Is there a free command line tool (preferably with a database) to find synonyms/antonyms in Linux? Where can I get it?
If you manage to find the necessary dictionary files in any 'open' format such as stardict, DSL, xdxf, Babylon BGL (this one is not really open, but there are tons of free dictionaries on their site), etc, then you can convert them to stardict format and use them from console using sdcv. Conversion can be done via makedict and/or dictconv.
Another option would be using the google dictionary:
I while ago I blogged about using dictionaries and google translate from console.