shadow is a file which contains the password information for the
       system's accounts and optional aging information.

       This file must not be readable by regular users if password security is
       to be maintained.

       Each line of this file contains 9 fields, separated by colons (":"), in
       the following order:

       login name
           It must be a valid account name, which exist on the system.

       encrypted password
           Refer to crypt(3) for details on how this string is interpreted.

           If the password field contains some string that is not a valid
           result of crypt(3), for instance ! or *, the user will not be able
           to use a unix password to log in (but the user may log in the
           system by other means).

           This field may be empty, in which case no passwords are required to
           authenticate as the specified login name. However, some
           applications which read the /etc/shadow file may decide not to
           permit any access at all if the password field is empty.

           A password field which starts with a exclamation mark means that
           the password is locked. The remaining characters on the line
           represent the password field before the password was locked.

       date of last password change
           The date of the last password change, expressed as the number of
           days since Jan 1, 1970.

           The value 0 has a special meaning, which is that the user should
           change her pasword the next time she will log in the system.

           An empty field means that password aging features are disabled.

       minimum password age
           The minimum password age is the number of days the user will have
           to wait before she will be allowed to change her password again.

           An empty field and value 0 mean that there are no minimum password

       maximum password age
           The maximum password age is the number of days after which the user
           will have to change her password.

           After this number of days is elapsed, the password may still be
           valid. The user should be asked to change her password the next
           time she will log in.
           An empty field and value 0 mean that there are no password warning

       password inactivity period
           The number of days after a password has expired (see the maximum
           password age above) during which the password should still be
           accepted (and the user should update her password during the next

           After expiration of the password and this expiration period is
           elapsed, no login is possible using the current user's password.
           The user should contact her administrator.

           An empty field means that there are no enforcement of an inactivity

       account expiration date
           The date of expiration of the account, expressed as the number of
           days since Jan 1, 1970.

           Note that an account expiration differs from a password expiration.
           In case of an acount expiration, the user shall not be allowed to
           login. In case of a password expiration, the user is not allowed to
           login using her password.

           An empty field means that the account will never expire.

           The value 0 should not be used as it is interpreted as either an
           account with no expiration, or as an expiration on Jan 1, 1970.

       reserved field
           This field is reserved for future use.

           User account information.

           Secure user account information.

           Backup file for /etc/shadow.

           Note that this file is used by the tools of the shadow toolsuite,
           but not by all user and password management tools.

       chage(1), login(1), passwd(1), passwd(5), pwck(8), pwconv(8),
       pwunconv(8), su(1), sulogin(8).

File Formats and Conversions      09/12/2012                         SHADOW(5)
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