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

If the password field is empty, the user can log in without a password. However, some applications that read the /etc/shadow file might block access if the password field is empty.

If the password field begins with an exclamation mark !, the password is locked. The remaining characters on the line represent the password field before the password was locked.

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

If the password field contains a string that is not a valid result of crypt(3), for instance ! or *, the user cannot use a UNIX password to log in. However, the user may log in the system by other means.

The date of the last password change, expressed as the number of days since 1970-01-01 00:00:00 UTC.

The value 0 indicates that the user must change their password the next time they log in to the system.

An empty field means that password aging features are disabled.

The minimum password age is the number of days the user must wait before they can change their password again.

An empty field and value 0 mean that there is no minimum password age.

The maximum password age is the number of days after which the user must change their password.

After this number of days has elapsed, the password may still be valid. The user is prompted to change their password at the next login.

An empty field means that there are no maximum password age, no password warning period, and no password inactivity period (see below).

If the maximum password age is lower than the minimum password age, the user cannot change her password.

The number of days before a password expires (see the maximum password age above) during which the user is warned.

An empty field and value 0 mean that there is no password warning period.

The number of days after a password expires (see the maximum password age above) during which the password is still accepted, and the user must update their password at the next login.

After the password expires and the password inactivity period elapses, the user cannot log in and must contact their administrator.

An empty string means that no inactivity period is enforced.

The date when the account expires, expressed as the number of days since 1970-01-01.

Note that account expiration differs from password expiration. Account expiration prevents the user from logging in, whereas password expiration only prevents users from logging in with their password.

An empty field means that the account never expires.

The value 0 should not be used, as it is interpreted either as an account with no expiration or as an expiration date of 1970-01-01.

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.