This task comes up regularly in my day job. It deserves a post.

We don't have photoshop and GIMP can't convert RGB to CMYK by default. ((I don't want to suffer the pain of installing GIMP plugin)) All I need is to convert an RGB TIFF image file to CMYK. No less, no more. ((I don't care the color conversion accuracy.))
The saviour? ImageMagick!

convert f3.tiff -colorspace CMYK -compress lzw f3_cmyk.tiff
identify -verbose f3_cmyk.tiff

The first line convert the color space of f3.tiff to CMYK and compress the image with LZW algoirthm. The second line is to confirm the color space has been changed.