GNU/Linux FAQ - GNU Project - Free Software Foundation
Linux or GNU/Linux is a Unix-like operating system for computers
The second line invokes the standard module. The "" argument here is what enables standard LinuxMD5 password hashes, though you have the option of using old-style DES56 hashesfor backwards compatibility with legacy Unix systems. "" tells to not bother doing any of itsown internal password checks, which duplicate many of the checks in , but instead accept thepassword that the user inputs after it's been thoroughly checked by .
GNU/Linux distro timeline by Andreas Lundqvist.
The first two fields are the username and user ID. The in the third field represents the number of old passwords currently beingstored for the user--this value is incremented by one every time a new hash isadded to the user's password history until ultimately equalsthe value of the ""parameter set on the configuration line. ,,...,are actually the MD5 password hashes for the user's old passwords.