Thursday, January 21, 2016

how to change mysql root password in rhel6 ?

I recently had to reset the MySQL root password due to the fact that initializing it the way I assumed it should did not work. The following procedure will work in CentOS/RHEL/Scientific Linux and Fedora.

After installing MySQL using

# yum install mysql-server
I can run the command

# mysqladmin -u root password 'new-password'
Trying to log in with the following failed

# mysql -u root -p
with the following error

Access denied for user 'root'@'localhost'
Decided to not spend more time as it’s a fresh MySQL installation. And did the following to reset the root password for MySQL.

Resetting the root password

1) Stopped the MySQL service.

# service mysqld stop
2) Started MySQL in safe mode.

# mysqld_safe --skip-grant-tables &
3) Logged in using root.

# mysql -u root
4) Reset the password.

> use mysql;
> update user set password=PASSWORD("mynewpassword") where User='root';
> flush privileges;
> quit
5) Stop MySQL in safe mode.

# service mysqld stop
6) Start MySQL.

# service mysqld start
7) Log in using the new password.

# mysql -u root -p

Friday, January 15, 2016

What is the command for finding the highest memory occupied file in Linux ?

# du –ah / | sort –n –r | head –n 1

Option Explanations :du  – estimate file space usage
a – write counts for all files, not just directories
h – print sizes in human readable format (eg. 1K 234M 2G)
sort – sort lines of text files
n – compare according to string numerical value
r – reverse the result of comparisons
head – output the first part of files

n – number of lines

How to change hostname in Red Hat 6 ?

There are several ways to change the hostname of a machine running Redhat 6.  These also works on CentOS, Fedora and older/other Redhat variants.

First: The "hostname" command.

You can use the hostname command to see the current host name of the system.

# hostname

You can also use the hostname command to change the host name of the machine.

# hostname

Then issue the hostname command again to see the changes.
# hostname

This only makes a temporary or non-persistent change of hostname.

Second: The /etc/sysconfig/network configuration file. (preferred method)

In order for the change to survive a reboot, or to make it persistent, you must change it in the /etc/sysconfig/network-scripts/ifconfig-eth0 file.

Open the file in your favorite editor and change the following line to reflect your desired hostname.

After making changing to the configuration file you need to restart the network service in order to read that file.

/etc/init.d/network restart
NOTE: Do not do this remotely (via ssh) or you will lose your connection.

If you issue the hostname command now, you will see the hostname has changed.

Third: The /proc/sys/kernel/hostname entry.

Another simple way to change the hostname is to echo the hostname into the /proc/sys/kernel/hostname file.

echo "" > /proc/sys/kernel/hostname

NOTE: Using the /etc/sysconfig/network file is the preferred method to set the permanent hostname of a system.  Anything in the /proc/sys/kernel/hostname file will be overridden by the /etc/sysconfig/network file during a reboot.