A system engineer's notes

Monday, 12 July 2021

Docker: Getting started

Introduction
In this article, we be covering a basic introduction to the Docker Tool along with its installation on a Centos 7 system. Docker has become synonymous with containerization of applications in Linux and thousands of applications have successfully been dockerized and deployed in Production since Docker's inception.

History of Docker:
Docker, Inc. is a technology company based in San Francisco. It was founded by French born american Solomon Hykes and start as a PaaS provider called dotCloud. dotCloud leveraged Linux containers and their internal tool used to manage containers was nick-named Docker. In 2013 dotCloud was re branded as Docker.

What is Docker?
To understand what is Docker we must first know what are containers. A container is a special type of process that is isolated from other processes. Containers are assigned resources that no other process can access, and they cannot access any resources not explicitly assigned to them. Docker provides a Runtime and Orchestration Engine to allow users to deploy and manage containers. There are tools available that also allow for the orchestration of containers.

Docker is available in the following main editions:

Enterprise Edition (EE): Paid and supported for 12 months. Provides additional features like certified images, same day support and vulnerability scanning of images.
Community Edition (CE) : Free and supported for 4 months.
Moby: Open-Source upstream project of Docker and breaks Docker down into more modular components

Why should you use Docker?

Dev and Production environment are the same

Bugs in Production can be replicated in Development

Lets you put your environment and configuration into code and deploy it

Allows the same Docker configuration to be used in a variety of environments

Build standards and repeatable processes

Developer Productivity

App Isolation

Server Consolidation

Installing Docker:

Before installing Docker we must install some of the pre-requisite packages as shown below:

yum install -y yum-utils device-mapper-persistent-data lvm2

Now we will add the stable repository for the Docker community edition of the system:

[root@dokcer-test ~]# yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo

Loaded plugins: fastestmirror

adding repo from: https://download.docker.com/linux/centos/docker-ce.repo

grabbing file https://download.docker.com/linux/centos/docker-ce.repo to /etc/yum.repos.d/docker-ce.repo

repo saved to /etc/yum.repos.d/docker-ce.repo

[root@dokcer-test ~]#

Now that the Docker repository has been configured, we can install Docker CE using yum.

yum -y install docker-ce

The above command installs the following packages:

===========================================================================================================================================================

Package Arch Version Repository Size

===========================================================================================================================================================

Installing:

docker-ce x86_64 3:19.03.1-3.el7 docker-ce-stable 24 M

Installing for dependencies:

container-selinux noarch 2:2.99-1.el7_6 extras 39 k

containerd.io x86_64 1.2.6-3.3.el7 docker-ce-stable 26 M

docker-ce-cli x86_64 1:19.03.1-3.el7 docker-ce-stable 39 M

After the packages are successfully installed, we'll start the docker service and also enable it so that it starts automatically on boot.

[root@dokcer-test ~]# systemctl status docker

● docker.service - Docker Application Container Engine

Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled; vendor preset: disabled)

Active: active (running) since Tue 2019-07-30 04:38:00 UTC; 6s ago

Docs: https://docs.docker.com

Main PID: 9008 (dockerd)

CGroup: /system.slice/docker.service

└─9008 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

Jul 30 04:37:58 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:58.649760132Z" level=info msg="Creating filesystem xfs on de...emapper

Jul 30 04:37:59 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:59.064964291Z" level=info msg="Successfully created filesyst...emapper

Jul 30 04:37:59 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:59.211113995Z" level=warning msg="[graphdriver] WARNING: the...elease"

Jul 30 04:37:59 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:59.269462315Z" level=info msg="Loading containers: start."

Jul 30 04:37:59 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:59.567854253Z" level=info msg="Default bridge (docker0) is a...ddress"

Jul 30 04:37:59 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:37:59.654696034Z" level=info msg="Loading containers: done."

Jul 30 04:38:00 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:38:00.354472432Z" level=info msg="Docker daemon" commit=74b1e89...19.03.1

Jul 30 04:38:00 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:38:00.355436639Z" level=info msg="Daemon has completed initialization"

Jul 30 04:38:00 dokcer-test.example.com systemd[1]: Started Docker Application Container Engine.

Jul 30 04:38:00 dokcer-test.example.com dockerd[9008]: time="2019-07-30T04:38:00.527177004Z" level=info msg="API listen on /var/run/docker.sock"

Hint: Some lines were ellipsized, use -l to show in full.

[root@dokcer-test ~]#

It is considered best practice to not manage containers directly as the root user. Therefore, I'll add a user named sahil to the docker group and use this user account for running and managing containers on this server.

usermod -aG docker sahil

Let's verify that we can run Docker related commands as the user sahil:

[sahil@docker-test ~]$ docker version

Client: Docker Engine - Community

Version: 19.03.1

API version: 1.40

Go version: go1.12.5

Git commit: 74b1e89

Built: Thu Jul 25 21:21:07 2019

OS/Arch: linux/amd64

Experimental: false

Server: Docker Engine - Community

Engine:

Version: 19.03.1

API version: 1.40 (minimum version 1.12)

Go version: go1.12.5

Git commit: 74b1e89

Built: Thu Jul 25 21:19:36 2019

OS/Arch: linux/amd64

Experimental: false

containerd:

Version: 1.2.6

GitCommit: 894b81a4b802e4eb2a91d1ce216b8817763c29fb

runc:

Version: 1.0.0-rc8

GitCommit: 425e105d5a03fabd737a126ad93d62a9eeede87f

docker-init:

Version: 0.18.0

GitCommit: fec3683

[sahil@docker-test ~]$

Conclusion

This concludes our introduction to Docker where we talked about the history of Docker along with it's most popular use cases and also installed Docker on a Centos 7 system.

Friday, 14 May 2021

Create VM in Google Cloud using cloud shell

Introduction

In this article, 'll explain how we can create a virtual machine in Google Cloud using the command line tool gcloud which is pre-installed in google cloud shell. Cloud Shell is a Debian-based virtual machine loaded with all the development tools you'll need (gcloud, git, and others) and offers a persistent 5-GB home directory. Let's get started.

Step 1: Open cloud Shell

To open cloud shell, click on the shell icon on the top let corner of the google cloud home page.

Step 2: Authenticate

After clicking on the shell icon, google cloud shell is launched in a terminal window towards the bottom of the home page. Here type 'gcloud auth list' to authenticate your user account to use cloud shell to access and manipulate google cloud resources. You may also type 'gcloud config list project' to check the project you are working under. The term project is analogous to the term organization in AWS.

Step 3: Create virtual machine

Now, we will create our virtual machine using the following command:

gcloud compute instances create gcelab2 --machine-type n1-standard-2 --zone us-central1-f

"compute instances" is the google cloud component we'd like to work with.
"create" is the action that we'd like to perform.
gcelab2 is the name of the VM
"machine-type" defines the CPU/Memory configuration that we would like to use and this is divided into entities called series wherein each series comprises CPU/Memory settings for VMs. In this example we've selected n1-standard-2.
"zone" attribute specifies the location where the VM will reside.

Step 4: Verify login to the VM

We should now be able see our VM in the VM instances section under the Compute Engine.

You can either click on ssh under the connect section to login to the VM or use the following command in the cloud shell:

gcloud compute ssh gcelab2 --zone us-central1-f

Conclusion

We hope that you found this post to be useful and we look forward towards your feedback and suggestions

Saturday, 26 September 2020

Setup and use lxc on Centos 8

Introduction

In this article, we will learn the basics of lxc containerization technology and launch our first lxc container on a Centos 8 machine.

What is lxc?

LXC (Linux Containers) is an OS-level virtualization technology that allows the creation and running of multiple isolated Linux virtual environments (VE) on a single control host. These isolation levels or containers can be used to either sandbox specific applications, or to emulate an entirely new host.

Now let's gor through the steps of installing lxc on our Centos 8 machine.

Step 1: Enable the EPEL repo

[root@sa-lxc ~]# yum install epel-release

Last metadata expiration check: 0:32:41 ago on Sat 26 Sep 2020 07:40:13 PM IST.

Dependencies resolved.

==========================================================================

Package Arch Version Repository Size

==========================================================================

Installing:

epel-release noarch 8-8.el8 extras 23 k

Transaction Summary

==========================================================================

Install 1 Package

Total download size: 23 k

Installed size: 32 k

Is this ok [y/N]: y

Downloading Packages:

epel-release-8-8.el8.noarch.rpm 19 kB/s | 23 kB 00:01

-----------------------------------------------------------------------------------------------------------------------

Total 9.3 kB/s | 23 kB 00:02

warning: /var/cache/dnf/extras-2770d521ba03e231/packages/epel-release-8-8.el8.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 8483c65d: NOKEY

CentOS-8 - Extras 1.3 MB/s | 1.6 kB 00:00

Importing GPG key 0x8483C65D:

Userid : "CentOS (CentOS Official Signing Key) <security@centos.org>"

Fingerprint: 99DB 70FA E1D7 CE22 7FB6 4882 05B5 55B3 8483 C65D

From : /etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

Is this ok [y/N]: y

Key imported successfully

Running transaction check

Transaction check succeeded.

Running transaction test

Transaction test succeeded.

Running transaction

Preparing : 1/1

Installing : epel-release-8-8.el8.noarch 1/1

Running scriptlet: epel-release-8-8.el8.noarch 1/1

Verifying : epel-release-8-8.el8.noarch 1/1

Installed:

epel-release-8-8.el8.noarch

Complete!

Step 2: Enable and increase user namespaces

To do so, we will need to modify the grub bootloader options and this will also entail a system reboot.

[root@sa-lxc ~]# grubby --args="namespace.unpriv_enable=1" --update-kernel="$(grubby --default-kernel)"

[root@sa-lxc ~]# echo "user.max_user_namespaces=2277484249" >> /etc/sysctl.d/99-userns.conf

Now reboot the system.

Step 3: Install snappy

The latest version of lxc is made available via the Snappy package manager. Snappy is a modern, cross-distribution, transactional package manager designed for working with self-contained, immutable packages. In order to install lxc we need to have the snappy package manager installed on the system and the corresponding services to be running.

[root@sa-lxc ~]# yum install snapd -y

Extra Packages for Enterprise Linux Modular 8 - x86_64 34 kB/s | 132 kB 00:03

Extra Packages for Enterprise Linux 8 - x86_64 2.0 MB/s | 8.0 MB 00:03

Dependencies resolved.

==========================================================================

Package Arch Version Repository Size

=========================================================================

Installing:

snapd x86_64 2.45.3.1-1.el8 epel 18 M

Upgrading:

libselinux x86_64 2.9-3.el8 BaseOS 166 k

libselinux-utils x86_64 2.9-3.el8 BaseOS 244 k

libsemanage x86_64 2.9-2.el8 BaseOS 165 k

libsepol x86_64 2.9-1.el8 BaseOS 340 k

policycoreutils x86_64 2.9-9.el8 BaseOS 377 k

policycoreutils-python-utils noarch 2.9-9.el8 BaseOS 251 k

python3-libselinux x86_64 2.9-3.el8 BaseOS 283 k

python3-libsemanage x86_64 2.9-2.el8 BaseOS 127 k

python3-policycoreutils noarch 2.9-9.el8 BaseOS 2.2 M

selinux-policy noarch 3.14.3-41.el8_2.6 BaseOS 615 k

selinux-policy-targeted noarch 3.14.3-41.el8_2.6 BaseOS 15 M

Installing dependencies:

snap-confine x86_64 2.45.3.1-1.el8 epel 3.3 M

snapd-selinux noarch 2.45.3.1-1.el8 epel 326 k

Transaction Summary

==========================================================================

Install 3 Packages

Upgrade 11 Packages

Total download size: 42 M

Downloading Packages:

(1/14): snapd-selinux-2.45.3.1-1.el8.noarch.rpm 231 kB/s | 326 kB 00:01

(2/14): snap-confine-2.45.3.1-1.el8.x86_64.rpm 2.1 MB/s | 3.3 MB 00:01

(3/14): snapd-2.45.3.1-1.el8.x86_64.rpm 9.6 MB/s | 18 MB 00:01

(4/14): libselinux-utils-2.9-3.el8.x86_64.rpm 549 kB/s | 244 kB 00:00

(5/14): libselinux-2.9-3.el8.x86_64.rpm 247 kB/s | 166 kB 00:00

(6/14): libsemanage-2.9-2.el8.x86_64.rpm 442 kB/s | 165 kB 00:00

(7/14): libsepol-2.9-1.el8.x86_64.rpm 1.1 MB/s | 340 kB 00:00

(8/14): policycoreutils-2.9-9.el8.x86_64.rpm 1.4 MB/s | 377 kB 00:00

(9/14): policycoreutils-python-utils-2.9-9.el8.noarch.rpm 1.9 MB/s | 251 kB 00:00

(10/14): python3-libsemanage-2.9-2.el8.x86_64.rpm 1.7 MB/s | 127 kB 00:00

(11/14): python3-libselinux-2.9-3.el8.x86_64.rpm 1.8 MB/s | 283 kB 00:00

(12/14): selinux-policy-3.14.3-41.el8_2.6.noarch.rpm 3.0 MB/s | 615 kB 00:00

(13/14): python3-policycoreutils-2.9-9.el8.noarch.rpm 8.1 MB/s | 2.2 MB 00:00

(14/14): selinux-policy-targeted-3.14.3-41.el8_2.6.noarch.rpm 17 MB/s | 15 MB 00:00

-----------------------------------------------------------------------------------------------------------------------

Total 7.2 MB/s | 42 MB 00:05

warning: /var/cache/dnf/epel-6519ee669354a484/packages/snap-confine-2.45.3.1-1.el8.x86_64.rpm: Header V4 RSA/SHA256 Signature, key ID 2f86d6a1: NOKEY

Extra Packages for Enterprise Linux 8 - x86_64 1.6 MB/s | 1.6 kB 00:00

Importing GPG key 0x2F86D6A1:

Userid : "Fedora EPEL (8) <epel@fedoraproject.org>"

Fingerprint: 94E2 79EB 8D8F 25B2 1810 ADF1 21EA 45AB 2F86 D6A1

From : /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-8

Key imported successfully

Running transaction check

Transaction check succeeded.

Running transaction test

Transaction test succeeded.

Running transaction

Preparing : 1/1

Running scriptlet: libsepol-2.9-1.el8.x86_64 1/1

Upgrading : libsepol-2.9-1.el8.x86_64 1/25

Running scriptlet: libsepol-2.9-1.el8.x86_64 1/25

Upgrading : libselinux-2.9-3.el8.x86_64 2/25

Running scriptlet: libselinux-2.9-3.el8.x86_64 2/25

Upgrading : libselinux-utils-2.9-3.el8.x86_64 3/25

Upgrading : libsemanage-2.9-2.el8.x86_64 4/25

Upgrading : policycoreutils-2.9-9.el8.x86_64 5/25

Running scriptlet: policycoreutils-2.9-9.el8.x86_64 5/25

Upgrading : python3-libselinux-2.9-3.el8.x86_64 6/25

Upgrading : python3-libsemanage-2.9-2.el8.x86_64 7/25

Upgrading : python3-policycoreutils-2.9-9.el8.noarch 8/25

Upgrading : policycoreutils-python-utils-2.9-9.el8.noarch 9/25

Upgrading : selinux-policy-3.14.3-41.el8_2.6.noarch 10/25

Running scriptlet: selinux-policy-3.14.3-41.el8_2.6.noarch 10/25

Running scriptlet: selinux-policy-targeted-3.14.3-41.el8_2.6.noarch 11/25

Upgrading : selinux-policy-targeted-3.14.3-41.el8_2.6.noarch 11/25

Running scriptlet: selinux-policy-targeted-3.14.3-41.el8_2.6.noarch 11/25

Running scriptlet: snapd-selinux-2.45.3.1-1.el8.noarch 12/25

Installing : snapd-selinux-2.45.3.1-1.el8.noarch 12/25

Running scriptlet: snapd-selinux-2.45.3.1-1.el8.noarch 12/25

Installing : snap-confine-2.45.3.1-1.el8.x86_64 13/25

Installing : snapd-2.45.3.1-1.el8.x86_64 14/25

Running scriptlet: snapd-2.45.3.1-1.el8.x86_64 14/25

Cleanup : selinux-policy-targeted-3.14.1-61.el8.noarch 15/25

Running scriptlet: selinux-policy-targeted-3.14.1-61.el8.noarch 15/25

Cleanup : selinux-policy-3.14.1-61.el8.noarch 16/25

Running scriptlet: selinux-policy-3.14.1-61.el8.noarch 16/25

Cleanup : policycoreutils-python-utils-2.8-16.1.el8.noarch 17/25

Cleanup : python3-policycoreutils-2.8-16.1.el8.noarch 18/25

Running scriptlet: policycoreutils-2.8-16.1.el8.x86_64 19/25

Cleanup : policycoreutils-2.8-16.1.el8.x86_64 19/25

Cleanup : python3-libsemanage-2.8-5.el8.x86_64 20/25

Cleanup : libsemanage-2.8-5.el8.x86_64 21/25

Cleanup : python3-libselinux-2.8-6.el8.x86_64 22/25

Cleanup : libselinux-utils-2.8-6.el8.x86_64 23/25

Cleanup : libselinux-2.8-6.el8.x86_64 24/25

Cleanup : libsepol-2.8-2.el8.x86_64 25/25

Running scriptlet: libsepol-2.8-2.el8.x86_64 25/25

Running scriptlet: snapd-selinux-2.45.3.1-1.el8.noarch 25/25

Running scriptlet: libsepol-2.8-2.el8.x86_64 25/25

Verifying : snap-confine-2.45.3.1-1.el8.x86_64 1/25

Verifying : snapd-2.45.3.1-1.el8.x86_64 2/25

Verifying : snapd-selinux-2.45.3.1-1.el8.noarch 3/25

Verifying : libselinux-2.9-3.el8.x86_64 4/25

Verifying : libselinux-2.8-6.el8.x86_64 5/25

Verifying : libselinux-utils-2.9-3.el8.x86_64 6/25

Verifying : libselinux-utils-2.8-6.el8.x86_64 7/25

Verifying : libsemanage-2.9-2.el8.x86_64 8/25

Verifying : libsemanage-2.8-5.el8.x86_64 9/25

Verifying : libsepol-2.9-1.el8.x86_64 10/25

Verifying : libsepol-2.8-2.el8.x86_64 11/25

Verifying : policycoreutils-2.9-9.el8.x86_64 12/25

Verifying : policycoreutils-2.8-16.1.el8.x86_64 13/25

Verifying : policycoreutils-python-utils-2.9-9.el8.noarch 14/25

Verifying : policycoreutils-python-utils-2.8-16.1.el8.noarch 15/25

Verifying : python3-libselinux-2.9-3.el8.x86_64 16/25

Verifying : python3-libselinux-2.8-6.el8.x86_64 17/25

Verifying : python3-libsemanage-2.9-2.el8.x86_64 18/25

Verifying : python3-libsemanage-2.8-5.el8.x86_64 19/25

Verifying : python3-policycoreutils-2.9-9.el8.noarch 20/25

Verifying : python3-policycoreutils-2.8-16.1.el8.noarch 21/25

Verifying : selinux-policy-3.14.3-41.el8_2.6.noarch 22/25

Verifying : selinux-policy-3.14.1-61.el8.noarch 23/25

Verifying : selinux-policy-targeted-3.14.3-41.el8_2.6.noarch 24/25

Verifying : selinux-policy-targeted-3.14.1-61.el8.noarch 25/25

Upgraded:

libselinux-2.9-3.el8.x86_64 libselinux-utils-2.9-3.el8.x86_64

libsemanage-2.9-2.el8.x86_64 libsepol-2.9-1.el8.x86_64

policycoreutils-2.9-9.el8.x86_64 policycoreutils-python-utils-2.9-9.el8.noarch

python3-libselinux-2.9-3.el8.x86_64 python3-libsemanage-2.9-2.el8.x86_64

python3-policycoreutils-2.9-9.el8.noarch selinux-policy-3.14.3-41.el8_2.6.noarch

selinux-policy-targeted-3.14.3-41.el8_2.6.noarch

Installed:

snapd-2.45.3.1-1.el8.x86_64 snap-confine-2.45.3.1-1.el8.x86_64 snapd-selinux-2.45.3.1-1.el8.noarch

Complete!

Step 4: Enable and start snappy

[root@sa-lxc ~]# ln -s /var/lib/snapd/snap /snap

[root@sa-lxc ~]# systemctl enable --now snapd.socket

Created symlink /etc/systemd/system/sockets.target.wants/snapd.socket → /usr/lib/systemd/system/snapd.socket.

[root@sa-lxc ~]# systemctl enable snapd.service

Created symlink /etc/systemd/system/multi-user.target.wants/snapd.service → /usr/lib/systemd/system/snapd.service.

[root@sa-lxc ~]# systemctl start snapd.service

Step 5: Install lxc

[root@sa-lxc ~]# snap install lxd

lxd 4.6 from Canonical✓ installed

Step 6: Configure lxc

To configure lxc we need to type the lxd init command and answer a couple of prompts as shown below.

[root@sa-lxc ~]# lxd init

Would you like to use LXD clustering? (yes/no) [default=no]:

Do you want to configure a new storage pool? (yes/no) [default=yes]:

Name of the new storage pool [default=default]: mylxc

Name of the storage backend to use (lvm, ceph, btrfs, dir) [default=btrfs]: lvm

Create a new LVM pool? (yes/no) [default=yes]: yes

Would you like to use an existing empty disk or partition? (yes/no) [default=no]:

Size in GB of the new loop device (1GB minimum) [default=5GB]:

Would you like to connect to a MAAS server? (yes/no) [default=no]:

Would you like to create a new local network bridge? (yes/no) [default=yes]:

What should the new bridge be called? [default=lxdbr0]:

What IPv4 address should be used? (CIDR subnet notation, “auto” or “none”) [default=auto]:

What IPv6 address should be used? (CIDR subnet notation, “auto” or “none”) [default=auto]: none

Would you like LXD to be available over the network? (yes/no) [default=no]: yes

Address to bind LXD to (not including port) [default=all]:

Port to bind LXD to [default=8443]:

Trust password for new clients:

Again:

No password set, client certificates will have to be manually trusted. Would you like stale cached images to be updated automatically? (yes/no) [default=yes]

Would you like a YAML "lxd init" preseed to be printed? (yes/no) [default=no]:

Step 7: Launch a container

Once lxd has been initialized we can go ahead and launch our first container using the lxc launch command.

[root@sa-lxc ~]# lxc launch ubuntu:18.04

Creating the instance

Instance name is: caring-dane

Starting caring-dane

To view currently running containers use the lxc list command.

[root@sa-lxc ~]# lxc list

+-------------+---------+-----------------------+------+-----------+-----------+

+-------------+---------+-----------------------+------+-----------+-----------+

+-------------+---------+-----------------------+------+-----------+-----------+

[root@sa-lxc ~]#

Note: Since this was just a lab setup, I had stopped the firewalld service on my Centos system. In a production setup, be sure to add the appropriate firewall rules to allow the setup to work.

Conclusion

We hope that this article has been helpful to you and we look forward to your suggestions and feedback.

Friday, 14 August 2020

Generate .pem file from .ppk file

Introduction

I recently came across a requirement from our DBA Team wherein they needed to access a server on Oracle Cloud and all they had was a .ppk file received from the App Team that managed the OCI server. Since the source server from which we had to connect did not have a GUI, we needed to generate a corresponding .pem file to use on the command line.

For this, I installed putty on a lab machine.

yum install putty

I then used the .ppk file to generate a .pem file using the following command:

puttygen oci.ppk -O private-openssh -o oci.pem

This step did prompt me for the passphrase for the .ppk file which I was are of.

To login to the OCI server, I used .pem file by specifying the -i option with the ssh command as follows.

-bash-4.1$ ssh -i oci.pem opc@11.59.17.209

Enter passphrase for key 'oci.pem':

Last login: Thu Aug 13 07:17:30 UTC 2020 from 10.22.88.26 on pts/0

Last login: Thu Aug 13 07:18:17 2020 from 10.22.88.26

The passphrase was the same one I was prompted for while generating the .pem file.

Conclusion

This concludes this post on how to generate a .pem file from a .ppk file. I hope you found the post to be useful.

Thursday, 21 May 2020

Add color to Message of the Day (MOTD in Centos/RHEL

Introduction

In this quick article, I'll demonstrate how to add some color to your message of the day (MOTD) which is some text that is displayed on the screen when you first log in.

We'll not be using the /etc/motd file for this so this file should be empty.

We'll create a script in /etc/profile.d/ that contains the text we'd like to print. Here I've named the script login-info.sh.

[ssuri@lab-node:~] $ cat /etc/profile.d/login-info.sh
#! /usr/bin/env bash

GREEN="\033[01;32m"
YELLOW="\033[01;33m"
MAGENTA="\033[01;35m"
CYAN="\033[01;36m"
NC="\033[00m"

echo -e "${GREEN}================================${NC}"
echo -e "${YELLOW}Welcome to the Ansible LAB Box${NC}"
echo -e "${GREEN}================================${NC}"
[ssuri@lab-node:~] $

I've used escape sequences with the echo command to display text in green and yellow colors when printed.

Next, we need to add the following line in /etc/ssh/sshd_config file and restart the sshd service.

PrintMotd no

That's it. The next time we login we'll be greeted with a colorful login message.

Conclusion

I've used green and yellow colors in this example but you can use a different color sequence. You may also use the output of commands within the script to make the login message more informative.

Monday, 12 August 2019

Bash: Use for loop to iterate over file without using cat

Introduction:

In this very brief post I just wanted to demonstrate a way to use the for loop to iterate over a file without using the cat command.

Here is a file containing 10 numbers, 1 number on each line.

cat seq.txt
1
2
3
4
5
6
7
8
9
10

If I wanted to loop over it using for loop in bash I could type:

for i in `cat seq.txt`; do echo $i ;done

But what if I don't want to read in the file using cat command?

I could use input redirection to read in the file contents as shown in the below example:

for i in $(<seq.txt); do echo $i ; done

One more way of avoiding the cat command would be to read the file into an array and then loop over the array as shown below.

readarray -t ARR < seq.txt

for i in "${ARR[@]}"; do echo $i; done

Conclusion

There is nothing wrong with using the cat command to read in file contents while iterating over it in a for loop. I just found redirecting directly from STDIN to be more elegant and I would try to use it more often.

Friday, 2 August 2019

Git cheat sheet

Introduction

This post contains a git cheat sheet that I've created for my use. The content is very succinct. I'll be writing more elaborate posts on installing and using git in the future. Also, this will be an ongoing post and will be periodically updated.

Here's the cheat sheet:

Initialize repository:

[sahil@lab-node:~/rep] $ git init .

Check for anything for ready to be staged or committed:
[sahil@lab-node:~/rep] $ git status

Staging changes:

[sahil@lab-node:~/rep] $ git add 1.bash
[sahil@lab-node:~/rep] $ git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
# (use "git rm --cached <file>..." to unstage)
#
# new file: 1.bash
#

Commit the changes to the repo:

[sahil@lab-node:~/rep] $ git commit -m "version 1"
[master (root-commit) 252f733] version 1
1 file changed, 10 insertions(+)
create mode 100644 1.bash

View log of commits:

[sahil@lab-node:~/rep] $ git log
commit 252f733ded22f7634725961bfd95ecdc0826a69c
Author: Sahil Suri <sahil.suri@emerson.com>
Date: Fri Aug 2 05:30:02 2019 +0000

version 1

View one line logs:

[sahil@lab-node:~/rep] $ git log --oneline
3ee84d7 version 2
252f733 version 1

View the difference between commits:

[sahil@lab-node:~/rep] $ git log --graph -p
* commit 3ee84d747f009d7cbbe6f12b3eb71bd39da14220
| Author: Sahil Suri <sahil.suri@emerson.com>
| Date: Fri Aug 2 05:32:29 2019 +0000
|
| version 2
|
| diff --git a/1.bash b/1.bash
| index e193461..2ab8d4e 100644
| --- a/1.bash
| +++ b/1.bash
| @@ -7,4 +7,4 @@
| #version:
| ##############################################################
|
| -echo "This is testing version 1"
| +echo "This is testing version 2"
|
* commit 252f733ded22f7634725961bfd95ecdc0826a69c
Author: Sahil Suri <sahil.suri@emerson.com>
Date: Fri Aug 2 05:30:02 2019 +0000

version 1

diff --git a/1.bash b/1.bash
new file mode 100644
index 0000000..e193461
--- /dev/null
+++ b/1.bash
@@ -0,0 +1,10 @@
+#!/bin/bash
+
+##############################################################
+#Author: Sahil Suri
+#Date:
+#Purpose:
+#version:
+##############################################################
+
+echo "This is testing version 1"

View differences b/w current version and a particular version:

#git diff <commit hash>

[sahil@lab-node:~/rep] $ git log --oneline
c9ef444 this is version 3'
3ee84d7 version 2
252f733 version 1

[sahil@lab-node:~/rep] $ git diff 3ee84d7
diff --git a/1.bash b/1.bash
index 2ab8d4e..c2b325e 100644
--- a/1.bash
+++ b/1.bash
@@ -7,4 +7,4 @@
#version:
##############################################################

-echo "This is testing version 2"
+echo "This is testing version 3"

[sahil@lab-node:~/rep] $ git diff 252f733
diff --git a/1.bash b/1.bash
index e193461..c2b325e 100644
--- a/1.bash
+++ b/1.bash
@@ -7,4 +7,4 @@
#version:
##############################################################

-echo "This is testing version 1"
+echo "This is testing version 3"
[sahil@lab-node:~/rep] $

Retrieving previous versions of files:

This has a couple of steps involved:

Retrieve version of 1.bash two commits before:

[sahil@lab-node:~/rep] $ git checkout HEAD~2 1.bash

To save this as the current version of a file, use git commit.

To go back to the updated version follow the below steps:

[sahil@lab-node:~/rep] $ git reset HEAD 1.bash
[sahil@lab-node:~/rep] $ git checkout 1.bash

Add a remote to git repository:

git remote add origin https://github.com/sahilsuri008/creating_shell_scripts_linux

To push updates to GitHub:

git push origin master

Conclusion

That's it for this post for now. Thank you for taking the time to read it.