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Linux 2.6 - man page for passwd (linux section 1)

PASSWD(1)				  User Commands 				PASSWD(1)

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 his/her 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 his/her 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. As a general guideline, passwords should
       consist of 6 to 8 characters including one or more characters from each of the following
       sets:

       o   lower case alphabetics

       o   digits 0 thru 9

       o   punctuation marks

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

   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.

       You can find advices 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
	   his/her 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 his/her password at any time.

       -q, --quiet
	   Quiet mode.

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

       -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 his/her 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.

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

SEE ALSO
       passwd(5), shadow(5), usermod(8).

User Commands				    06/24/2011					PASSWD(1)


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