shadowThe shadow file is mainly used for store Linux User's login passwords as encrypted. It located in /etc/shadow. It's encrypted passwords. So we can't use this passwords straightly for login. The root user only can access this file anything view are edit. Other user don't have permission to view this file.
Location : /etc/shadow
1 - UserName : It is name of user which we created or superuser or system user.
2 - Encrypted Password : It is current hash of the user's encrypted password.
3 - Last Password Change : It is date of last password changed from 1 Jan 1970. It is UNIX time format (Epoch). You can know more about it here. The encrypted password consists of 13 to 24 characters from the 64 characters alphabet a thru z, A thru Z, 0 thru 9, \. and /. Optionally it can start with a "$" character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with "$1$" it means the MD5-based algorithm was used. If in encrypted password edited with ! or *, the user will not be able to use a unix password to log in.
4 - Days until Password change allowed: Till this date the user can change their password.
5 - Days before password change required : After this date the user is forced to change password.
6 - Days warning for password change : How many days want to warn user before the password expire.
7 - Days before account inactive : How many number of days the user can active after password expire .
8 - Days since account expires : The correct day when the login disabled from Jan 1 1970.
9 - Reserved : This is reserved for future use.
* Use man pages for more help.