CHPASSWD(8)                    System Management Commands                   CHPASSWD(8)

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, NONE, and SHA256 or SHA512 if your libc
           support 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.

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

           The value 0 means that the system will choose the default number of rounds
           for the crypt method (5000).

           A minimal value of 1000 and a maximal value of 999,999,999 will be enforced.

           You can only use this option with the SHA256 or SHA512 crypt method.

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

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 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 (5000).

           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.

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).

shadow-utils 4.4                       05/17/2017                           CHPASSWD(8)

Usage: chpasswd [options]

Options:
  -c, --crypt-method METHOD     the crypt method (one of NONE DES MD5 SHA256 SHA512)
  -e, --encrypted               supplied passwords are encrypted
  -h, --help                    display this help message and exit
  -m, --md5                     encrypt the clear text password using
                                the MD5 algorithm
  -R, --root CHROOT_DIR         directory to chroot into
  -s, --sha-rounds              number of SHA rounds for the SHA*
                                crypt algorithms