I need the xpi_hash variable to be assigned only when update target's command is decided to execute. Then I'm using this variable as environment, exporting, etc..

If I put it outside of rule, it will be expanded firstly, before $(xpi) target is called, hence will not find that file.

update: $(xpi) $(target_update_rdf) 
    xpi_hash   := $(shell sha1sum $(xpi) | grep -Eow '^[^ ]+')
    @echo "Updating..."

and above of course is not correct, because for command part the shell is represented. So maybe another way to put this question is - how to bring variable as command output?