Ubuntu Manpage: passwd - change user password

Provided by: passwd_4.15.3-3ubuntu2_amd64

NAME

       passwd - change user password

SYNOPSIS

       passwd [options] [LOGIN]

DESCRIPTION

The passwd command changes passwords for user accounts. A normal user may only change the
password for their own account, while the superuser may change the password for any
account. passwd also changes the account or associated password validity period.

Password Changes
The user is first prompted for their old password, if one is present. This password is
then encrypted and compared against the stored password. The user has only one chance to
enter the correct password. The superuser is permitted to bypass this step so that
forgotten passwords may be changed.

After the password has been entered, password aging information is checked to see if the
user is permitted to change the password at this time. If not, passwd refuses to change
the password and exits.

The user is then prompted twice for a replacement password. The second entry is compared
against the first and both are required to match in order for the password to be changed.

Then, the password is tested for complexity. passwd will reject any password which is not
suitably complex. Care must be taken not to include the system default erase or kill
characters.

Hints for user passwords
The security of a password depends upon the strength of the encryption algorithm and the
size of the key space. The legacy UNIX System encryption method is based on the NBS DES
algorithm. More recent methods are now recommended (see ENCRYPT_METHOD). The size of the
key space depends upon the randomness of the password which is selected.

Compromises in password security normally result from careless password selection or
handling. For this reason, you should not select a password which appears in a dictionary
or which must be written down. The password should also not be a proper name, your license
number, birth date, or street address. Any of these may be used as guesses to violate
system security.

As a general guideline, passwords should be long and random. It's fine to use simple
character sets, such as passwords consisting only of lowercase letters, if that helps
memorizing longer passwords. For a password consisting only of lowercase English letters
randomly chosen, and a length of 32, there are 26^32 (approximately 2^150) different
possible combinations. Being an exponential equation, it's apparent that the exponent (the
length) is more important than the base (the size of the character set).

You can find advice on how to choose a strong password on
http://en.wikipedia.org/wiki/Password_strength

OPTIONS

The options which apply to the passwd command are:

-a, --all
This option can be used only with -S and causes show status for all users.

-d, --delete
Delete a user's password (make it empty). This is a quick way to disable a password
for an account. It will set the named account passwordless.

-e, --expire
Immediately expire an account's password. This in effect can force a user to change
their password at the user's next login.

-h, --help
Display help message and exit.

-i, --inactive INACTIVE
This option is used to disable an account after the password has been expired for a
number of days. After a user account has had an expired password for INACTIVE days,
the user may no longer sign on to the account.

-k, --keep-tokens
Indicate password change should be performed only for expired authentication tokens
(passwords). The user wishes to keep their non-expired tokens as before.

-l, --lock
Lock the password of the named account. This option disables a password by changing it
to a value which matches no possible encrypted value (it adds a ´!´ at the beginning
of the password).

Note that this does not disable the account. The user may still be able to login using
another authentication token (e.g. an SSH key). To disable the account, administrators
should use usermod --expiredate 1 (this set the account's expire date to Jan 2, 1970).

Users with a locked password are not allowed to change their password.

-n, --mindays MIN_DAYS
Set the minimum number of days between password changes to MIN_DAYS. A value of zero
for this field indicates that the user may change their password at any time.

-q, --quiet
Quiet mode.

-r, --repository REPOSITORY
change password in REPOSITORY repository

-R, --root CHROOT_DIR
Apply changes in the CHROOT_DIR directory and use the configuration files from the
CHROOT_DIR directory. Only absolute paths are supported.

-P, --prefix PREFIX_DIR
Apply changes to configuration files under the root filesystem found under the
directory PREFIX_DIR. This option does not chroot and is intended for preparing a
cross-compilation target. Some limitations: NIS and LDAP users/groups are not
verified. PAM authentication is using the host files. No SELINUX support.

-S, --status
Display account status information. The status information consists of 7 fields. The
first field is the user's login name. The second field indicates if the user account
has a locked password (L), has no password (NP), or has a usable password (P). The
third field gives the date of the last password change. The next four fields are the
minimum age, maximum age, warning period, and inactivity period for the password.
These ages are expressed in days.

-u, --unlock
Unlock the password of the named account. This option re-enables a password by
changing the password back to its previous value (to the value before using the -l
option).

-w, --warndays WARN_DAYS
Set the number of days of warning before a password change is required. The WARN_DAYS
option is the number of days prior to the password expiring that a user will be warned
that their password is about to expire.

-x, --maxdays MAX_DAYS
Set the maximum number of days a password remains valid. After MAX_DAYS, the password
is required to be changed.

Passing the number -1 as MAX_DAYS will remove checking a password's validity.

-s, --stdin
This option is used to indicate that passwd should read the new password from standard
input, which can be a pipe.

CAVEATS

       Password complexity checking may vary from site to site. The user is urged to select a
       password as complex as he or she feels comfortable with.

       Users may not be able to change their password on a system if NIS is enabled and they are
       not logged into the NIS server.

       passwd uses PAM to authenticate users and to change their passwords.

FILES

       /etc/passwd
           User account information.

       /etc/shadow
           Secure user account information.

       /etc/pam.d/passwd
           PAM configuration for passwd.

EXIT VALUES

       The passwd command exits with the following values:

       0
           success

       1
           permission denied

       2
           invalid combination of options

       3
           unexpected failure, nothing done

       4
           unexpected failure, passwd file missing

       5
           passwd file busy, try again

       6
           invalid argument to option