[visarga]

You can implement this and many other single purpose CLI tools with inline Python or Perl or other language. Easier to remember because it's your favorite language.

    python -c "import json; print(json.dumps(dict(a=10, b=False)))"

[Too]

While a handy trick, it doesn't entirely solve the original problem when the values have dynamic content. In your example, replace 10 with $FOO and then you are back to square one, having to escape python-strings within a shell-string. Better avoid the problem entirely by not using shell to begin with. To instead continue on the dirty track, replace 10 with int(sys.argv[1]) and call it as python -c "..." $FOO.