You shouldn't be at the command line. Very little, if any, of ordinary FreeNAS administration needs to be done at the command line, and nothing should be done at the command line that can be done in the GUI. If your boot device is full, either it's way too small (in which case the only solution is to reinstall to a larger device, and restore your saved config file), or you have too many saved boot environments, which can be deleted from the web GUI. The manual has instructions on this.
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