chpasswd man page on ElementaryOS

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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 occured 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.

       -S, --stdout
	   Report encrypted passwords to stdout instead of updating password
	   file.

       -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 4.1.5.1		  02/17/2014			   CHPASSWD(8)
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