NAME

       chpasswd - update passwords in batch mode

SYNOPSIS

       chpasswd [options]

DESCRIPTION

       The chpasswd command reads a list of user name and password pairs from standard input and
       uses this information to update a group of existing users. Each line is of the format:

       user_name:password

       By default the passwords must be supplied in clear-text, and are encrypted by chpasswd.
       Also the password age will be updated, if present.

       By default, passwords are encrypted by PAM, but (even if not recommended) you can select a
       different encryption method with the -e, -m, or -c options.

       Except when PAM is used to encrypt the passwords, chpasswd first updates all the passwords
       in memory, and then commits all the changes to disk if no errors occurred for any user.

       When PAM is used to encrypt the passwords (and update the passwords in the system
       database) then if a password cannot be updated chpasswd continues updating the passwords
       of the next users, and will return an error code on exit.

       This command is intended to be used in a large system environment where many accounts are
       created at a single time.

OPTIONS

       The options which apply to the chpasswd command are:

       -c, --crypt-method METHOD
           Use the specified method to encrypt the passwords.

           The available methods are DES, MD5, SHA256, SHA512, YESCRYPT and NONE if your libc
           supports these methods.

           By default, PAM is used to encrypt the passwords.

       -e, --encrypted
           Supplied passwords are in encrypted form.

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

       -m, --md5
           Use MD5 encryption instead of DES when the supplied passwords are not encrypted.

       -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, --sha-rounds ROUNDS
           Use the specified number of rounds to encrypt the passwords.

           You can only use this option with crypt method: SHA256 SHA512 YESCRYPT

           By default, the number of rounds for SHA256 or SHA512 is defined by the
           SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.

           A minimal value of 1000 and a maximal value of 999,999,999 will be enforced for SHA256
           and SHA512. The default number of rounds is 5000.

           By default, the number of rounds for YESCRYPT is defined by the YESCRYPT_COST_FACTOR
           in /etc/login.defs.

           A minimal value of 1 and a maximal value of 11 will be enforced for YESCRYPT. The
           default number of rounds is 5.

CAVEATS

       Remember to set permissions or umask to prevent readability of unencrypted files by other
       users.

CONFIGURATION

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

       SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)
           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 force 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 (5000), which is
           orders of magnitude too low for modern hardware.

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

           If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS values is set, then
           this value will be used.

           If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest value will be used.

           Note: This only affect the generation of group passwords. The generation of user
           passwords is done by PAM and subject to the PAM configuration. It is recommended to
           set this variable consistently with the PAM configuration.

       YESCRYPT_COST_FACTOR (number)
           When ENCRYPT_METHOD is set to YESCRYPT, this defines the cost factor used by the
           encryption algorithm by default (when the cost factor is not specified on the command
           line).

           With a high cost factor, it is more difficult to brute force the password. But note
           also that more CPU resources will be needed to authenticate users.

           The value must be inside the 1-11 range.

           Note: This only affect the generation of group passwords. The generation of user
           passwords is done by PAM and subject to the PAM configuration. It is recommended to
           set this variable consistently with the PAM configuration.

FILES

       /etc/passwd
           User account information.

       /etc/shadow
           Secure user account information.

       /etc/login.defs
           Shadow password suite configuration.

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

SEE ALSO

       passwd(1), newusers(8), login.defs(5), useradd(8).