Plain form as imperative

Is the plain form ever used to give a command? What does "sugu ni tatsu" mean?

Answer

Yes, 終止形 can be used as a command like in this manga. Translation: "Then stand up right away!"

It sounds like parents or school teachers ordering their kids to do something right away. It's weaker/friendlier than 「立て」 but stronger than 「立って下さい」.

I believe such use of 終止形 is not usually heard in military organizations, so the right person sounds rather friendly than dignified as a supervisor.

Negative form is also possible. Example: 「(教室で先生が)そこ、喋らない！」「ごちゃごちゃ反論しない！」）