gpasswd man page on Archlinux

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GPASSWD(1)			 User Commands			    GPASSWD(1)

       gpasswd - administer /etc/group and /etc/gshadow

       gpasswd [option] group

       The gpasswd command is used to administer /etc/group, and /etc/gshadow.
       Every group can have administrators, members and a password.

       System administrators can use the -A option to define group
       administrator(s) and the -M option to define members. They have all
       rights of group administrators and members.

       gpasswd called by a group administrator with a group name only prompts
       for the new password of the group.

       If a password is set the members can still use newgrp(1) without a
       password, and non-members must supply the password.

   Notes about group passwords
       Group passwords are an inherent security problem since more than one
       person is permitted to know the password. However, groups are a useful
       tool for permitting co-operation between different users.

       Except for the -A and -M options, the options cannot be combined.

       The options which apply to the gpasswd command are:

       -a, --add user
	   Add the user to the named group.

       -d, --delete user
	   Remove the user from the named group.

       -h, --help
	   Display help message and exit.

       -Q, --root CHROOT_DIR
	   Apply changes in the CHROOT_DIR directory and use the configuration
	   files from the CHROOT_DIR directory.

       -r, --remove-password
	   Remove the password from the named group. The group password will
	   be empty. Only group members will be allowed to use newgrp to join
	   the named group.

       -R, --restrict
	   Restrict the access to the named group. The group password is set
	   to "!". Only group members with a password will be allowed to use
	   newgrp to join the named group.

       -A, --administrators user,...
	   Set the list of administrative users.

       -M, --members user,...
	   Set the list of group members.

       This tool only operates on the /etc/group and /etc/gshadow files.  Thus
       you cannot change any NIS or LDAP group. This must be performed on the
       corresponding server.

       The 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 command line).

	   It can take one of these values: DES (default), MD5, SHA256,

	   Note: this parameter overrides the MD5_CRYPT_ENAB variable.

       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 same name,
	   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 NIS
	   groups 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 you
	   really 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 the
	   MD5-based algorithm compatible with the one used by recent releases
	   of FreeBSD. It supports passwords of unlimited length and longer
	   salt strings. Set to no if you need to copy encrypted passwords to
	   other systems which don't understand the new algorithm. Default is

	   This variable is superseded by the ENCRYPT_METHOD variable or by
	   any command line option used to configure the encryption algorithm.

	   This variable is deprecated. You should use ENCRYPT_METHOD.

	   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

	   The values must be inside the 1000-999,999,999 range.

	   values is set, then this value will be used.

	   will be used.

	   Group account information.

	   Secure group account information.

       newgrp(1), groupadd(8), groupdel(8), groupmod(8), grpck(8), group(5),

shadow-utils		  05/25/2012			    GPASSWD(1)

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