Manual: chpasswd

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, 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).

shadow-utils 4.17.4					 02/02/2026						 CHPASSWD(8)