#Bash 的 passwd 命令

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 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
       https://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 sets 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. No PAM support. 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

SEE ALSO
       chpasswd(8), makepasswd(1), passwd(5), shadow(5), usermod(8).

       The following web page comically (yet correctly) compares the strength
       of two different methods for choosing a password:
       "https://xkcd.com/936/"

shadow-utils 4.17.4                02/02/2026                         PASSWD(1)