannot change this script, it is supplied in my current environment.

Further I have a Makefile that looks like this:

SHELL := /bin/bash

all: set_env
    env | grep MY_VAR    

set_env:
    eval ./set_env.py

With this makefile I expected the grep to list my two variables, however it seems the environment is not set.

I suspect this is because make creates a sub-environment for each line and so the variables set on the first line will not be available in the second.

So the question is, how would I go about exporting the environment from the untouchable script in my makefile ?

Answer 1

Actually, the output of the python is valid make . One option then is to read the output of the python directly into the makefile. The only fly in the ointment is that $(shell) doesn't cut the mustard.

include Environment.mk

PHONY: test
test:
    env | grep MY_VAR

Environment.mk:
    ./set_env.py >$@-tmp
    mv $@-tmp $@

How does this work? The first thing that make tries to do is to ensure the makefile itself is up-to-date. Since we have told it to include Environment.mk , make must ensure that is up-to-date too.

• Make finds a rule for Environment.mk
• The python is run, creating Environment.mk
• Environment.mk is read in, creating two make variables with the export attribute
• The makefile is now up-to-date, so make proceeds on to the target ( test in this case)
• Make runs test 's recipe, exporting any variables with the export attribute.

No recursion, but you should ensure the python always spits out make compatible syntax.

Answer 2

Recursive make is your friend I'm afraid.

.PHONY: all
all:
    eval $$(./set_env.py) && ${MAKE} test

.PHONY: test
test:
    env | grep MY_VAR    

There are a few moving parts here.

• make all executes the shell command eval $(./set_env.py) && make test
• The shell first does the command substitution
  • $(./set_env.py) is replaced by the export commands
• The export commands are passed to the (shell) eval command
  • The environment variables are defined, but only for this shell invocation
• The eval succeeds, so the execution passes to the command after the &&
• Make runs recursively, but this second make has an augmented environment