Clean up CI bash scripts, add docs/STYLE_GUIDE.md

docs/STYLE_GUIDE.md

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+# Style Guide
+
+The majority of our style recommendations are automatically enforced via our
+automated linters. This document has guidelines that are less easy to lint for.
+
+## Shell scripts
+
+Shell scripts are OK for small tasks or wrappers, but prefer to use Python for
+anything with a hint of complexity or in general.
+
+Shell scripts should be written against bash, starting with this shebang:
+```
+#!/usr/bin/env bash
+```
+
+Note that the version of bash available on OS X by default is quite old, so be
+careful when using new features.
+
+Scripts should enable a few options at the top for robustness:
+```
+set -o errexit
+set -o nounset
+set -o pipefail
+```
+
+Quote all variables, using the full form: `"${SOME_VARIABLE}"`.
+
+Use `"$(some-command)"` instead of backticks for command substitution. Note
+that these should be quoted as well.