-a

Add the user to the named group.

-d

Remove the user from the named group.

-h

Display help message and exit.

-Q

Apply changes in the CHROOT_DIR directory and use the configuration files from the CHROOT_DIR directory.

-r

Remove the password from the named group. The group password will be empty. Only group members will be allowed to use newgrp tojoin the named group.

-R

Restrict the access to the named group. The group password is set to "!". Only group members with a password will be allowed touse newgrp to join the named group.

-A

Set the list of administrative users.

-M

Set the list of group members.CAVEATSThis tool only operates on the /etc/group and /etc/gshadow files. Thus you cannot change any NIS or LDAP group. This must beperformed on the corresponding server.CONFIGURATIONThe following configuration variables in /etc/login.defs change the behavior of this tool:ENCRYPT_METHOD (string)This defines the system default encryption algorithm for encrypting passwords (if no algorithm are specified on the commandline).It can take one of these values: DES (default), MD5, SHA256, SHA512.Note: this parameter overrides the MD5_CRYPT_ENAB variable.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.MAX_MEMBERS_PER_GROUP (number)Maximum members per group entry. When the maximum is reached, a new group entry (line) is started in /etc/group (with the samename, same password, and same GID).The default value is 0, meaning that there are no limits in the number of members in a group.This feature (split group) permits to limit the length of lines in the group file. This is useful to make sure that lines for NISgroups are not larger than 1024 characters.If you need to enforce such limit, you can use 25.Note: split groups may not be supported by all tools (even in the Shadow toolsuite). You should not use this variable unless youreally need it.MD5_CRYPT_ENAB (boolean)Indicate if passwords must be encrypted using the MD5-based algorithm. If set to yes, new passwords will be encrypted using theMD5-based algorithm compatible with the one used by recent releases of FreeBSD. It supports passwords of unlimited length andlonger salt strings. Set to no if you need to copy encrypted passwords to other systems which don't understand the new algorithm.Default is no.This variable is superseded by the ENCRYPT_METHOD variable or by any command line option used to configure the encryptionalgorithm.This variable is deprecated. You should use ENCRYPT_METHOD.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.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/groupGroup account information./etc/gshadowSecure group account information.