Unix useful commands

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Unix useful commands

Sometime we need a user who can do everything in our server as root does. So we may do the following:

  1. Create a new user with the same privileges as root
  2. Grant same same privileges to existing user as root

Case 1: Lets say we need to add a new user and grant him root privileges :

Use the following commands to create the new user temp, grand him the same privileges as root and set him a password :

[root@m1 ~]# useradd -ou 0 -g 0 temp

[root@m1 ~]# passwd temp

Changing password for user temp.

New password:

BAD PASSWORD: it is based on a dictionary word

BAD PASSWORD: is too simple

Retype new password:

passwd: all authentication tokens updated successfully.

We’ve just created the user temp, with UID 0 and GID 0, so he is in the same group and has the same permissions as root.

Case 2: Grant ROOT Privileges to an Existing USER: 
Perhaps you already have some user temp and you would like to give root permissions to a normal user.

[root@m1 ~]# grep temp1 /etc/passwd


Solu 1:

Edit /etc/passwd file and grant root permissions to the user temp1 by changing User and Group IDs to UID 0 and GID 0. 

Solu 2: Create a group and assign this existing user to that group. Also grant that group to sudo access.

[root@m1 ~]# groupadd test

[root@m1 ~]# usermod -g test temp1

[temp2@m1 ~]$ id temp1

uid=1006(temp1) gid=1007(test) groups=1007(test)

Edit /etc/sudoers file and add %test ALL=(ALL)       NOPASSWD: ALL line to file. 

[root@m1 ~]# grep -C4 test /etc/sudoers

# %wheel ALL=(ALL) ALL

## Same thing without a password


%test ALL=(ALL)       NOPASSWD: ALL

[root@m1 ~]# su temp1

[temp1@m1 ~]$ sudo su – hdfs

[hdfs@m1 ~]$ exit


[temp1@m1 ~]$ sudo su – root

[root@m1 ~]# exit


Delete a USER Account with UID 0 : You won’t be able to delete second root user with another UID 0 using userdel command.
[root@m1 ~]# userdel temp
userdel: user temp is currently used by process 1

To delete user temp with UID 0, open /etc/passwd file and change temp’s UID.
[root@m1 ~]# vi /etc/passwd
[root@m1 ~]# id temp

Now, you’ll be able to delete user temp with userdel command :
[root@m1 ~]# userdel temp
[root@m1 ~]# id temp

id: temp: No such user


How to make sure /etc/resolv.conf Never Get Updated By DHCP Client in centos 6 :

I using GNU/Linux with the Internet Systems Consortium DHCP Client. It also updates my /etc/resolv.conf file each time my laptop connects to different network or after restart machine. I would like to keep my existing nameservers. How do I skip /etc/resolv.conf update on a Linux based system?

The DHCP protocol allows a host to contact a central server which maintains a list of IP addresses which may be assigned on one or more subnets. This protocol reduces system administration workload, allowing devices to be added to the network with little or no manual configuration. There are various method to fix this issue but I would prefer to use the following one.

We have to modify our interface configuration file such as /etc/sysconfig/network-scripts/ifcfg-eth0 file and append the following option:


[root@m1 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0








PEERDNS=NO ## change it to No from Yes and the following DNS accordingly. 







NAME=”System eth0″

Save and close the file. Where,

1. PEERDNS=yes|no – Modify /etc/resolv.conf if peer uses msdns extension (PPP only) or DNS{1,2} are set, or if using dhclient. default to “yes”.

2. DNS{1,2}=<ip address> – Provide DNS addresses that are dropped into the resolv.conf file if PEERDNS is not set to “no”.


I hope this will help you, please feel free to give your valuable suggestion or feedback.

1 Comment

piyush chauhan

April 2, 2017 at 1:26 pm

Hi Bro,
Piyush from MCA Kiet ..Good Job keep it up

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