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
A reserved field
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. If the password field contains some string that is not valid result of crypt(3), for instance ! or *, the user will not be able to use a unix password to log in, subject to pam(7).
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.