unix_update
The purpose of the helper is to enable tighter confinement of login and password changing services. The helper is thus called only when SELinux is enabled and in the enforcing mode on the system.
The interface of the helper - command line options, and input/output data format are internal to the pam_unix module and it should not be called directly from applications.
pam_unix(8)
Written by Tomas Mraz and other various people.