1. First need to run
   unshadow passwdfile shadowfile >unshadowed.txt
2. Then crack with
   john --rules --wordlist=/usr/share/seclists/rockyou.txt unshadowed.txt
3. Or straight use hashcat:
   hashcat -m 1800 -a 0 -o cracked.txt hashes.txt /usr/share/seclists/rockyou.txt -O
   (use -m 500 for even old password shadow hash)