Ch5. Manage Local Users and Groups

 

Users:

[root@master ~]# ls -l
[root@master ~]# ps aux
[root@master ~]# useradd --help
[root@master ~]# useradd abeer
[root@master ~]# passwd abeer
[root@master ~]# useradd -G admin -u 1005 -s /usr/sbin/nologin abeer
[root@master ~]# useradd -c "ahmed hosni" -e 2016-12-31 -s /bin/csh ahmed
[root@master ~]# id
[root@master ~]# id abeer
[root@master ~]# id -u abeer          (UID for abeer)
[root@master ~]# tail -n 1 /etc/passwd

username:password:UID:GID:GECOS:/home/dir:shell
Note: GECOS field is arbitrary text, which usually includes the user's real name.

[root@master ~]# tail -n 1 /etc/shadow
	name:password:lastchange:minage:maxage:warning:inactive:expire:b1ank

The number 1 indicates an MD5 hash. The number 6 appears when a SHA-512 hash is used.
[root@master ~]# authconfig --passalgo=<descrypt|bigcrypt|md5|sha256|sha512>    (to change the hashing type)

Note: !! indicates that the user has no password

Groups:

-Primary group is the user Private Group (UPG).
[root@master ~]# groupadd sales
[root@master ~]# groupadd -g 1005 admin

To verify:
[root@master ~]# id
[root@master ~]# id abeer
[root@master ~]# grep sales /etc/group

groupname:password:GID:list,of,users,in,this,group

[root@master ~]# groupmod -g 2000 admin
[root@master ~]# groupadd old
[root@master ~]# groupmod -n new old       (rename a group)

Switching users with su:

[root@master ~]# su abeer
[abeer@master root]$ exit
exit
[root@master ~]# su - abeer
[abeer@master ~]$ 
[abeer@master ~]$ su 
[abeer@master ~]$ su -

Running commands as root with sudo:

[root@master ~]# vim /etc/sudoers
abeer   ALL=(ALL)       ALL
%sales  ALL=(ALL)       ALL

[abeer@master ~]$ sudo passwd ahmed
[abeer@master ~]$ sudo passwd -l ahmed

[root@master ~]# tail -f /var/log/secure

Modify users:

**[root@master ~]# usermod -L abeer          (lock the user)
[root@master ~]# usermod -U abeer          (unlock the user)
[root@master ~]# usermod -G sales abeer    (overwrite secondary group)
[root@master ~]# usermod -aG admin abeer   (append to secondary group)

[root@master ~]# vim /etc/group

[root@master ~]# id abeer**

Delete users & groups:

[root@master ~]# userdel abeer
[root@master ~]# userdel -r test       (removes home directory)

Note: from RHEL7 the useradd command assigns new users the first free UID number available in the range starting from UID 1000 or above. 

[root@master ~]#useradd abeer
[root@master ~]# ls /home/ -l
	drwx------. 5 abeer abeer 4096 Jun 21 21:36 abeer
[root@master ~]#userdel abeer
[root@master ~]#useradd ahmed
[root@master ~]# ls /home/ -l
	drwx------. 5 ahmed ahmed    74 Jun 23 05:00 abeer
	drwx------. 3 ahmed ahmed    74 Jun 23 02:55 ahmed

[root@master ~]# groupdel admin

UID ranges:

UID 0 is always assigned to the superuser account, root.
UID 1-200 is a range of "system users" assigned statically to system processes by Red Hat.
UID 201-999 is a range of "system users" used by system processes that do not own files on the file system.
UID 1000+ is the range available for assignment to regular users.

To change the default:
[root@master ~]# vim /etc/login.defs

Password aging:

[root@master ~]# chage -l abeer           (list info about the user)
[root@master ~]# chage -E 2017-1-1 abeer  (expire the user on the specified address)
[root@master ~]# chage -m 1 abeer         (set minimum number of days before password change) 
[root@master ~]# chage -M 120 abeer       (set maximim number of days before password change)
[root@master ~]# passwd -x 90 abeer       (the password will expire after 90 days)

Create a default file or directory in the user's home directory:

[root@master ~]# touch /etc/skel/new_file

GUI tool to manage users and groups:

[root@master ~]# yum install system-config-users
[root@master ~]# system-config-users

---