user relation with group in linux

The tutorial is about the user relation with group in linux. Many users can be listed in single group in linux.
Groups allow to set permissions on the group level instead
of having to grant permissions for every individual user
It specifies database structure with lots of ease

There are many useful commands for user and group management
in linux which helps to maintain groups very easily

• groups

  This command lists all the group names from which the user belongs
For example
• root@kali:~#   groups
  root

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• groupadd

  This command is used to create groups,
  so that user can be listed in that group
  For example
  • root@kali:~#   groupadd   sandy
  • It will create group name sandy in   /etc/group   file

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• groupmod

  It is used to change the particular group definition in the system database
  
  For example,to change the name of any group
  • root@kali:~#   groupmod  -n  linux  sandy
  • -n   is to change name
    Hence group sandy is changed to linux

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• groupdel

  This command is used to permanently delete the particular group
  For example
  • root@kali:~#  groupdel   sandy
  • It will delete the group named sandy from the system database

---

• useradd

  This command is used to add user to any particular group.
  For example
  • root@kali:/etc#   useradd  -g  linux  user1
  • It will add user named user1 to the group linux

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• usermod

  It is used to modify the user account within the system database.
  It modifies the system account files to show the
  changes that are specified on the command line.
  For example
  • usermod -a -G sandy,linux user1
  • user1 will be member of groups sandy and linux>
  • -a   is used to add the user to the supplementary group(s).
  • -G   represents list of supplementary groups which the user is also a member of.
    If the user is currently a member of a group which is not listed in the command,
    then the user will be removed from that group.

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• gpasswd

  It is used to give control of group membership to another user with the gpasswd command.
  It is used to set administrators to group,
  who can control the membership of different users
  For example
  • root@kali:~#   gpasswd  -A  sandeep  sandy
  • It will make sandeep the administrator of group sandy
  • root@kali:~#   su  -  sandeep
  • It will make sandeep the super user
  • $  id  user1 
  • uid=1005(user1)  gid=1004(sandy)  groups=1004(sandy)
  • $  gpasswd  -a  google  sandy
  • Adding  user  google  to  group  sandy
  • $  id  google
  • uid=1007(google)  gid=1004(sandy)  groups=1004(sandy)
  • $  tail  -1  /etc/group
  • sandy:x:1004:google,sandeep
  • Information  about  group  administrators  is  kept  in  the  /etc/gshadow  file.

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• newgrp

  It helps to start a child shell with new primary group.
  Thus it helps to change the current group with another named group
  For example
  • root@kali:~#   mkdir  sandy
  • root@kali:~# cd   sandy/
  • root@kali:~/sandy#   touch  operators.txt
  • root@kali:~/sandy#  ls   -lh
    total 0
    -rw-r--r-- 1 root root 0 Jun 12 16:50 operators.txt
  • echo  $SHLVL
    1
    "It shows that we are in parent shell"
  • root@kali:/etc#   newgrp  sandeep
  • root@kali:/etc#   echo  $SHLVL
    2
    "It shows that we are in child shell"
  • root@kali:~/sandy#   ls  -lh
    total  0
    -rw-r--r-- 1 root sandeep 0 Jun 12 17:08 operators2.txt
    -rw-r--r-- 1 root root 0 Jun 12 16:50 operators.txt
    
  • root@kali:~/sandy#   exit
    exit
    "exiting from child shell"