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It is a good idea to force users to change passwords on a regular basis. The commands above will set all active accounts (except the root account) to force password changes every 91 days (13 weeks), and then prevent password changes for seven days (one week) thereafter. Users will begin receiving warnings 28 days (4 weeks) before their password expires. Sites also have the option of expiring idle accounts after a certain number of days (see the on-line manual page for the usermod command, particularly the -f option).