passwd man page on Archlinux

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PASSWD(1)			 User Commands			     PASSWD(1)

       passwd - change user password

       passwd [options] [LOGIN]

       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:

       ·   lower case alphabetics

       ·   digits 0 thru 9

       ·   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

   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

       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

       -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

       -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

       -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

       -R, --root CHROOT_DIR
	   Apply changes in the CHROOT_DIR directory and use the configuration
	   files from the CHROOT_DIR directory.

       -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.

       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

       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.

       The following configuration variables in /etc/login.defs change the
       behavior of this tool:

       ENCRYPT_METHOD (string)
	   This defines the system default encryption algorithm for encrypting
	   passwords (if no algorithm are specified on the command line).

	   It can take one of these values: DES (default), MD5, SHA256,

	   Note: this parameter overrides the MD5_CRYPT_ENAB variable.

       MD5_CRYPT_ENAB (boolean)
	   Indicate if passwords must be encrypted using the MD5-based
	   algorithm. If set to yes, new passwords will be encrypted using the
	   MD5-based algorithm compatible with the one used by recent releases
	   of FreeBSD. It supports passwords of unlimited length and longer
	   salt strings. Set to no if you need to copy encrypted passwords to
	   other systems which don't understand the new algorithm. Default is

	   This variable is superseded by the ENCRYPT_METHOD variable or by
	   any command line option used to configure the encryption algorithm.

	   This variable is deprecated. You should use ENCRYPT_METHOD.

       OBSCURE_CHECKS_ENAB (boolean)
	   Enable additional checks upon password changes.

       PASS_ALWAYS_WARN (boolean)
	   Warn about weak passwords (but still allow them) if you are root.

       PASS_CHANGE_TRIES (number)
	   Maximum number of attempts to change password if rejected (too

       PASS_MAX_LEN (number), PASS_MIN_LEN (number)
	   Number of significant characters in the password for crypt().
	   PASS_MAX_LEN is 8 by default. Don't change unless your crypt() is
	   better. This is ignored if MD5_CRYPT_ENAB set to yes.

	   When ENCRYPT_METHOD is set to SHA256 or SHA512, this defines the
	   number of SHA rounds used by the encryption algorithm by default
	   (when the number of rounds is not specified on the command line).

	   With a lot of rounds, it is more difficult to brute forcing the
	   password. But note also that more CPU resources will be needed to
	   authenticate users.

	   If not specified, the libc will choose the default number of rounds

	   The values must be inside the 1000-999,999,999 range.

	   values is set, then this value will be used.

	   will be used.

	   User account information.

	   Secure user account information.

	   Shadow password suite configuration.

       The passwd command exits with the following values:


	   permission denied

	   invalid combination of options

	   unexpected failure, nothing done

	   unexpected failure, passwd file missing

	   passwd file busy, try again

	   invalid argument to option

       chpasswd(8), passwd(5), shadow(5), login.defs(5), usermod(8).

shadow-utils		  05/25/2012			     PASSWD(1)

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