How to find the IP address of a linux server ?

To check the IP address of a Linux server, type the following command in the terminal/commandline.

ifconfig

The below command also will help in finding the ip address.

ip addr

CentOS 8 Released Last week.

CentOS 8 released last week and I just downloaded the iso file locally. It is around 6.6 GB.

centos_8_download

There are a lot of exciting updates in this new release. I will share my experience after installing it.

 

How to find and kill a process locking a particular port in Linux?

Sometimes because of some issue or bug, our application may stop working, but the port will be locked. This kind of issue is very common with MySQL server, Elasticsearch, WebServices, Tomcat etc. In such scenarios, we have to find the zombie process and kill it to free up the locked port.

How to find the process that locks the port?

Use the following command

netstat -tulpn | grep <port>

This output of this command will contain the process id. Now we just need to kill the process.

Verify the process

Before killing the process, figure out what process it is and ensure we are not killing any required processes.

ps -aux | grep <process id>

The output of the above command will give the details of the process.

How to Kill a process ?

After confirming the details, you can kill the process

kill -9 <process id>

Now verify whether the port is still locked or not by executing the netstat command again

Linux commands to check the disk utilization, size of directory or file

  • Command to check the disk utilization
df -h

The ‘-h’ option will provide the utilization in human readable format.

  • Command to check the size of a directory
du -sh <directory name>
  • Command to check the size of a file
du -sh <file name>
  • Command to check the size of files in a directory

Go inside the directory and execute the following command

du -sh *

 

How to check whether a Raspberry Pi is 32 bit or 64 bit ?

The latest version of Raspberry Pi comes with 64 bit CPU, but prior to that it was with 32 bit CPU. Some softwares and applications are dependent on CPU and OS architecture.

There are various options to check the architecture.

Method 1:

type the following command and check the response

uname -m

You will get a response something like armv7l or armv8.

ARMv7 and below are 32-bit. AMRv8 introduces the 64-bit instruction set.

Method 2:

Install lshw using the command

apt-get install lshw

Then type the command lshw.  You will be able to find the architecture from the response of the command.

How to clear/delete the cached Kerberos ticket ?

In Linux

kdestroy

 

In Windows

klist purge

How to check the Java architecture from command line ?

To check the Java architecture whether it is 32 bit or 64 bit, the following commands will be helpful.

Execute the following commands in the command line and check the results

java -d32 -version

java -d64 -version

If any of the above command is giving an error message similar to “Error: This Java instance does not support a xx-bit JVM.” If it is not supporting 32 bit, then we can say it is a 64 bit Java. If it is not supporting 64 bit, we can say it is a 32-bit Java. The screenshot of the same is attached below.

java_screen.PNG

Hope this will help .. 🙂

Error: Cannot retrieve metalink for repository: epel. Please verify its path and try again

Problem:

Error: Cannot retrieve metalink for repository: epel. Please verify its path and try again

Faced this error in CentOS 6.3. This issue started after installing the epel-repo

Solution:

You need to update ca-certificates package. Before that disable all the repos with https that are failing.

Here in my case,  epel-repo is failing, so I have to disable only epel repo:

yum --disablerepo=epel -y update  ca-certificates

The above fix helped me to resolve the issue.

How to verify/validate the entries in fstab without a system reboot ?

/etc/fstab contains information about the disks. It has the details about where the partitions and storage devices should be mounted. We usually configure automount, disk quota, mount points etc in this fstab.

Inorder to test the entries or modifications in fstab without restart the following commands will be helpful

mount -a

The above command will mount all the filesystems mentioned in the fstab. This is just like a refresh command to activate the entries in fstab.

mount -fav

The above command will help if you don’t want to apply the modifications in the fstab and want to validate the entries only.  This will just fake the entries in the fstab without applying the changes. This is a very useful command.

 

 

How to attach a new EBS to an EC2 instance

Nowadays majority of us are using some cloud services. Amazon Web Services is one of the popular provider among all the other cloud service providers. Just like we upgrade our harddisk or mounting new drives to physical machines, we can attach new block storages to Amazon EC2 also. Amazon provides a service called EBS (Elastic Block Storage). There are various types of EBS with various speed and cost. Example are magnetic, SSD etc.

Attaching a new EBS to a running EC2 instance is very simple. We can do this programatically as well as using the console. Here I am explaining the basic steps to perform this operation using the console.

  1. Launch an EBS in the same region and same availability zone as that of the EC2 instance
  2. Note down the instance id of the EC2 instance
  3. Attach the EBS to the EC2. This can be done by using the attach option available in the EBS. The EBS will be listed under the Volumes section in EC2 service page of AWS console.
  4. Login to the EC2 instance and switch to the root user
  5. Type lsblk to list all the block devices
  6. Identify the new block device.
  7. Create a new directory to mount the EBS.
  8. Format the newly mounted storage. The command is mkfs -t ext4 /dev/<device-name>
  9. Mount the EBS on the directory. The command is mount /dev/<device-name>  <mount-dir>
  10. Check for the new storage. The command is df -h