Has the word halleluyah always been both a command to praise and a praise itself, or was there a historical development from the word as command to the word as the praise itself? Thank you for your assistance.

Both explanations are found in the interesting ancient tradition to write the word “Halleluyah” found at the end of Psalms 104, as a new line, right in the middle between the previous sentence, and the beginning of the next chapter, 105. As the end of the previous psalm, it is like an exclamation: “He is praised”. On the other hand, as the beginning of the next chapter, it serves as a command or request: “Let’s Praise Him” (as in the beginning of Psalms 117). In Psalm 106, we find both usages, at the beginning and at the end. In other words, it is both a command, and praise in unto itself. In all probability, it began as a command (as two words: Hallilu Y-ah (similar to the beginning of Psalms 117), and eventually became also a praise, becoming one word. With Love of Israel, Rav Ari Shvat