On a Linux system with shadow passwords enabled, the file that contains the password hashes of all local users is /etc/shadow. This file is a replacement for the password field in /etc/passwd, which is a world-readable file that contains basic information about users. The /etc/shadow file is not readable by regular users, and it stores the encrypted passwords (or hashes) of each user, along with other information such as password expiration dates, minimum and maximum password ages, and password warning periods. The /etc/shadow file has nine colon-delimited fields for each user:

Username: The name used when the user logs into the system.

Password: The encrypted password of the user, or a special character that indicates the password status. For example, an asterisk (*) means the account is locked, and an exclamation mark (!) means the password is expired.

Last Password Change: The date of the last password change, expressed as the number of days since January 1, 1970.

Minimum Password Age: The minimum number of days required between password changes. A zero means the password can be changed anytime.

Maximum Password Age: The maximum number of days the password is valid. After this number of days, the password must be changed. A zero means the password never expires.

Password Warning Period: The number of days before the password expires that the user will be warned. A zero means no warning is given.

Password Inactivity Period: The number of days after the password expires that the account will be disabled. A negative value means the account is never disabled.

Account Expiration Date: The date when the account will be disabled, expressed as the number of days since January 1, 1970. A zero means the account never expires.

Reserved Field: A field for future use.

The /etc/shadow file can be modified by using the commands passwd and chage, which are used to change the password and the password aging information of a user, respectively. The /etc/shadow file should not be edited directly, but always through the tools provided by the distribution. For more details, see the shadow manual page.

References:

LPIC-1 Exam 102 Objectives, Topic 110: Security, Subtopic 110.2: Use sudo to manage access to the root account, Weight: 2, Key Knowledge Areas: Configure sudo and sudoers. Use sudo to execute commands as another user.

LPIC-1 Exam 102 Learning Materials, Topic 110: Security, Subtopic 110.2: Use sudo to manage access to the root account, Section 110.2.1: sudo and sudoers, Page 3-5.