unshadow

Utility provided by the John the Ripper project to obtain the traditional Unix password file if the system uses shadow passwords. More information: https://www.openwall.com/john/.

Combine the /etc/shadow and /etc/passwd of the current system:

sudo unshadow /etc/passwd /etc/shadow

Combine two arbitrary shadow and password files:

sudo unshadow {{path/to/passwd}} {{path/to/shadow}}