"Off-topic" doesn't always mean off-topic. It's (currently) a catch-all category for things which don't meet community guidelines or are lacking in other respects. Your topic is on-topic, but your post could use improvement.
What's Wrong with Your Post
In my opinion, your post has the following issues:
- It isn't clear what your question is.
- You have built a really large "wall of code" into your post.
- You haven't demonstrated an understanding of the problem.
- Your post isn't succinct or informative because you haven't really isolated the issue.
Ultimately, your question is committing the cardinal sin of boring the reader through lack of focus. By the time future meta users go to look at it, it may have already been edited or improved. However, that's how it looks right now.
How to Improve Your Post
Luckily, your post can be improved with just a bit of work on your part. Here are some suggestions to get you started:
Clarify your question.
"Please find my bug" is a terrible premise for anything on Stack Overflow. Be targeted. You have a stack trace and some triggering code. Hone in on that.
Trim your code.
Reduce your code to the minimum necessary to reproduce your state in a REPL. The exercise of doing that might even allow you to solve the problem yourself; if not, at least it will direct readers' attention to the right place.
Demonstrate your understanding of the problem.
Even if your conclusions are wrong, people want to know what you think is wrong and what you've already considered. No one wants to spend time writing an answer only to have you say you already tried that and it didn't work, either.
Make your title count!
A good title is the most important part of your question, because a well-written title attracts the right readers. "Starting a Python script from within another - odd behavior" is extremely generic; something like "Why is OS.system using /bin/sh?" might be better if it addresses your core question or draws in people who might know something about the method call you're using.
Make your post as succinct as possible.
You have a backtrace that shows a syntax error. You should probably focus your post on that.
/bin/sh: 1: Syntax error: "(" unexpected (expecting "}") seems like it should be the point of your post. Posting the backtrace, along with just enough code to reproduce the problem, would improve your question tremendously.
Reference code, not line numbers.
Unless your code sample includes line numbers, don't reference lines of code that way. Instead of saying "[t]he error is now on line 29" you should use properly-formatted code inline. For example:
multiprocessing.Process(os.system('./conf/set_data.py3')) results in the following error:
/bin/sh: 1: Syntax error: "(" unexpected (expecting "}")
would be a lot more informative for the readers, would also go a long way towards showing folks what you think is wrong, and what sort of solution you might be looking for.
Your question is salvageable, and I think you should make the effort to fix it up. You may or may not reverse any existing down-votes, but you'll be more likely to get a useful answer, and less likely to fall afoul of the system's ban-hammer.
Also, cleaning up your question might help you find your own answer. If that happens, you'll be more in keeping with the Stack Overflow spirit if you post that answer to your own question and help future visitors.