Although John the Ripper has been packaged for debian and ubuntu, it seems that as of August 2015 the packaged version doesn't actually work. Recent versions of these systems encrypt passwords using the sha512 hash function, but support for that hash function is only currently available through a user-supported version of the program. The symptom of this problem is the generic error message "No password hashes loaded." How do you get it to work?
Go to http://www.openwall.com/john/ and find the URL of the latest community-enhanced version, which is in xz format.
wget http://www.openwall.com/john/j/john-1.8.0-jumbo-1.tar.xz tar xf john-1.8.0-jumbo-1.tar.xz sudo apt-get install libssl-dev cd john-1.8.0-jumbo-1/src && ./configure && make cd ../run sudo ./unshadow /etc/passwd /etc/shadow > ~bcrowell/mypasswd.txt ./john mypasswd.txt rm mypasswd.txt