guard duty
guard , ***
sentry duty
sentry go

I won't be able to spare engineers for sentry duty once we begin.

Standing on sentry duty, the big African looked like a loyal guard.

He was making ready for his turn at sentry duty.

"I want all the men within whistle sound that aren't on sentry duty."

"They immediately start doing sentry duty and caring for the young."

The others were put to work on sentry duty or cleaning up.

"We must return to the camp to begin sentry duty."

Yet there had been only one man on sentry duty.

After the second time she tried to sneak out, I sent her to bed and took up sentry duty here by the door.

He would never have to do patrols or sentry duties.