必须是有效的账户名，且已经存在于系统中。

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

If the password field contains some string that is not a valid result of crypt(3), for instance ! or *, the user will not be able to use a unix password to log in (but the user may log in the system by other means).

This field may be empty, in which case no passwords are required to authenticate as the specified login name. However, some applications which read the /etc/shadow file may decide not to permit any access at all if the password field is empty.

A password field which starts with an exclamation mark means that the password is locked. The remaining characters on the line represent the password field before the password was locked.

The date of the last password change, expressed as the number of days since Jan 1, 1970 00:00 UTC.

The value 0 has a special meaning, which is that the user should change her password the next time she will log in the system.

空字段表示密码年龄功能被禁用。

最小密码年龄是指，用户一次更改密码之后，要等多长时间才再次被允许更改密码。

空字段或 0 表示没有最小密码年龄。

最大密码年龄是指，这写天之后，用户必须更改密码。

这写天之后，密码仍然可用。用户将会在下次登录的时候被要求更改密码。

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。

密码过期之前，提前警告用户的的天数(请参考上边的密码的最大年龄)。

空字段或者 0 表示没有密码警告期。

密码过期(查看上边的密码最大年龄)后，仍然接受此密码的天数(在此期间，用户应该在下次登录时修改密码)。

After expiration of the password and this expiration period is elapsed, no login is possible for the user. The user should contact her administrator.

空字段表示没有强制密码过期。

The date of expiration of the account, expressed as the number of days since Jan 1, 1970 00:00 UTC.

Note that an account expiration differs from a password expiration. In case of an account expiration, the user shall not be allowed to login. In case of a password expiration, the user is not allowed to login using her password.

空字段表示账户永不过期。

应该避免使用 0，因为它既能理解成永不过期也能理解成在1970年1月1日过期。

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。