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$ sed 's/=/ /' complex_file | join -a 1 -1 1 -2 3 simple_file -

$ mv simple_file simple_file.bk; sort simple_file.bk > simple_file
$ mv complex_file comple_file.bk; sed 's/=/\t/' complex_file.bk | sort
+ -k 3,3r | sed 's/\t/=/' > complex_file