How do I convert epub to mobi using calibre?

Yes, Calibre package contains a script called ebook-convert to convert ebooks between formats. You can install Calibre with

sudo apt-get install calibre

then

ebook-convert "book.epub" "book.mobi"

NB: Other formats are supported, too:

ebook-convert "book.azw3" "book.pdf"

I found a script for batch conversion:

for book in *.epub; do echo "Converting $book"; ebook-convert "$book" "$(basename "$book" .epub).mobi"; done

It works good for me, but the process is pretty slow.

You can also do this without installing Calibre. This would be really handy on a Chromebook, for example. There is a free, online site called EPUB Converter that will let you upload the epub and it lets you then download the mobi equivalent.