Not sure - I've never used it before.
It does look like it's tring to either execute something (the /bin/sh), or a script inside is messed up.
[time passes...]
I just downloaded it, and it seems that the version I got is compiled against a different GLib than I have, but for some reason "rar" works...
The rar options would be :
rar e archive.rar
I got this info by just running "rar" - it should give you a list of possible commands.