Linux "chpasswd" Command Line Options and Examples
update passwords in batch mode

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.


chpasswd [options]

Command Line Options:

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.
chpasswd -c ...
Supplied passwords are in encrypted form.
chpasswd -e ...
Display help message and exit.
chpasswd -h ...
Use MD5 encryption instead of DES when the supplied passwords are not encrypted.
chpasswd -m ...
Apply changes in the CHROOT_DIR directory and use the configuration files from the CHROOT_DIR directory.
chpasswd -R ...
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.CAVEATSRemember to set permissions or umask to prevent readability of unencrypted files by other users.CONFIGURATIONThe 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 toauthenticate 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 PAMconfiguration. It is recommended to set this variable consistently with the PAM configuration.FILES/etc/passwdUser account information./etc/shadowSecure user account information./etc/login.defsShadow password suite configuration./etc/pam.d/chpasswdPAM configuration for chpasswd.
chpasswd -s ...