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.

空字段表示密码年龄功能被禁用。

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.

空字段表示没有最大密码年龄，没有密码警告时间段，没有密码禁用时间段(请看下边)。

如果最大密码年龄小于最小密码年龄，用户将会不能更改密码。

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.

此字段保留作将来使用。

用户账户信息。

安全用户账户信息。

/etc/shadow 的备份文件。

注意，此文件由 shadow 工具集使用，而不是所有的用户和密码管理工具都会使用。