Dear Tom, 


> ERROR:  While executing gem ... (Errno::EACCES)                 
>     Permission denied - /home/tomc/.ri/cache/.doc_dirs

you don't have sufficient writing rights on that file, probably
because you installed the gems as normal user rather than with sudo
privileges.
There is by default no "root" user in Ubuntu like in other Linuxes,
but you can create one and use "gem install" as root:

sudo passwd root



 - and then do all sorts of (potentially dangerous things ..)

(look here:  https://help.ubuntu.com/community/RootSudo)


Best regards,

Axel 
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