Playing Groups in Linux

[root@localhost ws]# more /etc/group
root:x:0:
bin:x:1:bin,daemon
daemon:x:2:bin,daemon
sys:x:3:bin,adm
adm:x:4:adm,daemon
[group_name, Password (x), Group ID (GID), Group List (multiple user names are separated by commas)]

[root@localhost ws]# id
uid=0(root) gid=0(root) groups=0(root),490(sfcb)

[root@localhost ws]# id ajay
uid=506(ajay) gid=507(ajay) groups=507(ajay)

Create a New Group
[root@localhost ws]# groupadd -g 601 HSS

Add the users in new group
[root@localhost ws]# usermod -G HSS ajay

[root@localhost ws]# id ajay
uid=506(ajay) gid=507(ajay) groups=507(ajay),601(HSS)


[root@localhost ws]# more /etc/login.defs
# Password aging controls:
#
#       PASS_MAX_DAYS   Maximum number of days a password may be used.
#       PASS_MIN_DAYS   Minimum number of days allowed between password changes.
#       PASS_MIN_LEN    Minimum acceptable password length.
#       PASS_WARN_AGE   Number of days warning given before a password expires.
#
PASS_MAX_DAYS   99999
PASS_MIN_DAYS   0
PASS_MIN_LEN    5
PASS_WARN_AGE   7
#
# Min/max values for automatic uid selection in useradd
#
UID_MIN                   500
UID_MAX                 60000
#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN                   500
GID_MAX                 60000


Grant administrative rights to a normal user
[root@localhost ws]# vim /etc/sudoers
## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL
ajay    ALL=(ALL)       ALL

Change group of file/directory.
[root@localhost ws]# ls -lrt
total 4
-rwxr-xr-x 1 root ajay 878 Jun 14 11:22 glist.sh

[root@localhost ws]# chgrp HSS glist.sh
[root@localhost ws]# ls -lrt
total 4
-rwxr-xr-x 1 root HSS 878 Jun 14 11:22 glist.sh

Permission
You can use octal number to represent mode/permission:

• r: 4
• w: 2
• x: 1

For example, for file owner you can use octal mode as follows. Read, write and execute (full) permission on a file in octal is
0+r+w+x = 0+4+2+1 = 7

output of ls command will show permissions as below
-  = - for file d means directory
next three (bit 2,3,4) for user eg. rwx
next three (bit 5,6,7) for group eg. rwx
next three (bit 8,9,10) for others eg. rwx

Change the permission of file for user/group/others
[root@localhost ws]# chmod 775 glist.sh
[root@localhost ws]# ls -lrt
total 4
-rwxrwxr-x 1 root HSS 878 Jun 14 11:22 glist.sh