Hello again this time I just got a shedinja, and as most of you know it only has 1 HP. so if you let it hold a focus band, can it survive forever?
No. As soon as it is hit once, the Focus Sash is used up, just like with any other Pokemon.
Focus Band - 10% chance of effectivity
Focus Sash - 100% chance of effectivity, one-time-use-only