उपकोसलो ह वै कामलायनः सत्यकामे जाबाले ब्रह्मचार्यमुवास तस्य ह द्वादश वार्षाण्यग्नीन्परिचचार स ह स्मान्यानन्तेवासिनः समावर्तयंस्तं ह स्मैव न समावर्तयति ॥ ४.१०.१ ॥
In plain words
Upakosala Kāmalāyana lived as a student of sacred knowledge with Satyakāma Jābāla. For twelve years he tended his fires. And though the teacher let other pupils return home, he did not let him return.
The commentators
Chāndogyopaniṣadbhāṣyam
पुनर्ब्रह्मविद्यां प्रकारान्तरेण वक्ष्यामीत्यारभते गतिं च तद्विदोऽग्निविद्यां च । आख्यायिका पूर्ववच्छ्रद्धातपसोर्ब्रह्मविद्यासाधनत्वप्रदर्शनार्था—
उपकोसलो ह वै नामतः कमलस्यापत्यं कामलायनः सत्यकामे जाबाले ब्रह्मचर्यमुवास । तस्य, ह ऐतिह्यार्थः, तस्य आचार्यस्य द्वादश वर्षाणि अग्नीन्परिचचार अग्नीनां परिचरणं कृतवान् । स ह स्म आचार्यः अन्यान्ब्रह्मचारिणः स्वाध्यायं ग्राहयित्वा समावर्तयन् तमेवोपकोसलमेकं न समावर्तयति स्म ह ॥
In plain words
The text now begins to impart the knowledge of Brahman through a different approach, along with the path of the knowers and the science of the sacred fires. As before, this story is introduced to show that faith and austerity are the true stepping stones to the knowledge of Brahman.
Upakosala, son of Kamala and hence known as Kāmalāyana, lived as a celibate student with Satyakāma, the son of Jabālā. The word 'tasya' here indicates a traditional account: for twelve years he diligently served that teacher's sacred fires. Yet while the teacher would have the other students complete their Vedic studies and send them home to their families, he consistently left Upakosala behind without sending him home.
Close to the Sanskrit
Beginning again, [the text] undertakes to teach the knowledge of Brahman through another method, as well as the path [of the soul] and the fire-knowledge of those who know that. The narrative is meant to show, as before, that faith and austerity are the means to the knowledge of Brahman.
Upakosala, by name the offspring of Kamala, hence Kāmalāyana, lived as a student of sacred knowledge under Satyakāma, the son of Jabālā. 'Tasyāha' has the sense of tradition: he tended the fires of that teacher for twelve years, performing the service of the fires. That teacher, having made the other students of sacred knowledge grasp their Vedic recitation and sending them home, did not send Upakosala alone home.