Always quote shell script variable expansions using double quotes to prevent word splitting issues and ensure consistent behavior. This is particularly important in conditionals and when passing variables as command arguments. Bad: ```bash rm -rf /usr/local/lib/python${PYTHON_MAJOR_MINOR_VERSION}/site-packages/ if [[ ${UPGRADE_SQLALCHEMY=} != "true" ]]; then ``` Good: ```bash rm -rf /usr/local/lib/python"${PYTHON_MAJOR_MINOR_VERSION}"/site-packages/ if [[ "${UPGRADE_SQLALCHEMY}" != "true" ]]; then ``` Unquoted variables can lead to unexpected behavior when they contain spaces, are empty, or include special characters. Consistently applying proper quoting improves script robustness and prevents subtle bugs that might only appear in edge cases.