Unless you work in a big printing place then you should ALWAYS work in RGB and leave the CMYK nonsense to the experts in wasting dead-trees. Pro printers and their software will do a better job of CMYK than you will. Besides, do you really want to fill up your disks twice as fast with extra-dull CMYK-ness? And then when you have a colleague put those images online that they don't show up in some browsers because they end up as half-supported CMYK-jpeg's?
You have heard 'if you cannot do, then teach', well, there are a lot of teachers out there killing the minds of keen art students by instilling the overstated importance of CMYK into their brains. Nobody really cares about dead-tree CMYK no more except these 'teachers' and those they brainwashed.
If you do happen to have to work with a batch of CMYK images that you NEED to get into RGB, then Photoshop 'batch' is your best option, however, if Linux is your preferred tool of choice then apt-get install imagemagick and put your own command-line batch file together.