function be on duty be on duty (of police/military/etc.)
A policeman has to keep on his toes all the time he´s on duty.
警察在执勤时必须时刻保持警覺。
—— 给力词典精选The striped band round a policeman´s sleeve is to signify that he is on duty.
警察袖子上的袖标标明他正在执勤。
—— 给力词典精选Half the garrison are on duty.
有半数卫戍部队在执勤。
—— 辞典例句Half the garrison is / are on duty.
有半数卫戍部队在执勤。
—— 辞典例句I related to the duty officer what I´d seen and was told to telephone the main station.
我把我所看到的告诉了执勤官,他让我给总站打电话。
—— 《柯林斯英汉双解大词典》