Bhagavad Gita · Jnana Karma Sannyasa YogaChapter 4, Shloka 13 (4.13)

The two parts of the verse. (1) The creation of the fourfold varna order — according to the division of gunas and actions; (2) the Lord is the non-doer even while being the doer.

The traditional commentaries — as they are written:

• Shankaracharya: the four varnas were created by Ishvara — according to the Purusha Sukta (brāhmaṇo 'sya mukham āsīt). The division by gunas: the brahmin, in whom sattva predominates (with actions such as shama, dama, tapas — calm, self-control, austerity); the kshatriya, in whom rajas predominates with sattva secondary (valour, vigour); the vaishya, in whom rajas predominates with tamas secondary (farming and so on); the shudra, in whom tamas predominates with rajas secondary (śuśrūṣaiva karma — service alone). Then the main question: is the Lord bound by this act of creation? The answer: māyā-saṃvyavahāreṇa — though the doer in the workings of maya, paramārthataḥ akartāram — in ultimate truth he is the non-doer.
• Madhusudana Sarasvati: the same kind of division of varnas by gunas; and he says that śarīrārambhaka-guṇa-vaiṣamyād api na sarve samāna-svabhāvāḥ — because of the inequality of the gunas that form the body, not all have the same nature.
• Madhva: the division by gunas in the same order, and the division by actions will be explained from 18.42 onwards.
• Acharya Ramanuja: a broad reading — cāturvarṇya-pramukhaṃ brahmādi-stamba-paryantaṃ kṛtsnaṃ jagat — the entire universe, from Brahma down to a blade of grass, of which the four varnas are the chief example, I created through the division of the gunas, sattva and the rest, and of the actions that correspond to them — and I alone protect and dissolve it. Though I am its maker, know me to be the non-doer (how — 4.14).

An honest conclusion: The scriptural commentators connect this verse with the social order of their time (which in practice was tied to birth), and they assign to the shudra the work of "service". This is a record of their view — not this book's endorsement.