shadow is a file which contains the password information for the
system's accounts and optional aging information.
This file must not be readable by regular users if password
security is to be maintained.
Each line of this file contains 9 fields, separated by colons
(':'), in the following order:
login name
It must be a valid account name, which exist on the system.
encrypted password
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.
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).
date of last password change
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.
An empty field means that password aging features are
disabled.
minimum password age
The minimum password age is the number of days the user will
have to wait before she will be allowed to change her
password again.
An empty field and value 0 mean that there is no minimum
password age.
maximum password age
The maximum password age is the number of days after which
the user will have to change her password.
After this number of days is elapsed, the password may still
be valid. The user should be asked to change her password the
next time she will log in.
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.
password warning period
The number of days before a password is going to expire (see
the maximum password age above) during which the user should
be warned.
An empty field and value 0 mean that there are no password
warning period.
password inactivity period
The number of days after a password has expired (see the
maximum password age above) during which the password should
still be accepted (and the user should update her password
during the next login).
After expiration of the password and this expiration period
is elapsed, no login is possible for the user. The user
should contact her administrator.
An empty field means that there are no enforcement of an
inactivity period.
account expiration date
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.
An empty field means that the account will never expire.
The value 0 should not be used as it is interpreted as either
an account with no expiration, or as an expiration on Jan 1,
1970.
reserved field
This field is reserved for future use.
