shadow
contains the encrypted password information for user's accounts
and optional the password aging information.
Included is
Days since Jan 1, 1970 that password was last changed
Days before password may be changed
Days after which password must be changed
Days before password is to expire that user is warned
Days after password expires that account is disabled
Days since Jan 1, 1970 that account is disabled
The password field must be filled.
The encryped password consists of 13 to 24 characters from the
64 character alphabet
a thru z, A thru Z, 0 thru 9, \. and /.
Refer to crypt(3) for details on how this string is
interpreted.
The date of the last password change is given as the number
of days since Jan 1, 1970.
The password may not be changed again until the proper number
of days have passed, and must be changed after the maximum
number of days.
If the minimum number of days required is greater than the
maximum number of day allowed, this password may not be
changed by the user.
An account is considered to be inactive and is disabled if
the password is not changed within the specified number of
days after the password expires.
An account will also be disabled on the specified day
regardless of other password expiration information.
This information supercedes any password or password age
information present in /etc/passwd.
This file must not be readable by regular users if password
security is to be maintained.