The 2nd and 3rd examples are plural. You don't need an article for plural nouns. "Fix bayonets." and "Fix the bayonet." are standard grammar. "Fix bayonet." isn't.
Well, hands up is lacking a verb, and fix bayonets is in a funny passive tense - or something - because it seems to say "generally go around looking for bayonets to fix", but means specifically "fix your bayonets". In fact hands up is like that too, the intent is "put your hands up", not just "put hands up" in the abstract.
Then there's informational signs, too. Wet floor is not an instruction. Labels generally aren't sentences.
Or instructions on signs: ring bell for assistance, return tray to counter, close gate after use.
> Or instructions on signs: ring bell for assistance, return tray to counter, close gate after use.
I have never seen this.
I have seen plenty of "Please close the gate" or "Keep the gate closed". Sometimes, the article is eluded when the noun is subject "Gate must be kept closed" but imperative + noun without an article on a sign seem highly unusual to me. It feels weird so I would definitely notice.
I have seen "ring bell for assistance" however. It's jarring everytime. I must be the strange one.
This kind of phrasing is so common (in American English directions) that I remember examples from when I was very young:
(on toothpaste) "Squeeze tube from the bottom and flatten it as you go up."
(on a kerosene heater) "Rotate wick adjuster knob clockwise until it stops."
Australians tend to prefer more conversationally phrased directions from what I've seen, e.g., the rail station signs that read "Keep off the tracks and use the walkways provided to cross. Or catch a $100 fine. Don't say we didn't warn you, mate!"