import jenkins.model.Jenkins 
Jenkins.instance.pluginManager.plugins.each{ 
  plugin ->  
    println ("${plugin.getDisplayName()} (${plugin.getShortName()}): ${plugin.getVersion()}") 
}

node('linux') {
stage('Linux :: NodeJS :: Default' ) { 
    sh 'node --version' 
}
stage('Linux :: NodeJS :: v8.9.0' ) {
    def nodejs = tool name: 'Linux NodeJS v8.9.0', type: 'jenkins.plugins.nodejs.tools.NodeJSInstallation'
    sh "$ {nodejs}/bin/node --version"
}
stage('Linux :: NodeJS :: v8.9.0:: withEnv' ) {
    withEnv(["PATH+NODE=${tool 'Linux NodeJS v8.9.0'}/bin"]){ 
        sh 'node --version' 
        }
    }
}

def labels = [
    'node1',
	'node2'
    ]
def builders = [:]
for (x in labels) {
    def label = x 
    // Create a map to pass in to the 'parallel' step so we can fire all the builds at once
    builders[label] = {
      node(label) {
        // build steps that should happen on all nodes go here
        }
    }
}
parallel builders

export JAVA_HOME=/home/sriram/Downloads/jdk-8u191-linux-x64/jdk1.8.0_191
$JAVA_HOME/bin/java -jar jenkins.war &

sriram@optimus-prime:~/.jenkins$ pwd
/home/sriram/.jenkins
sriram@optimus-prime:~/.jenkins$ ll
total 104
drwxr-xr-x 12 sriram sriram  4096 nov 10 12:56 ./
drwxr-xr-x 44 sriram sriram  4096 nov 10 12:51 ../
-rw-r--r--  1 sriram sriram  1644 nov 10 12:56 config.xml
-rw-r--r--  1 sriram sriram   156 nov 10 12:56 hudson.model.UpdateCenter.xml
-rw-r--r--  1 sriram sriram   370 nov 10 12:51 hudson.plugins.git.GitTool.xml
-rw-------  1 sriram sriram  1712 nov 10 12:49 identity.key.enc
-rw-r--r--  1 sriram sriram    94 nov 10 12:49 jenkins.CLI.xml
-rw-r--r--  1 sriram sriram     7 nov 10 12:53 jenkins.install.InstallUtil.lastExecVersion
-rw-r--r--  1 sriram sriram     7 nov 10 12:53 jenkins.install.UpgradeWizard.state
-rw-r--r--  1 sriram sriram   179 nov 10 12:53 jenkins.model.JenkinsLocationConfiguration.xml
-rw-r--r--  1 sriram sriram   171 nov 10 12:49 jenkins.telemetry.Correlator.xml
drwxr-xr-x  2 sriram sriram  4096 nov 10 12:49 jobs/
drwxr-xr-x  3 sriram sriram  4096 nov 10 12:49 logs/
-rw-r--r--  1 sriram sriram   907 nov 10 12:56 nodeMonitors.xml
drwxr-xr-x  2 sriram sriram  4096 nov 10 12:49 nodes/
drwxr-xr-x 75 sriram sriram 12288 nov 10 12:51 plugins/
-rw-r--r--  1 sriram sriram   129 nov 10 12:55 queue.xml.bak
-rw-r--r--  1 sriram sriram    64 nov 10 12:49 secret.key
-rw-r--r--  1 sriram sriram     0 nov 10 12:49 secret.key.not-so-secret
drwx------  4 sriram sriram  4096 nov 10 12:53 secrets/
drwxr-xr-x  2 sriram sriram  4096 nov 10 12:50 updates/
drwxr-xr-x  2 sriram sriram  4096 nov 10 12:49 userContent/
drwxr-xr-x  3 sriram sriram  4096 nov 10 12:53 users/
drwxr-xr-x 11 sriram sriram  4096 nov 10 12:49 war/
drwxr-xr-x  2 sriram sriram  4096 nov 10 12:51 workflow-libs/
sriram@optimus-prime:~/.jenkins$

syntax:
docker run jenkins/jnlp-slave -url http://jenkins-server:port <secret> <agent name>

example:
docker run jenkins/jnlp-slave -url http://192.168.2.8:8080 8302b7d76d0828b629bdd1460d587268af64616fe464d69f34c9119f5670f1f3 docker-agent-1

[nexusrepo]
name=Nexus Repository
baseurl=http://localhost:8081/repository/yum-google-chrome/
enabled=1
gpgcheck=1
gpgkey=https://dl-ssl.google.com/linux/linux_signing_key.pub
priority=1

docker pull sonarqube
docker run -d --name sonarqube -p 9000:9000 <image_name>

# once the container has started successfully, you can open below url to access sonaqube.
# http://localhost:9000/

[root@10 ~]# sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     actual (secure)
Max kernel policy version:      31

[root@10 ~]# cat /etc/selinux/config

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these three values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected.
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted

ssh-keygen -t rsa -b 2048 -C "your_email@example.com"

ssh keygen will create 2 files in .ssh default path which are id_rsa and id_rsa.pub  
id_rsa file will have private key  
id_rsa.pub will have the public key

ssh-copy-id user@host
# This will add public key from ~/.ssh/id_rsa.pub on your system to /home/USER/authorized_keys on target HOST

echo | openssl s_client -connect www.google.com:443 2>&1 | sed --quiet '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > google.crt

sriram@sriram-Inspiron-5567:~$ openssl s_client -connect google.com:443 -servername google.com -cipher ALL -brief
CONNECTION ESTABLISHED
Protocol version: TLSv1.3
Ciphersuite: TLS_AES_256_GCM_SHA384
Peer certificate: C = US, ST = California, L = Mountain View, O = Google LLC, CN = *.google.com
Hash used: SHA256
Signature type: ECDSA
Verification: OK
Server Temp Key: X25519, 253 bits

openssl s_client -connect google.com:443 -servername google.com -cipher ALL -showcerts

openssl x509 -pubkey -noout -in cert.pem  > pubkey.pem

echo | openssl s_client -connect google.com:443 2>/dev/null | openssl x509 -noout -dates

# Generate a private key
openssl genrsa -out mykey.pem 2048

# Extract public key from above generated private key
openssl rsa -in mykey.pem -pubout > mykey.pub

sriram@sriram-Inspiron-5567:~$ curl abnamro.com -L -v
* Rebuilt URL to: abnamro.com/
*   Trying 88.221.24.80...
* TCP_NODELAY set
* Connected to abnamro.com (88.221.24.80) port 80 (#0)
> GET / HTTP/1.1
> Host: abnamro.com
> User-Agent: curl/7.58.0
> Accept: */*
< HTTP/1.1 301 Moved Permanently
< Server: AkamaiGHost
< Content-Length: 0
< Location: https://www.abnamro.com/
< Expires: Fri, 09 Aug 2019 04:24:52 GMT
< Cache-Control: max-age=0, no-cache, no-store
< Pragma: no-cache
< Date: Fri, 09 Aug 2019 04:24:52 GMT
< Connection: keep-alive
* Connection #0 to host abnamro.com left intact
* Issue another request to this URL: 'https://www.abnamro.com/'
*   Trying 88.221.24.96...
* TCP_NODELAY set
* Connected to www.abnamro.com (88.221.24.96) port 443 (#1)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* (304) (OUT), TLS handshake, Client hello (1):
* (304) (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
* ALPN, server accepted to use h2
* Server certificate:
*  subject: jurisdictionC=NL; jurisdictionST=NH; jurisdictionL=Amsterdam; businessCategory=Private Organization; serialNumber=34334259; C=NL; ST=NH; L=Amsterdam; O=ABN AMRO Bank N.V.; OU=Internet Banking; CN=www.abnamro.com
*  start date: Sep 24 13:22:58 2018 GMT
*  expire date: Sep 24 13:31:00 2020 GMT
*  subjectAltName: host "www.abnamro.com" matched cert's "www.abnamro.com"
*  issuer: C=BM; O=QuoVadis Limited; CN=QuoVadis EV SSL ICA G1
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x5602954b8580)
> GET / HTTP/2
> Host: www.abnamro.com
> User-Agent: curl/7.58.0
> Accept: */*

# requires elevated access either root or sudo
# install required dependencies (tested on Raspberry Pi 4)
sudo apt-get -y install libffi-dev libssl-dev python3-dev python3 python3-pip
sudo curl -sSL https://get.docker.com | sh

# To run docker as non sudo/root user, add the user to docker group
sudo usermod -aG docker <user> #logout and login after this command.

# Testing
docker run hello-world

# Installing dockerCE
yum install -y yum-utils \
  device-mapper-persistent-data \
  lvm2

yum-config-manager \
    --add-repo \
    https://download.docker.com/linux/centos/docker-ce.repo

yum install docker-ce

# post docker install steps
# to run docker as non root user
usermod -aG docker <user_id>

[root@centos7vm ~]# systemctl enable docker
Created symlink from /etc/systemd/system/multi-user.target.wants/docker.service to /usr/lib/systemd/system/docker.service.

[root@centos7vm ~]# chkconfig docker on
Note: Forwarding request to 'systemctl enable docker.service'.

service docker start

docker ps

#Example, portainer an opensource web interface to manage containers using bind mount
$ docker container run -d \
  -p 9000:9000 \
  -v /var/run/docker.sock:/var/run/docker.sock \ #bind mount
  portainer/portainer

sudo vi /lib/systemd/system/docker.service

# Modify the line that starts with ExecStart
ExecStart=/usr/bin/docker daemon -H fd:// -H tcp://0.0.0.0:4243

systemctl daemon-reload
sudo service docker restart

#Testing
sriram@optimus-prime:~$ curl http://localhost:4243/version
{"Platform":{"Name":""},"Components":[{"Name":"Engine","Version":"18.03.1-ce","Details":{"ApiVersion":"1.37","Arch":"amd64","BuildTime":"2018-04-26T07:15:45.000000000+00:00","Experimental":"false","GitCommit":"9ee9f40","GoVersion":"go1.9.5","KernelVersion":"4.15.0-38-generic","MinAPIVersion":"1.12","Os":"linux"}}],"Version":"18.03.1-ce","ApiVersion":"1.37","MinAPIVersion":"1.12","GitCommit":"9ee9f40","GoVersion":"go1.9.5","Os":"linux","Arch":"amd64","KernelVersion":"4.15.0-38-generic","BuildTime":"2018-04-26T07:15:45.000000000+00:00"}

ENV
FROM
LABEL maintainer=""
      version=""  
WORKDIR
RUN
VOLUME
EXPOSE
ENTRYPOINT --> Executes custom scripts while starting a docker container
           --> Do not add layer to docker image
CMD

# Example dockerfile
COPY docker-entrypoint.sh /
RUN chmod +x /docker-entrypoint.sh
ENTRYPOINT ["/docker-entrypoint.sh"]

#docker image build -t <image_name>:<version_tag> .
docker image build -t ansible:v1.0 .

docker container run -d -t --name ansible ansible:v1.0 bin/bash

$ docker container exec -it <container_id> bash
# (or) to run shell command directly on a running container, then
$ docker container exec <container_name/id> cat /appl/readme.txt

docker container run --rm -itd --name <container_name> -v $PWD:/appl -v /appl/data

docker container run --rm -itd --name <dest-container> --volumes-from <src_container_name_from_which_data_is_accessed>

# To stop all running containers in one go, below command can be used
docker container stop $(docker container ls -a -q)

# By default, docker will add all running containers to default bridge network
# To inspect docker bridge network, use below command
docker network inspect bridge

# To run a docker container and join to a custom bridge network, use --net flag
docker container run --rm --itd --name <container_name> --net <network_name>

docker exec <container_name> ifconfig
docker exec <container_name> ip -a

chown -R 200 /home/sriram.yeluri/Data/NEXUS_DATA  

docker run -d \
-p 8081:8081 \
--name nexus \
-v /home/sriram.yeluri/Data/NEXUS_DATA:/nexus-data \
sonatype/nexus3

docker run -p 8080:8080 -p 50000:50000 \
--name jenkins \
-v /home/sriram.yeluri/Data/JENKINS_HOME:/var/jenkins_home \
jenkins

docker run -p 8080:8080 -p 50000:50000 \
--name jenkins \
-v /home/sriram.yeluri/Data/JENKINS_HOME:/var/jenkins_home \
jenkins/jenkins:lts

docker run -p 8089:8080 -p 50001:50000 \
--name cjoc \
-v /home/sriram.yeluri/Data/JENKINS_OC_HOME:/var/jenkins_home \
cloudbees/jenkins-operations-center

#Initial secret can be found at below path
/var/jenkins_home/secrets/initialAdminPassword

docker run \
--name postgres \
-e POSTGRES_PASSWORD=secret \
-d postgres \
-v /home/sriram.yeluri/Data/PG_DATA:/var/lib/postgresql/data

$ kubectl create deployment hello-node --image=gcr.io/hello-minikube-zero-install/hello-node

deployment.apps/hello-node created

$ kubectl get deployments

NAME         READY   UP-TO-DATE   AVAILABLE   AGE
hello-node   1/1     1            1           19s

$ kubectl get pods
NAME                          READY   STATUS    RESTARTS   AGE
hello-node-55b49fb9f8-bkmnb   1/1     Running   0          43s


$ kubectl get events

LAST SEEN   TYPE     REASON                    OBJECT                             MESSAGE

94s         Normal   Scheduled                 pod/hello-node-55b49fb9f8-bkmnb    Successfully assigned default/hello-node-55b49fb9f8-bkmnb to minikube
92s         Normal   Pulling                   pod/hello-node-55b49fb9f8-bkmnb    Pulling image "gcr.io/hello-minikube-zero-install/hello-node"
91s         Normal   Pulled                    pod/hello-node-55b49fb9f8-bkmnb    Successfully pulled image "gcr.io/hello-minikube-zero-install/hello-node"
90s         Normal   Created                   pod/hello-node-55b49fb9f8-bkmnb    Created container hello-node
90s         Normal   Started                   pod/hello-node-55b49fb9f8-bkmnb    Started container hello-node
94s         Normal   SuccessfulCreate          replicaset/hello-node-55b49fb9f8   Created pod: hello-node-55b49fb9f8-bkmnb
94s         Normal   ScalingReplicaSet         deployment/hello-node              Scaled up replica set hello-node-55b49fb9f8 to 1
4m34s       Normal   NodeHasSufficientMemory   node/minikube                      Node minikube status is now: NodeHasSufficientMemory
4m34s       Normal   NodeHasNoDiskPressure     node/minikube                      Node minikube status is now: NodeHasNoDiskPressure
4m34s       Normal   NodeHasSufficientPID      node/minikube                      Node minikube status is now: NodeHasSufficientPID
4m11s       Normal   RegisteredNode            node/minikube                      Node minikube event: Registered Node minikube in Controller
4m6s        Normal   Starting                  node/minikube                      Starting kube-proxy.

$ kubectl config view

apiVersion: v1
clusters:
- cluster:
    certificate-authority: /root/.minikube/ca.crt
    server: https://172.17.0.27:8443
  name: minikube
contexts:
- context:
    cluster: minikube
    user: minikube
  name: minikube
current-context: minikube
kind: Config
preferences: {}
users:
- name: minikube
  user:
    client-certificate: /root/.minikube/client.crt
    client-key: /root/.minikube/client.key

# nginx-deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

$ kubectl apply -f nginx-deployment.yaml
deployment.apps/nginx-deployment created
$ kubectl get events
$ kubectl get pods
$ kubectl delete deployment nginx-deployment
deployment.extensions "nginx-deployment" deleted

$ kubectl get deployments

NAME         READY   UP-TO-DATE   AVAILABLE   AGE
hello-node   1/1     1            1           4m16s

$ kubectl delete deployment hello-node
deployment.extensions "hello-node" deleted

$ kubectl get deployments
No resources found.

$ kubectl get pods
No resources found.

# This script is the modified version from k8s documentation
RELEASE="$(curl -sSL https://dl.k8s.io/release/stable.txt)"

mkdir -p /usr/bin
cd /usr/bin

curl -L --remote-name-all https://storage.googleapis.com/kubernetes-release/release/${RELEASE}/bin/linux/amd64/{kubeadm,kubelet,kubectl}
chmod +x {kubeadm,kubelet,kubectl}

curl -sSL "https://raw.githubusercontent.com/kubernetes/kubernetes/${RELEASE}/build/debs/kubelet.service" > /etc/systemd/system/kubelet.service

mkdir -p /etc/systemd/system/kubelet.service.d

curl -sSL "https://raw.githubusercontent.com/kubernetes/kubernetes/${RELEASE}/build/debs/10-kubeadm.conf" > /etc/systemd/system/kubelet.service.d/10-kubeadm.conf

[root@10 ~]# swapoff -a
[root@10 ~]# kubeadm init
[init] Using Kubernetes version: v1.15.3
[preflight] Running pre-flight checks
	[WARNING Firewalld]: firewalld is active, please ensure ports [6443 10250] are open or your cluster may not function correctly
	[WARNING Service-Docker]: docker service is not enabled, please run 'systemctl enable docker.service'
[preflight] Pulling images required for setting up a Kubernetes cluster
[preflight] This might take a minute or two, depending on the speed of your internet connection
[preflight] You can also perform this action in beforehand using 'kubeadm config images pull'
[kubelet-start] Writing kubelet environment file with flags to file "/var/lib/kubelet/kubeadm-flags.env"
[kubelet-start] Writing kubelet configuration to file "/var/lib/kubelet/config.yaml"
[kubelet-start] Activating the kubelet service
[certs] Using certificateDir folder "/etc/kubernetes/pki"
[certs] Generating "ca" certificate and key
[certs] Generating "apiserver-kubelet-client" certificate and key
[certs] Generating "apiserver" certificate and key
[certs] apiserver serving cert is signed for DNS names [10.0.2.15 kubernetes kubernetes.default kubernetes.default.svc kubernetes.default.svc.cluster.local] and IPs [10.96.0.1 10.0.2.15]
[certs] Generating "etcd/ca" certificate and key
[certs] Generating "etcd/server" certificate and key
[certs] etcd/server serving cert is signed for DNS names [10.0.2.15 localhost] and IPs [10.0.2.15 127.0.0.1 ::1]
[certs] Generating "etcd/healthcheck-client" certificate and key
[certs] Generating "apiserver-etcd-client" certificate and key
[certs] Generating "etcd/peer" certificate and key
[certs] etcd/peer serving cert is signed for DNS names [10.0.2.15 localhost] and IPs [10.0.2.15 127.0.0.1 ::1]
[certs] Generating "front-proxy-ca" certificate and key
[certs] Generating "front-proxy-client" certificate and key
[certs] Generating "sa" key and public key
[kubeconfig] Using kubeconfig folder "/etc/kubernetes"
[kubeconfig] Writing "admin.conf" kubeconfig file
[kubeconfig] Writing "kubelet.conf" kubeconfig file
[kubeconfig] Writing "controller-manager.conf" kubeconfig file
[kubeconfig] Writing "scheduler.conf" kubeconfig file
[control-plane] Using manifest folder "/etc/kubernetes/manifests"
[control-plane] Creating static Pod manifest for "kube-apiserver"
[control-plane] Creating static Pod manifest for "kube-controller-manager"
[control-plane] Creating static Pod manifest for "kube-scheduler"
[etcd] Creating static Pod manifest for local etcd in "/etc/kubernetes/manifests"
[wait-control-plane] Waiting for the kubelet to boot up the control plane as static Pods from directory "/etc/kubernetes/manifests". This can take up to 4m0s
[apiclient] All control plane components are healthy after 20.014010 seconds
[upload-config] Storing the configuration used in ConfigMap "kubeadm-config" in the "kube-system" Namespace
[kubelet] Creating a ConfigMap "kubelet-config-1.15" in namespace kube-system with the configuration for the kubelets in the cluster
[upload-certs] Skipping phase. Please see --upload-certs
[mark-control-plane] Marking the node 10.0.2.15 as control-plane by adding the label "node-role.kubernetes.io/master=''"
[mark-control-plane] Marking the node 10.0.2.15 as control-plane by adding the taints [node-role.kubernetes.io/master:NoSchedule]
[bootstrap-token] Using token: n2e7ii.lp571oh88qidwzdj
[bootstrap-token] Configuring bootstrap tokens, cluster-info ConfigMap, RBAC Roles
[bootstrap-token] configured RBAC rules to allow Node Bootstrap tokens to post CSRs in order for nodes to get long term certificate credentials
[bootstrap-token] configured RBAC rules to allow the csrapprover controller automatically approve CSRs from a Node Bootstrap Token
[bootstrap-token] configured RBAC rules to allow certificate rotation for all node client certificates in the cluster
[bootstrap-token] Creating the "cluster-info" ConfigMap in the "kube-public" namespace
[addons] Applied essential addon: CoreDNS
[addons] Applied essential addon: kube-proxy

Your Kubernetes control-plane has initialized successfully!

To start using your cluster, you need to run the following as a regular user:

  mkdir -p $HOME/.kube
  sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
  sudo chown $(id -u):$(id -g) $HOME/.kube/config

You should now deploy a pod network to the cluster.
Run "kubectl apply -f [podnetwork].yaml" with one of the options listed at:
  https://kubernetes.io/docs/concepts/cluster-administration/addons/

Then you can join any number of worker nodes by running the following on each as root:

kubeadm join 10.0.2.15:6443 --token n2e7ii.lp571oh88qidwzdj \
    --discovery-token-ca-cert-hash sha256:0957baa4cdea8fda244c159cf2a038a2afe2c0b20fb922014472c5c7918dac81

[root@10 ~]# service kubelet status
Redirecting to /bin/systemctl status kubelet.service
● kubelet.service - kubelet: The Kubernetes Node Agent
   Loaded: loaded (/etc/systemd/system/kubelet.service; enabled; vendor preset: disabled)
  Drop-In: /etc/systemd/system/kubelet.service.d
           └─10-kubeadm.conf
   Active: active (running) since zo 2019-09-01 21:04:42 CEST; 1min 9s ago
     Docs: http://kubernetes.io/docs/
 Main PID: 4682 (kubelet)
   CGroup: /system.slice/kubelet.service
           └─4682 /usr/bin/kubelet --bootstrap-kubeconfig=/etc/kubernetes/bootstrap-kubelet.conf --kubeconfig=/etc/kubernetes/kubelet.conf --c...

sep 01 21:05:35 10.0.2.15 kubelet[4682]: E0901 21:05:35.780354    4682 summary_sys_containers.go:47] Failed to get system container stats for ...
sep 01 21:05:35 10.0.2.15 kubelet[4682]: E0901 21:05:35.780463    4682 summary_sys_containers.go:47] Failed to get system container stats for ...
sep 01 21:05:37 10.0.2.15 kubelet[4682]: W0901 21:05:37.277274    4682 cni.go:213] Unable to update cni config: No networks found in /...ni/net.d
sep 01 21:05:40 10.0.2.15 kubelet[4682]: E0901 21:05:40.725221    4682 kubelet.go:2169] Container runtime network not ready: NetworkRe...tialized
sep 01 21:05:42 10.0.2.15 kubelet[4682]: W0901 21:05:42.277894    4682 cni.go:213] Unable to update cni config: No networks found in /...ni/net.d
sep 01 21:05:45 10.0.2.15 kubelet[4682]: E0901 21:05:45.728937    4682 kubelet.go:2169] Container runtime network not ready: NetworkRe...tialized
sep 01 21:05:45 10.0.2.15 kubelet[4682]: E0901 21:05:45.813742    4682 summary_sys_containers.go:47] Failed to get system container stats for ...
sep 01 21:05:45 10.0.2.15 kubelet[4682]: E0901 21:05:45.813868    4682 summary_sys_containers.go:47] Failed to get system container stats for ...
sep 01 21:05:47 10.0.2.15 kubelet[4682]: W0901 21:05:47.278690    4682 cni.go:213] Unable to update cni config: No networks found in /...ni/net.d
sep 01 21:05:50 10.0.2.15 kubelet[4682]: E0901 21:05:50.733668    4682 kubelet.go:2169] Container runtime network not ready: NetworkRe...tialized
Hint: Some lines were ellipsized, use -l to show in full.

#run the following as a regular user, to start using the cluster
mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config

[root@10 ~]# kubectl get nodes
NAME        STATUS     ROLES    AGE     VERSION
10.0.2.15   NotReady   master   6m38s   v1.15.3

[root@10 ~]# kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 | tr -d '\n')"
serviceaccount/weave-net created
clusterrole.rbac.authorization.k8s.io/weave-net created
clusterrolebinding.rbac.authorization.k8s.io/weave-net created
role.rbac.authorization.k8s.io/weave-net created
rolebinding.rbac.authorization.k8s.io/weave-net created
daemonset.extensions/weave-net created

# kubernetes master is now in Ready status after deploying weave-net
[root@10 ~]# kubectl get nodes
NAME        STATUS   ROLES    AGE   VERSION
10.0.2.15   Ready    master   4d    v1.15.3

[root@10 ~]# kubectl get pods -n kube-system
NAME                                READY   STATUS    RESTARTS   AGE
coredns-5644d7b6d9-9gp2m            0/1     Running   0          23m
coredns-5644d7b6d9-x5f9f            0/1     Running   0          23m
etcd-10.0.2.15                      1/1     Running   0          22m
kube-apiserver-10.0.2.15            1/1     Running   0          22m
kube-controller-manager-10.0.2.15   1/1     Running   0          22m
kube-proxy-xw6mq                    1/1     Running   0          23m
kube-scheduler-10.0.2.15            1/1     Running   0          22m
weave-net-8hmqc                     2/2     Running   0          2m6s

[root@10 ~]# kubectl get pods -A
NAMESPACE     NAME                                READY   STATUS    RESTARTS   AGE
kube-system   coredns-5644d7b6d9-9gp2m            0/1     Running   0          23m
kube-system   coredns-5644d7b6d9-x5f9f            0/1     Running   0          23m
kube-system   etcd-10.0.2.15                      1/1     Running   0          22m
kube-system   kube-apiserver-10.0.2.15            1/1     Running   0          22m
kube-system   kube-controller-manager-10.0.2.15   1/1     Running   0          22m
kube-system   kube-proxy-xw6mq                    1/1     Running   0          23m
kube-system   kube-scheduler-10.0.2.15            1/1     Running   0          22m
kube-system   weave-net-8hmqc                     2/2     Running   0          2m9s

[root@10 ~]# kubectl get componentstatuses
NAME                 STATUS    MESSAGE             ERROR
controller-manager   Healthy   ok
scheduler            Healthy   ok
etcd-0               Healthy   {"health":"true"}

[root@10 ~]# kubectl get events
LAST SEEN   TYPE     REASON                    OBJECT           MESSAGE
4d          Normal   NodeHasSufficientMemory   node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasSufficientMemory
4d          Normal   NodeHasNoDiskPressure     node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasNoDiskPressure
4d          Normal   NodeHasSufficientPID      node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasSufficientPID
4d          Normal   RegisteredNode            node/10.0.2.15   Node 10.0.2.15 event: Registered Node 10.0.2.15 in Controller
4d          Normal   Starting                  node/10.0.2.15   Starting kube-proxy.
38m         Normal   Starting                  node/10.0.2.15   Starting kubelet.
38m         Normal   NodeHasSufficientMemory   node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasSufficientMemory
38m         Normal   NodeHasNoDiskPressure     node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasNoDiskPressure
38m         Normal   NodeHasSufficientPID      node/10.0.2.15   Node 10.0.2.15 status is now: NodeHasSufficientPID
38m         Normal   NodeAllocatableEnforced   node/10.0.2.15   Updated Node Allocatable limit across pods
38m         Normal   Starting                  node/10.0.2.15   Starting kube-proxy.
38m         Normal   RegisteredNode            node/10.0.2.15   Node 10.0.2.15 event: Registered Node 10.0.2.15 in Controller
12m         Normal   NodeReady                 node/10.0.2.15   Node 10.0.2.15 status is now: NodeReady

[root@10 kubernetes]# tree /etc/kubernetes
/etc/kubernetes
├── admin.conf
├── controller-manager.conf
├── kubelet.conf
├── manifests
│   ├── etcd.yaml
│   ├── kube-apiserver.yaml
│   ├── kube-controller-manager.yaml
│   └── kube-scheduler.yaml
├── pki
│   ├── apiserver.crt
│   ├── apiserver-etcd-client.crt
│   ├── apiserver-etcd-client.key
│   ├── apiserver.key
│   ├── apiserver-kubelet-client.crt
│   ├── apiserver-kubelet-client.key
│   ├── ca.crt
│   ├── ca.key
│   ├── etcd
│   │   ├── ca.crt
│   │   ├── ca.key
│   │   ├── healthcheck-client.crt
│   │   ├── healthcheck-client.key
│   │   ├── peer.crt
│   │   ├── peer.key
│   │   ├── server.crt
│   │   └── server.key
│   ├── front-proxy-ca.crt
│   ├── front-proxy-ca.key
│   ├── front-proxy-client.crt
│   ├── front-proxy-client.key
│   ├── sa.key
│   └── sa.pub
└── scheduler.conf

3 directories, 30 files

[root@10 kubernetes]# docker images
REPOSITORY                           TAG                 IMAGE ID            CREATED             SIZE
k8s.gcr.io/kube-proxy                v1.15.3             232b5c793146        13 days ago         82.4 MB
k8s.gcr.io/kube-apiserver            v1.15.3             5eb2d3fc7a44        13 days ago         207 MB
k8s.gcr.io/kube-scheduler            v1.15.3             703f9c69a5d5        13 days ago         81.1 MB
k8s.gcr.io/kube-controller-manager   v1.15.3             e77c31de5547        13 days ago         159 MB
k8s.gcr.io/coredns                   1.3.1               eb516548c180        7 months ago        40.3 MB
k8s.gcr.io/etcd                      3.3.10              2c4adeb21b4f        9 months ago        258 MB
k8s.gcr.io/pause                     3.1                 da86e6ba6ca1        20 months ago       742 kB

[root@10 ~]# kubeadm reset
[reset] Reading configuration from the cluster...
[reset] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[reset] WARNING: Changes made to this host by 'kubeadm init' or 'kubeadm join' will be reverted.
[reset] Are you sure you want to proceed? [y/N]: y
[preflight] Running pre-flight checks
[reset] Removing info for node "10.0.2.15" from the ConfigMap "kubeadm-config" in the "kube-system" Namespace
W0901 11:56:16.802756   29062 removeetcdmember.go:61] [reset] failed to remove etcd member: error syncing endpoints with etc: etcdclient: no available endpoints
.Please manually remove this etcd member using etcdctl
[reset] Stopping the kubelet service
[reset] Unmounting mounted directories in "/var/lib/kubelet"
[reset] Deleting contents of config directories: [/etc/kubernetes/manifests /etc/kubernetes/pki]
[reset] Deleting files: [/etc/kubernetes/admin.conf /etc/kubernetes/kubelet.conf /etc/kubernetes/bootstrap-kubelet.conf /etc/kubernetes/controller-manager.conf /etc/kubernetes/scheduler.conf]
[reset] Deleting contents of stateful directories: [/var/lib/etcd /var/lib/kubelet /etc/cni/net.d /var/lib/dockershim /var/run/kubernetes]

The reset process does not reset or clean up iptables rules or IPVS tables.
If you wish to reset iptables, you must do so manually.
For example:
iptables -F && iptables -t nat -F && iptables -t mangle -F && iptables -X

If your cluster was setup to utilize IPVS, run ipvsadm --clear (or similar)
to reset your system's IPVS tables.

The reset process does not clean your kubeconfig files and you must remove them manually.
Please, check the contents of the $HOME/.kube/config file.
[root@10 ~]#

# swapoff -a
# systemctl start kubelet
# systemctl status kubelet

# This command will create a new token and display the connection string
[root@10 ~]# kubeadm token create --print-join-command
kubeadm join 10.0.2.15:6443 --token 1clhim.pk9teustr2v1gnu2     --discovery-token-ca-cert-hash sha256:efe18b97c7a320e7173238af7126e33ebe76f3877255c8f9aa055f292dbf3755

# Other toke commands
kubeadm token list
kubeadm token delete <TOKEN>

# vi /var/log/messages
Sep  1 20:57:25 localhost kubelet: F0901 20:57:25.706063    2874 server.go:198] failed to load Kubelet config file /var/lib/kubelet/config.yaml, error failed to read kubelet config file "/var/lib/kubelet/config.yaml", error: open /var/lib/kubelet/config.yaml: no such file or directory
Sep  1 20:57:25 localhost systemd: kubelet.service: main process exited, code=exited, status=255/n/a
Sep  1 20:57:25 localhost systemd: Unit kubelet.service entered failed state.
Sep  1 20:57:25 localhost systemd: kubelet.service failed.
Sep  1 20:57:35 localhost systemd: kubelet.service holdoff time over, scheduling restart.
Sep  1 20:57:35 localhost systemd: Stopped kubelet: The Kubernetes Node Agent.
Sep  1 20:57:35 localhost systemd: Started kubelet: The Kubernetes Node Agent.
Sep  1 20:57:35 localhost kubelet: F0901 20:57:35.968939    2882 server.go:198] failed to load Kubelet config file /var/lib/kubelet/config.yaml, error failed to read kubelet config file "/var/lib/kubelet/config.yaml", error: open /var/lib/kubelet/config.yaml: no such file or directory
Sep  1 20:57:35 localhost systemd: kubelet.service: main process exited, code=exited, status=255/n/a
Sep  1 20:57:35 localhost systemd: Unit kubelet.service entered failed state.
Sep  1 20:57:35 localhost systemd: kubelet.service failed.

# Set go path, kind path and KUBECONFIG path

export PATH=$PATH:$HOME/go/bin:$HOME/k8s/bin
kind get kubeconfig-path)
# output: /home/sriram/.kube/kind-config-kind
export KUBECONFIG="$(kind get kubeconfig-path)"

sriram@sriram-Inspiron-5567:~$ kind create cluster
Creating cluster "kind" ...
 ✓ Ensuring node image (kindest/node:v1.15.0) 🖼 
 ✓ Preparing nodes 📦 
 ✓ Creating kubeadm config 📜 
 ✓ Starting control-plane 🕹️ 
 ✓ Installing CNI 🔌 
 ✓ Installing StorageClass 💾 
Cluster creation complete. You can now use the cluster with:

export KUBECONFIG="$(kind get kubeconfig-path --name="kind")"
kubectl cluster-info

kind create cluster
 kubectl cluster-info
 Kubernetes master is running at https://localhost:37933
 KubeDNS is running at https://localhost:37933/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy

# To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

kind delete cluster
Deleting cluster "kind" ...
$KUBECONFIG is still set to use /home/sriram/.kube/kind-config-kind even though that file has been deleted, remember to unset it

$ kubectl version

Client Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.2", GitCommit:"f6278300bebbb750328ac16ee6dd3aa7d3549568", GitTreeState:"clean", BuildDate:"2019-08-05T09:23:26Z", GoVersion:"go1.12.5", Compiler:"gc", Platform:"linux/amd64"}

Server Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.0", GitCommit:"e8462b5b5dc2584fdcd18e6bcfe9f1e4d970a529", GitTreeState:"clean", BuildDate:"2019-06-19T16:32:14Z", GoVersion:"go1.12.5", Compiler:"gc", Platform:"linux/amd64"}

$ kubectl cluster-info
Kubernetes master is running at https://172.17.0.45:8443
KubeDNS is running at https://172.17.0.45:8443/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy
To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

$ kubectl get nodes
NAME       STATUS   ROLES    AGE   VERSION
minikube   Ready    master   11m   v1.15.0

# list nodes with more information
$ kubectl get nodes -o=wide
NAME       STATUS   ROLES    AGE   VERSION   INTERNAL-IP   EXTERNAL-IP   OS-IMAGE              KERNEL-VERSION   CONTAINER-RUNTIME
minikube   Ready    master   13m   v1.15.0   10.0.2.15     <none>        Buildroot 2018.05.3   4.15.0           docker://18.9.6

kubectl create namespace dev

Kubectl create configmap \
 <config-name> --from-literal=<key>=<value>

(or)
Kubectl create configmap \
 <config-name> --from-file=<path_to_file>

Kubectl create configmap \
app-color-config –from-literal=APP_COLOR=blue \
--from-literal=APP_MOD=prod

Kubectl create configmap \
app-config  --from-file=app-config.properties

sriram@sriram-Inspiron-5567:~/k8s$ minikube start
😄  minikube v1.2.0 on linux (amd64)
💿  Downloading Minikube ISO ...
 129.33 MB / 129.33 MB [============================================] 100.00% 0s
🔥  Creating virtualbox VM (CPUs=2, Memory=2048MB, Disk=20000MB) ...
🐳  Configuring environment for Kubernetes v1.15.0 on Docker 18.09.6
💾  Downloading kubeadm v1.15.0
💾  Downloading kubelet v1.15.0
🚜  Pulling images ...
🚀  Launching Kubernetes ... 
⌛  Verifying: apiserver proxy etcd scheduler controller dns
🏄  Done! kubectl is now configured to use "minikube"

$ minikube status
host: Running
kubelet: Running
apiserver: Running
kubectl: Correctly Configured: pointing to minikube-vm at 192.168.99.100

$ minikube service list

|-------------|------------|--------------|
|  NAMESPACE  |    NAME    |     URL      |
|-------------|------------|--------------|
| default     | kubernetes | No node port |
| kube-system | kube-dns   | No node port |
|-------------|------------|--------------|

$ minikube stop
✋  Stopping "minikube" in virtualbox ...
🛑  "minikube" stopped.

$ minikube status
host: Stopped
kubelet:
apiserver:
kubectl:

sriram@sriram-Inspiron-5567:~/k8s$ minikube start
😄  minikube v1.2.0 on linux (amd64)
💡  Tip: Use 'minikube start -p <name>' to create a new cluster, or 'minikube delete' to delete this one.
🔄  Restarting existing virtualbox VM for "minikube" ...
⌛  Waiting for SSH access ...
🐳  Configuring environment for Kubernetes v1.15.0 on Docker 18.09.6
🔄  Relaunching Kubernetes v1.15.0 using kubeadm ... 
⌛  Verifying: apiserver proxy etcd scheduler controller dns
🏄  Done! kubectl is now configured to use "minikube"

$ minikube addons enable metrics-server
✅  metrics-server was successfully enabled

sriram@sriram-Inspiron-5567:~/k8s$ kubectl get apiservices
NAME                                   SERVICE                      AVAILABLE   AGE
v1.                                    Local                        True        31m
v1.apps                                Local                        True        31m
v1.authentication.k8s.io               Local                        True        31m
v1.authorization.k8s.io                Local                        True        31m
v1.autoscaling                         Local                        True        31m
v1.batch                               Local                        True        31m
v1.coordination.k8s.io                 Local                        True        31m
v1.networking.k8s.io                   Local                        True        31m
v1.rbac.authorization.k8s.io           Local                        True        31m
v1.scheduling.k8s.io                   Local                        True        31m
v1.storage.k8s.io                      Local                        True        31m
v1beta1.admissionregistration.k8s.io   Local                        True        31m
v1beta1.apiextensions.k8s.io           Local                        True        31m
v1beta1.apps                           Local                        True        31m
v1beta1.authentication.k8s.io          Local                        True        31m
v1beta1.authorization.k8s.io           Local                        True        31m
v1beta1.batch                          Local                        True        31m
v1beta1.certificates.k8s.io            Local                        True        31m
v1beta1.coordination.k8s.io            Local                        True        31m
v1beta1.events.k8s.io                  Local                        True        31m
v1beta1.extensions                     Local                        True        31m
v1beta1.metrics.k8s.io                 kube-system/metrics-server   True        95s
v1beta1.networking.k8s.io              Local                        True        31m
v1beta1.node.k8s.io                    Local                        True        31m
v1beta1.policy                         Local                        True        31m
v1beta1.rbac.authorization.k8s.io      Local                        True        31m
v1beta1.scheduling.k8s.io              Local                        True        31m
v1beta1.storage.k8s.io                 Local                        True        31m
v1beta2.apps                           Local                        True        31m
v2beta1.autoscaling                    Local                        True        31m
v2beta2.autoscaling                    Local                        True        31m

export HTTP_PROXY=http://<proxy hostname:port>
export HTTPS_PROXY=https://<proxy hostname:port>
export NO_PROXY=localhost,127.0.0.1,10.96.0.0/12,192.168.99.0/24,192.168.39.0/24

minikube start

$ kubectl describe pods

Name:           kubernetes-bootcamp-5b48cfdcbd-5ddlwNamespace:      defaultPriority:       0

Node:           minikube/172.17.0.90
Start Time:     Mon, 26 Aug 2019 11:54:05 +0000
Labels:         pod-template-hash=5b48cfdcbd
                run=kubernetes-bootcamp

Annotations:    <none>

Status:         Running
IP:             172.18.0.5
Controlled By:  ReplicaSet/kubernetes-bootcamp-5b48cfdcbd

Containers:

  kubernetes-bootcamp:
    Container ID:   docker://016f25827984c14dc74e5cbaafe43b0fb77b20b8838b5818d1e9a90376b8ad5d
    Image:          gcr.io/google-samples/kubernetes-bootcamp:v1
    Image ID:       docker-pullable://jocatalin/kubernetes-bootcamp@sha256:0d6b8ee63bb57c5f5b6156f446b3bc3b3c143d233037f3a2f00e279c8fcc64af
    Port:           8080/TCP
    Host Port:      0/TCP
    State:          Running
      Started:      Mon, 26 Aug 2019 11:54:06 +0000
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-5wbkl (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             True
  ContainersReady   True
  PodScheduled      True
Volumes:
  default-token-5wbkl:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-5wbkl
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age    From               Message
  ----    ------     ----   ----               -------
  Normal  Scheduled  6m58s  default-scheduler  Successfully assigned default/kubernetes-bootcamp-5b48cfdcbd-5ddlw to minikube
  Normal  Pulled     6m57s  kubelet, minikube  Container image "gcr.io/google-samples/kubernetes-bootcamp:v1" already present on machine
  Normal  Created    6m57s  kubelet, minikube  Created container kubernetes-bootcamp
  Normal  Started    6m57s  kubelet, minikube  Started container kubernetes-bootcamp

apiVersion: v1
kind: Pod
metadata:
  name: label-demo
  labels:
    environment: production
    app: nginx
spec:
  containers:
  - name: nginx
    image: nginx:1.7.9
    ports:
    - containerPort: 80

apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
data:
  username: User
  password: **********

$ kubectl apply -f ./secret.yaml
$ kubectl get secrets
NAME                  TYPE                                  DATA   AGE
default-token-prh24   kubernetes.io/service-account-token   3      27m
mysecret              Opaque                                2      14m

$ export POD_NAME=$(kubectl get pods -o go-template --template '{{range .items}}{{.metadata.name}}{{"\n"}}{{end}}')

$ echo $POD_NAME
kubernetes-bootcamp-5b48cfdcbd-5ddlw

$ kubectl exec $POD_NAME env

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=kubernetes-bootcamp-5b48cfdcbd-5ddlw
KUBERNETES_SERVICE_HOST=10.96.0.1
KUBERNETES_SERVICE_PORT=443
KUBERNETES_SERVICE_PORT_HTTPS=443
KUBERNETES_PORT=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP_PROTO=tcp
KUBERNETES_PORT_443_TCP_PORT=443
KUBERNETES_PORT_443_TCP_ADDR=10.96.0.1
NPM_CONFIG_LOGLEVEL=info
NODE_VERSION=6.3.1
HOME=/root

# Start a bash session in the Pod’s container:
 ## name of the container can be omitted if we only have a single container in the Pod

kubectl exec -ti $POD_NAME bash

# gopath can be any directory on your system  
Edit your `~/.bash_profile` and add the line: `export GOPATH=$HOME/go`  
source your bash_profile `source ~/.bash_profile`

# Set the GOBIN path to generate a binary file when go install is executed.
 `export GOBIN=$HOME/go/bin`

env GOOS=target-OS GOARCH=target-architecture go build <package-import-path>
env GOOS=windows GOARCH=amd64 go build <path_to_go_src>

sriram@optimus-prime:~$ go tool dist list
android/386
android/amd64
android/arm
android/arm64
darwin/386
darwin/amd64
darwin/arm
darwin/arm64
dragonfly/amd64
freebsd/386
freebsd/amd64
freebsd/arm
linux/386
linux/amd64
linux/arm
linux/arm64
linux/mips
linux/mips64
linux/mips64le
linux/mipsle
linux/ppc64
linux/ppc64le
linux/s390x
nacl/386
nacl/amd64p32
nacl/arm
netbsd/386
netbsd/amd64
netbsd/arm
openbsd/386
openbsd/amd64
openbsd/arm
plan9/386
plan9/amd64
plan9/arm
solaris/amd64
windows/386
windows/amd64

# Install prerequisites
$ sudo apt-get install build-essential 
$ sudo apt-get install libreadline-gplv2-dev libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev zlib1g-dev

# Download and extract python source tar
cd /tmp
wget https://www.python.org/ftp/python/3.8.5/Python-3.8.5.tar.xz
tar -xvf Python-3.8.5.tar.xz
cd Python-3.8.5
./configure --prefix=/appl/Python_3_8_5 --enable-optimizations
make
make install

# load json file 
with open('data_file.json') as json_file: 
    data = json.load(json_file) 
 
print(json.dumps(data,indent=4)) 

for groups in data['values']: 
     print(groups.items()) 

class MyClass: 
    def __init__(self,f_name,l_name): 
        print("MyClass is instantiated successfully") 
        print(f'{f_name}_{l_name}') 
        self.f_name = f_name 
        self.l_name = l_name 
 
if __name__ == '__main__': 
    print('file is called directly') 
else: 
    print('test2 file is imported') 
 
print(MyClass.__dict__) 

# https://docs.atlassian.com/bitbucket-server/rest/6.10.0/bitbucket-rest.html 
# https://pynative.com/parse-json-response-using-python-requests-library/ 
 
import requests 
 
session = requests.Session() 
limit = 25 
start = 0 
isLastPage = False 
json_response = [] 
admin_groups = [] 
 
try: 
    while not isLastPage: 
        url = f'https://bitbucket_url:7999/rest/api/1.0/admin/groups?limit={limit}&start={start}'
        # print(url) 
        r = session.get(url,auth=('USer_ID', 'Passwd')) 
        json_response.append(r.json()) 
        isLastPage = r.json()['isLastPage'] 
        if isLastPage == True: 
            break 
        start = r.json()['nextPageStart'] 
except Exception as err: 
    print(f'error: {err}') 
 
# json_response is a list with dictionary of values 
# iterate through list and get the dictionary 
for item in json_response: 
    for names in item['values']: 
        admin_groups.append(names['name'])      # Add the admin group names to list 
 
# Total number of groups 
print(f'Total Number of groups : {len(admin_groups)}') 
# iterate through admins list and print the admin group names 
for admin in admin_groups: 
    print(admin)

import requests 
import argparse 
 
 
def check_app_status(url): 
    r = requests.get(url) 
    try: 
        response = r.json() 
    except Exception as e: 
        print(f'Exception occured : {e}') 
    if (r.status_code == 200 and response['state'] == "RUNNING"): 
        print(f'Application is up and running') 
    else: 
        print(f'Application is not reachable') 

 
def init_argument_parser(argument_list=None): 
    parser = argparse.ArgumentParser() 
    parser.add_argument('-url', '--url', help='URL of Application ', required=True) 
    return parser.parse_args() 
 
if __name__ == '__main__': 
    args = init_argument_parser() 
    # print(f'{args.url}') 
    check_app_status(args.url) 

yum update -y && \
yum install ansible -y

 git config --global user.email "email_id"
 git config --global user.name "User Name"
 git config --list

 # Omit --global to set the identity only to a repository.

 git init (initialize a local repository)  
 git add --all (or)  
 git add [filename] (or)
 git add . [all changed files for commit]  
 git status (show the status of working tree)  
 git commit -m "commit message"  
 git push  (push to remote repository)  
 git pull (fetch changes from remote repository)  
 git clone [git repo url]  
 git fetch (fetch branches, tags from remote repository)

 git reset file_name  (or)  
 git reset  (to remove all files from staging area)  

 git tag -a v1.0 -m "Reason for tagging"
 git push origin v1.0

# If there are multiple tags, then use --tags flag  
 git push origin --tags (to push all tags)  

# To list out all the available tags
 git tag
 git tag -l (or) --list  (optional)

 git remote -v  

# To display all branches that are present remotely and locally
 git branch -a
# To create a new branch
 git branch branch_name  
 git checkout branch_name  

# discard all local changes/commits and pull from upstream

git reset --hard origin/master
git pull origin master

GIT_VERSION=2.33.1
wget https://mirrors.edge.kernel.org/pub/software/scm/git/git-${GIT_VERSION}.tar.gz
tar -xvf git-${GIT_VERSION}.tar.gz
cd git-${GIT_VERSION}
make configure
./configure --prefix=/appl/Git/${GIT_VERSION} --with-curl --with-expat
make all
make install

cd /appl
tar -cvf git-${GIT_VERSION}.tar Git/${GIT_VERSION}

# Reference : https://wiki.debian.org/iwlwifi

apt edit-sources
# add below non-free sources of debian to the list
# deb http://deb.debian.org/debian buster main contrib non-free
# deb-src http://deb.debian.org/debian buster main contrib non-free

apt update

apt install wireless-tools
apt install firmware-iwlwifi

modprobe -r iwlwifi
modprobe iwlwifi

root@sriram-pc:~# lspci
00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers (rev 02)
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 620 (rev 02)
00:04.0 Signal processing controller: Intel Corporation Skylake Processor Thermal Subsystem (rev 02)
00:14.0 USB controller: Intel Corporation Sunrise Point-LP USB 3.0 xHCI Controller (rev 21)
00:14.2 Signal processing controller: Intel Corporation Sunrise Point-LP Thermal subsystem (rev 21)
00:15.0 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #0 (rev 21)
00:15.1 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #1 (rev 21)
00:16.0 Communication controller: Intel Corporation Sunrise Point-LP CSME HECI #1 (rev 21)
00:17.0 SATA controller: Intel Corporation Sunrise Point-LP SATA Controller [AHCI mode] (rev 21)
00:1c.0 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #5 (rev f1)
00:1c.5 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #6 (rev f1)
00:1f.0 ISA bridge: Intel Corporation Sunrise Point-LP LPC Controller (rev 21)
00:1f.2 Memory controller: Intel Corporation Sunrise Point-LP PMC (rev 21)
00:1f.3 Audio device: Intel Corporation Sunrise Point-LP HD Audio (rev 21)
00:1f.4 SMBus: Intel Corporation Sunrise Point-LP SMBus (rev 21)
01:00.0 Network controller: Intel Corporation Wireless 3165 (rev 79)
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8101/2/6E PCI Express Fast/Gigabit Ethernet controller (rev 07)

# List all the available interfaces
ip a

# Install net-tools
apt-get install net-tools

# execute the commands as root
vi /etc/network/interfaces

# Add below lines to the interface file
auto enp0s8
iface enp0s8 inet dhcp

# Start the network interface
ifup enp0s8

# Check the status of enp0s8
ip a show enp0s8

# https://github.com/jgraph/drawio-desktop/releases/

cd /tmp
wget https://github.com/jgraph/drawio-desktop/releases/download/v13.6.2/draw.io-amd64-13.6.2.deb
sudo dpkg -i draw.io-amd64-13.6.2.deb

# Add to sources
echo "deb https://dl.bintray.com/getinsomnia/Insomnia /" \
    | sudo tee -a /etc/apt/sources.list.d/insomnia.list

# Add public key used to verify code signature
wget --quiet -O - https://insomnia.rest/keys/debian-public.key.asc \
    | sudo apt-key add -

# Refresh repository sources and install Insomnia
sudo apt-get update
sudo apt-get install insomnia

dnf -y install epel-release
dnf update -y

[root@192 ~]# dnf install epel-release
Last metadata expiration check: 1:50:34 ago on Fri 17 Jul 2020 11:34:52 AM CEST.
Dependencies resolved.
================================================================================================================
 Package                       Architecture            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                                                  98 kB/s |  23 kB     00:00    
----------------------------------------------------------------------------------------------------------------
Total                                                                            71 kB/s |  23 kB     00:00     
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                                                               1.6 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 products updated.

Installed:
  epel-release-8-8.el8.noarch                                                                                   

Complete!


[root@192 ~]# dnf update
Extra Packages for Enterprise Linux Modular 8 - x86_64                          122 kB/s |  82 kB     00:00    
Extra Packages for Enterprise Linux 8 - x86_64                                  1.1 MB/s | 7.4 MB     00:06    
Dependencies resolved.
Nothing to do.
Complete!

# check the latest available release of draw.io from github before installing.
sudo dnf install https://github.com/jgraph/drawio-desktop/releases/download/v13.4.5/draw.io-x86_64-13.4.5.rpm

edit '/etc/sysconfig/network-scripts/ifcfg-enp0s8'
change onboot parameter to yes, and restart the interface
'ONBOOT=YES'
# Restart the interface
ifdown ifcfg-enp0s8
ifup ifcfg-enp0s8

#nmcli d (List the available interfaces)
#nmtui
1. open Network manager, and choose Edit connection
2. choose you network interfaces and click “Edit”
3. Choose “Automatic” in IPv4 CONFIGURATION and check Automatically connect check box and press OK and quit from Network manager.
4. Restart network service 'systemctl restart NetworkManager.service'

[root@10 ~]# nmcli dev status
[or]
[root@10 ~]# nmcli d
DEVICE  TYPE      STATE      CONNECTION
enp0s3  ethernet  connected  enp0s3
enp0s8  ethernet  connected  enp0s8

# vim /etc/sysconfig/network-scripts/ifcfg-eth0

## Default Configuration
DEVICE="eth0"
ONBOOT=yes
NETBOOT=yes
UUID="41171a6f-bce1-44de-8a6e-cf5e782f8bd6"
IPV6INIT=yes
BOOTPROTO=dhcp
HWADDR="00:08:a2:0a:ba:b8"
TYPE=Ethernet
NAME="eth0"

## Configuration for Static IP
HWADDR=00:08:A2:0A:BA:B8
TYPE=Ethernet
BOOTPROTO=static
# Server IP #
IPADDR=192.168.2.203
# Subnet #
PREFIX=24
# Set default gateway IP #
GATEWAY=192.168.2.254
# Set dns servers #
DNS1=192.168.2.254
DNS2=8.8.8.8
DNS3=8.8.4.4
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
# Disable ipv6 #
IPV6INIT=no
NAME=eth0
# This is system specific and can be created using `uuidgen eth0` command #
UUID=41171a6f-bce1-44de-8a6e-cf5e782f8bd6
DEVICE=eth0
ONBOOT=yes

# Restart network interface
systemctl restart NetworkManager

# Verify new IP settings:
ip a s eth0

# Verify new routing settings:
ip r

# Verify DNS servers settings
cat /etc/resolv.conf

# Error message : "Your system does not seem to be set up to build kernel modules"
# Solution:
yum clean all
yum install gcc-c++
yum install kernel-devel
yum install kernel-headers

ubuntu@myberry:/etc/netplan$ cat config.yaml
network:
  version: 2
  renderer: networkd
  wifis:
    wlan0:
      dhcp4: no
      dhcp6: no
      addresses: [192.168.2.40/24]
      gateway4: 192.168.2.1
      nameservers:
              addresses: [8.8.8.8,192.168.2.1]
      access-points:
        "ACCESSPOINT_NAME":
          password: "PASSWORD"

ubuntu@myberry:/etc/netplan$ ip r
default via 192.168.2.1 dev wlan0 proto static
192.168.2.0/24 dev wlan0 proto kernel scope link src 192.168.2.40

sudo apt remove openssh-server openssh-client --purge \
&& sudo apt autoremove \
&& sudo apt autoclean \
&& sudo apt update \
&& sudo apt install openssh-server openssh-client

sudo systemctl enable ssh
sudo systemctl daemon-reload
sudo systemctl status ssh

ubuntu@myberry:/etc/netplan$ systemctl status ssh
● ssh.service - OpenBSD Secure Shell server
     Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
     Active: active (running) since Sat 2020-07-04 19:44:17 UTC; 21min ago
       Docs: man:sshd(8)
             man:sshd_config(5)
   Main PID: 1880 (sshd)
      Tasks: 1 (limit: 9255)
     CGroup: /system.slice/ssh.service
             └─1880 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups

Jul 04 19:44:17 myberry systemd[1]: Starting OpenBSD Secure Shell server...
Jul 04 19:44:17 myberry sshd[1880]: Server listening on 0.0.0.0 port 22.
Jul 04 19:44:17 myberry sshd[1880]: Server listening on :: port 22.
Jul 04 19:44:17 myberry systemd[1]: Started OpenBSD Secure Shell server.
Jul 04 19:47:28 myberry sshd[2195]: Accepted password for ubuntu from 192.168.2.13 port 36716 ssh2
Jul 04 19:47:28 myberry sshd[2195]: pam_unix(sshd:session): session opened for user ubuntu by (uid=0)

# To start firewalld
systemctl unmask firewalld
systemctl enable firewalld.service
systemctl start firewalld

# To stop firewalld
systemctl stop firewalld
systemctl disable firewalld
systemctl mask firewalld

# Quick command to check whether the firewall is enabled or disabled
systemctl is-enabled firewalld

[root@centos8 ~]# systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
   Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
   Active: active (running) since Mon 2019-10-28 15:05:45 CET; 1min 25s ago
     Docs: man:firewalld(1)
 Main PID: 772 (firewalld)
    Tasks: 2 (limit: 11525)
   Memory: 36.2M
   CGroup: /system.slice/firewalld.service
           └─772 /usr/libexec/platform-python -s /usr/sbin/firewalld --nofork --nopid

Oct 28 15:05:44 centos8 systemd[1]: Starting firewalld - dynamic firewall daemon...
Oct 28 15:05:45 centos8 systemd[1]: Started firewalld - dynamic firewall daemon.

# Examples:

# How to add a service to firewall
yum install tftp-server
firewall-cmd --add-service=tftp

# How to add and open port to firewall
## The command below will open the port effective immediately, but will not persist across reboots:
firewall-cmd --add-port=<YOUR PORT>/tcp
## The following command will create a persistent rule, but will not be put into effect immediately:
firewall-cmd --permanent --add-port=<YOUR PORT>/tcp

sriram@sriram-Inspiron-5567:~$ sudo fdisk -l /dev/sda
Disk /dev/sda: 111,8 GiB, 120034123776 bytes, 234441648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 833807FE-A7E1-46DA-B629-ECC1B32A087E

Device         Start       End   Sectors   Size Type
/dev/sda1       2048   1050623   1048576   512M EFI System
/dev/sda2    1050624 217874431 216823808 103,4G Linux filesystem
/dev/sda3  217874432 234440703  16566272   7,9G Linux swap

[root@10 ~]# ls -ld /tmp
drwxrwxrwt. 8 root root 216 Oct 27 11:18 /tmp

ifup eth0  (deprecated)
ifdown eth0 (deprecated)

[root@10 ~]# nmcli d
DEVICE  TYPE      STATE      CONNECTION
enp0s3  ethernet  connected  enp0s3
enp0s8  ethernet  connected  enp0s8
lo      loopback  unmanaged  --

nc -zv www.google.com 21 22 80 443 3000
nc: connect to www.google.com port 21 (tcp) failed: Connection timed out
nc: connect to www.google.com port 21 (tcp) failed: Connection timed out
nc: connect to www.google.com port 22 (tcp) failed: Connection timed out
nc: connect to www.google.com port 22 (tcp) failed: Connection timed out
Connection to www.google.com 80 port [tcp/http] succeeded!
Connection to www.google.com 443 port [tcp/https] succeeded!
nc: connect to www.google.com port 3000 (tcp) failed: Connection timed out

~$ nmap -sP 192.168.2.1/24
Starting Nmap 7.80 ( https://nmap.org ) at 2020-07-04 22:14 CEST
Nmap scan report for wn3000rpv3.home (192.168.2.1)
Host is up (0.098s latency).
Nmap scan report for 192.168.2.6 (192.168.2.6)
Host is up (0.098s latency).
Nmap scan report for 192.168.2.11 (192.168.2.11)
Host is up (0.011s latency).
Nmap scan report for sriram-inspiron-5567.home (192.168.2.13)
Host is up (0.00024s latency).
Nmap scan report for 192.168.2.40 (192.168.2.40)
Host is up (0.064s latency).
Nmap scan report for router.home (192.168.2.254)
Host is up (0.088s latency).
Nmap done: 256 IP addresses (6 hosts up) scanned in 11.17 seconds

dnf -h  
dnf --help

[root@192 ~]# dnf history
ID     | Command line             | Date and time    | Action(s)      | Altered
-------------------------------------------------------------------------------
     5 | install transmission-gtk | 2020-07-17 13:26 | Install        |    3   
     4 | install epel-release     | 2020-07-17 13:25 | Install        |    1   
     3 |                          | 2020-07-17 11:33 | Install        |    8   
     2 |                          | 2020-07-17 11:27 | Removed        |    1   
     1 |                          | 2020-07-17 11:15 | Install        | 1476 EE

[root@192 ~]# dnf repolist
repo id                             repo name
AppStream                           CentOS-8 - AppStream
BaseOS                              CentOS-8 - Base
epel                                Extra Packages for Enterprise Linux 8 - x86_64
epel-modular                        Extra Packages for Enterprise Linux Modular 8 - x86_64
extras                              CentOS-8 - Extras
google-chrome                       google-chrome

#search package details for the given string

[root@192 ~]# dnf search chrome
Last metadata expiration check: 0:31:24 ago on Fri 17 Jul 2020 01:25:55 PM CEST.
======================================== Name & Summary Matched: chrome ========================================
google-chrome-stable.x86_64 : Google Chrome
google-chrome-beta.x86_64 : Google Chrome (beta)
google-chrome-unstable.x86_64 : Google Chrome (unstable)
chromedriver.x86_64 : WebDriver for Google Chrome/Chromium
============================================= Name Matched: chrome =============================================
chrome-gnome-shell.x86_64 : Support for managing GNOME Shell Extensions through web browsers
mathjax-winchrome-fonts.noarch : Fonts used by MathJax to display math in the browser
=========================================== Summary Matched: chrome ============================================
webextension-token-signing.x86_64 : Chrome and Firefox extension for signing with your eID on the web

# clear all cached packages from the system
[root@192 ~]# dnf clean all
44 files removed

### YUM not working on Centos and gave below error
could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_64&repo=extras&infra=stock error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

### To Fix above issue : go to /etc/resolve.conf and add/update nameserver
nameserver 8.8.8.8

yum clean all
rm -f /var/lib/rpm/__db*
rpm --rebuilddb
yum update

# from baremetal
sriram@sriram-Inspiron-5567:~$ sudo fdisk -l /dev/sda
Disk /dev/sda: 111,8 GiB, 120034123776 bytes, 234441648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 833807FE-A7E1-46DA-B629-ECC1B32A087E

Device         Start       End   Sectors   Size Type
/dev/sda1       2048   1050623   1048576   512M EFI System
/dev/sda2    1050624 217874431 216823808 103,4G Linux filesystem
/dev/sda3  217874432 234440703  16566272   7,9G Linux swap

# From Virtual Machine
[root@CentosServer1910 ~]# fdisk -l
Disk /dev/sda: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xec47036c

Device     Boot   Start      End  Sectors Size Id Type
/dev/sda1  *       2048  2099199  2097152   1G 83 Linux
/dev/sda2       2099200 20971519 18872320   9G 8e Linux LVM

Disk /dev/mapper/cl_centosserver1910-root: 8 GiB, 8585740288 bytes, 16769024 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk /dev/mapper/cl_centosserver1910-swap: 1 GiB, 1073741824 bytes, 2097152 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

[sriram@CentosServer1910 ~]$ sudo blkid /dev/sda*
/dev/sda: PTUUID="ec47036c" PTTYPE="dos"
/dev/sda1: UUID="64b6f04f-d510-4c39-9a37-cacfeeec774b" TYPE="ext4" PARTUUID="ec47036c-01"
/dev/sda2: UUID="wi21xh-Sj3l-ocKe-Q6Qr-aRLv-n3Nm-hcYhL5" TYPE="LVM2_member" PARTUUID="ec47036c-02"

$ adduser --system -s usr/sbin/nologin subversion
# The -r flag will create a system user - one which does not have a password, a home dir and is unable to login.
# -s flag is for shell , /bin/nologin prevents to have a shell for this user.

## Testing
$ sudo adduser --system --no-create-home --shell /usr/sbin/nologin subversion
Adding system user `subversion' (UID 109) ...
Adding new user `subversion' (UID 109) with group `nogroup' ...
Not creating home directory `/home/subversion'.

$ sudo grep subversion /etc/passwd /etc/shadow
/etc/passwd:subversion:x:109:65534::/home/subversion:/usr/sbin/nologin
/etc/shadow:subversion:*:18628:0:99999:7:::

## Check if account is usuable
$ sudo -u subversion whoami
subversion

$ sudo -u subversion date
Fri 01 Jan 2021 07:35:20 PM CET

dd bs=4M if=<path to your image.iso> of=<path to your USB> status=progress

 yum groupinstall gnome-desktop x11 fonts  
 yum groupinstall "Server with GUI"  
 systemctl set-default graphical.target  
 systemctl start graphical.target  

awk '/keyword/ {print}'
cat /etc/passwd | awk -F: '/bin/ {print}'
cat /etc/passwd | awk -F: '/bin\/false/ {print}'
cat /etc/passwd | awk -F: '/usr\/sbin\/nologin/ {print $1}'

cat /etc/passwd | awk -F: '/bin/ {print}'

awk -F'[/=]' '{print $3 "\t" $5 "\t" $8}' filename

SYNTAX: -F"<separator1>|<separator2>|..."
awk -F"/|=" '{print $3 "\t" $5 "\t" $8}' filename

#!/bin/bash
# Check if arguments are given, $0 is the script name #
if [ $# -lt 3 ]
then
        echo "Missing Arguments"
        echo "Usage : $0 arg1 arg2 arg3"
        exit
fi

# $1 is the first argument, expecting a string
if [ -z $1 ]; then
   echo "You must specify a string in first argument"
   exit
fi

# using translate command 'tr', Translate any uppercase characters into lowercase #
test=$( echo "$1" | tr -s  '[:upper:]'  '[:lower:]' )

# Check if the given file exists where arg3 is the given filename to check #
if [ ! -f $3 ]
then
        echo "Filename given \"$3\" doesn't exist"
        exit
fi

For string comparision use,
==  
!=  
<  

For numeric comparision use,
-gt
-lt
-eq
-ne

#!/bin/bash
for variable in {list}
do
    <commands>
done

# Example
for i in {1..10}
do
    echo $i
done

# Bash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax
#!/bin/bash
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
  do
     echo "Count $i times"
 done

#output
Bash version 4.4.20(1)-release...
Count 0 times
Count 2 times
Count 4 times
Count 6 times
Count 8 times
Count 10 times

# C Style for loop
#!/bin/bash
for (( i=1; i<=5; i++ ))
do  
   echo "Welcome $i times"
done

# Infinite loop
#!/bin/bash
for (( ; ; ))
do
   echo "infinite loops [ hit CTRL+C to stop]"
done

BOOKS=('Title-1' \
     'Title-2' \
     'Title-3' \
     'Title-4')
for book in "${BOOKS[@]}"
do
  echo "Book: $book"
done

# Output
Book: Title-1
Book: Title-2
Book: Title-3
Book: Title-4

for i in {1..10}
do
  statements1      #Executed for all values of ''i'', up to a disaster-condition if any.
  statements2
  if (disaster-condition)
  then
     break            #Abandon the loop.
  fi
  statements3         #While good and, no disaster-condition.
done

# Example
# This shell script will go though all files stored in /etc directory.
# The for loop will be abandon when /etc/resolv.conf file found.

#!/bin/bash
for file in /etc/*
do
     if [ "${file}" == "/etc/resolv.conf" ]
     then
          countNameservers=$(grep -c nameserver /etc/resolv.conf)
          echo "Total  ${countNameservers} nameservers defined in ${file}"
          break
     fi
done

#!/bin/bash
while [[condition]]  
do
    <commands>
done

# Example
num=1
while [ $num -le 5 ]
do
   echo "$num"
   num=$((num+1))
done

for I in 1 2 3 4 5
do
  statements1      #Executed for all values of ''I'', up to a disaster-condition if any.
  statements2
  if (condition)
  then
     continue   #Go to next iteration of I in the loop and skip statements3
  fi
  statements3
done

# Example
# This script will make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.
#!/bin/bash
FILES="$@"
for f in $FILES
do
        # if .bak backup file exists, read next file
     if [ -f ${f}.bak ]
     then
          echo "Skiping $f file..."
          continue  # read next file and skip the cp command
     fi
     /bin/cp $f $f.bak
done