Cfrtjryk

On Debian, the adduser package contains a deluser program which removes a user from a group if you pass both as arguments:

deluser user group

If your distribution doesn't have adduser, you can edit /etc/group and /etc/gshadow manually.

vigr

vigr -s

usermod -G "" username

removes all secondary/supplementary groups from username, leaving them as a member of only their primary group.
this worked in Solaris 5.9

This is the “old school” approach...

Most *nix systems maintain group information into a plain text file /etc/group, where

• 
  

  each line contains the fields

  
  
  
  
  
  • group_name
  
  
  • password
  
  
  • GID, and
  
  
  • user_list
  
  
  
  

  delimited by the : character.

  
  


• the user_list field is a list of user names, separated by commas.

Now suppose you want to remove a user named thisuser
from a group named thatgroup.  Start by backing up /etc/group,
then use the editor of your preference with su privileges
to edit the file /etc/group
and remove the thisuser reference from the thatgroup line entry, e.g.,

original line is something like this:

thatgroup:x:1274:someuser,thisuser,anotheruser

after editing should be left like this:

thatgroup:x:1274:someuser,anotheruser

As with all the other answers, this will not affect the user's current session(s), if any (i.e., if the user is currently logged in). 
The change will take effect the next time the user logs in.

You can use the below command on SUSE distributions
(and, apparently, no others).

usermod -R group user_name

where group is the group that you want to remove the user from
and user_name the user that you want to remove from the group.
For example,

usermod -R root imnottheroot

Consider:

• username: abc2
  



• group name: newgroup11




• Task: Removing user abc2 from group newgroup11

[root@home1 ~]# groups abc2

abc2 : abc2

[root@home1 ~]# usermod -G newgroup11 abc2

[root@home1 ~]# groups abc2

abc2 : abc2 newgroup11

[root@home1 ~]# usermod -G newgroup11 abc2

[root@home1 ~]# usermod -G abc2 abc2

[root@home1 ~]# groups abc2

abc2 : abc2

** Kindly correct me if I am wrong. **

Suppose that username=student and groupname=research, therefore to remove student user from research group it's need to do following:

gpasswd -d student research

To continue using usermod in a distro (like Fedora) which does not have a remove option, where user=bob and group=deletethisgroup, command would be:

usermod -G `cat /etc/group |  grep bob | grep -v deletethisgroup | cut -d ':' -f 1 | tr 'n' ',' | sed 's/,$//'` bob

The pipes (1) get all group entries user belongs to, (2) take out the one which needs to be removed, (3) returns first column (group name), replaces newline with comma, and removes trailing comma.

Of course, you could put all that in a bash script which takes user and group to be deleted as parameters. awk could be used to shorten the end but I wanted to stick to grep, cut, tr and sed.

You can remove user from group by executing usermod command whithout -a option.
Example, by executing "usermod -G group1 username" will add the user to the group1, and will remove it from any other groups where it is. Remember, you can keep user in various groups by listing group's names, separated with comma.

pw groupmod "groupname|gid" -d "username|uid"

A solution if you are using CSH, for whatever reason.

To remove a user from a group gpasswd is the best utility for this IMO.

Command Example:

sudo gpasswd -d group user

* Help Info *

Usage: gpasswd [option] GROUP



Options:

  -a, --add USER                add USER to GROUP

  -d, --delete USER             remove USER from GROUP

  -h, --help                    display this help message and exit

  -Q, --root CHROOT_DIR         directory to chroot into

  -r, --delete-password         remove the GROUP's password

  -R, --restrict                restrict access to GROUP to its members

  -M, --members USER,...        set the list of members of GROUP

  -A, --administrators ADMIN,...

                                set the list of administrators for GROUP

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