How to convert a text file from Shift JIS to UTF-8 and back from the terminal

To convert from Shift JIS to UTF-8 on linux just do:

iconv -f SHIFT-JIS -t UTF-8 <infile> > <outfile>

and to go the other way just reverse the arguments:

iconv -f UTF-8 -t SHIFT-JIS <infile> > <outfile>

I don't know OSX but it may well have the iconv command as well, in which case something similar should work.