I had always thought that you need to post a bond in order to obtain an injunction, both in federal and state court. It turns out that I was wrong.
The federal rule seems to require a bond. It says:
(c) Security. The court may issue a preliminary injunction or a temporary restraining order only if the movant gives security in an amount that the court considers proper to pay the costs and damages sustained by any party found to have been wrongfully enjoined or restrained. The United States,
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