How to solve git merge conflict?

e.g. git merge conflict issue
source: private branch
target: develop branch

1st step:
$ git clone http://blabla --branch private

2nd step:
$ cd <working folder>

3rd step:
$ git checkout private
-> if not specified branch by default while cloning branch

4th step:
$ git merge develop

Error occurred in here
Auto-merging my_new_file.yml
CONFLICT (content): Merge conflict in my_new_file.yml
Auto-merging my_new_file.yml
Automatic merge failed; fix conflicts and then commit the result.

5th step:
-> make necessary changes on my_new_file.yml

6th step:
$ git status
-> check the file

7th step:
$ git branch
-> check the branch before commit

8th step:
$ git add --all

9th step:
$ git commit -m "blabla"

10th step:
$ git push

Ansible Commands Book

-> valid syntax of the playbook
$ ansible-playbook -i myhosts example.yml -v --syntax-check

-> show elapsed time of the playbook
$ time ansible-playbook -i myhosts example.yml -v

-> maintain output verbosity (-v, -vv, -vvv, -vvv)
$ time ansible-playbook -i myhosts example.yml -vvvv

-> list tags/tasks of the playbook
$ ansible-playbook -i myhosts example.yml --list-tags
$ ansible-playbook -i myhosts example.yml --list-tasks

-> run the playbook by getting vault password/sudo password from user
$ ansible-playbook -i myhosts example.yml --ask-vault-pass
$ ansible-playbook -i myhosts example.yml --ask-sudo-pass

-> run the playbook with sudo password 
$ ansible-playbook -i myhosts example.yml -e "ansible_sudo_pass=alper"
(-e, --extra-vars)

How to drop database/user on logical replicated database?

1st step
on 2nd node:
$ echo "DROP SUBSCRIPTION exampledb_node_2" | psql

on 1st node:
$ echo "DROP PUBLICATION exampledb_node_1" | psql

2nd step
on 1st node:
$ echo -e "DROP DATABASE exampledb \n" | psql
$ echo "DROP USER \"user-example\"" | psql

on 2nd node:
$ echo -e "DROP DATABASE exampledb \n" | psql
$ echo "DROP USER \"user-example\"" | psql

Kubernetes label usage

e.g. namespace; kube-system

-> List labels

# kubectl get pods --all-namespaces --show-labels

# kubectl get pods -n kube-system --show-labels

-> List pods via labels
# kubectl get pods -n kube-system -l tier=control-plane

-> Describe pod/pods via labels
# kubectl describe pod -n kube-system -l component=kube-scheduler
# kubectl describe pod -n kube-system -l tier=control-plane

-> Get logs via labels

# kubectl logs -n kube-system -l component=kube-scheduler

# kubectl logs -n kube-system -l tier=control-plane

-> Delete pods via labels

# kubectl delete pods -n kube-system -l component=example

# kubectl delete pods -n kube-system -l 'component in (example,example2)'

# kubectl delete pods -n kube-system -l 'component notin (example3, example4)'

Examples of "escape sequences" usage on echo command

To use interpretation of backslash escapes, just need to put "-e" parameters afterward echo command.

Info: "-e" means enable escape sequences

Examples of backslash escapes:
New line;
$ echo -e "Hello World\n"
-> Hello World

Form feed;
$ echo -e "Hello World\fWhat a lovely day\fI like this\f"
-> Hello World
            What a lovely day
                              I like this

Carriage return;
$ echo -e "Hello World\rWorld"
-> World World

Horizontal tab;
$ echo -e "Hello World\tWhat a lovely day"
-> Hello World     What a lovely day

Vertical tab;
$ echo -e "Hello World\vWhat a lovely day"
-> Hello World
           What a lovely day

Null;
$ echo -e "Hello World\0 What a lovely day"
-> Hello World What a lovely day

Clear after all;
$ echo -e "Hello World\c What a lovely day"
-> Hello World

Escape character;

$ echo -e "Go to \\home directory"

-> Go to \home directory

$ echo -e "Go to \"home\" directory"

-> Go to "home" directory

Unicode usage;
$ echo -e "This is turkish lira symbol-> \u20ba"
-> This is turkish lira symbol-> ₺

Linux Terminal Commands Book

* chage (manage user password settings)
* hostnamectl (show hostname settings)
* date (manage system date and time)
* rpm (means RedHat Package Manager)
* timedatectl (show timezone settings)
* tcpdump (analyze network packages)
* tar (compress the files for archiving)
* tree (see the files as tree format)
* wc (means word count)
* yum (manage RPM packages on RedHat distro)

How to fix "connection refused" error?

Error
e.g.
The connection to the server 10.10.10.10:6443 was refused - did you specify the right host or port?

Solution
On kubernetes control nodes, try to "stop/start" docker service
# service docker restart
or
# systemctl restart docker

What are the differences of su/ su -/ sudo / sudo su in linux system?

Info: 
su means, switch user or substitute user
sudo means, super user do
to use sudo command, related user must be defined in sudoers file

$ su -
(switch user to root then executes all /etc/profile, .profile and .bashrc files by root)

$ su
(switch user to root user then executes only .bashrc file by root)

$ sudo su -
(switch user to root then executes all /etc/profile, .profile and .bashrc files by root if current user defined in sudoers file)

$ sudo su
(switch user to root then executes only .bashrc file by root if current user defined in sudoers file)

Ref: https://ozsoyler.blogspot.com/2016/09/how-to-gain-root-access-without_15.html

How to change timezone?

-> Determine your timezone
# cd /usr/share/zoneinfo
# tzselect
e.g.
Europe/Istanbul

-> Remove current localtime soft link
# rm /etc/localtime 

-> Define your localtime soft link as new
# ln -s /usr/share/zoneinfo/Europe/Istanbul /etc/localtime

-> Check your latest timezone setting
# timedatectl

How to connect into local VM via SSH?

* Set a new Port Forwarding Rule in Network Settings

Step 1: Settings -> Network -> Adapter 1 -> Advanced -> Adapter Type -> "Paravirtualized network adapter (virtio-net)" -> OK

Step 2: Settings -> Network -> Adapter 1 -> Advanced -> Keep it as "NAT" -> Click "Port Forwarding" -> Define a Rule -> OK
e.g.
Name Protocol Host Port Guest Port
SSH TCP      1022 22

SSH Connection Host/Port Settings:
Host: 127.0.0.1
Port: 1022

WSREP "Failed to open channel" solution

Error
ERROR WSREP "Failed to open channel 'my_mariadb_cluster'"

Detailed Error
WSREP: gcs/src/gcs.cpp:gcs_open():1458: Failed to open channel 'my_mariadb_cluster' at 'gcomm://10.10.10.10,10.10.10.11': -110 (Connection timed out)

Solution
On the first node: (e.g. node-1)
-> Stop mariadb service if it hangs
# systemctl stop mariadb.service

-> Make sure "safe_to_bootstrap" value is "1"
# cat /var/lib/mysql/grastate.dat
safe_to_bootstrap: 1

-> Run following command for starting cluster as new
# galera_new_cluster

-> mariadb service should be started automatically
# systemctl status mariadb.service

-> note wsrep_last_committed number by looking system table
# mysql -u root -p
> show status like 'wsrep_%';

-> note uuid number
# cat /var/lib/mysql/grastate.dat

On the second node: (e.g. node-2)
-> Stop mariadb service if it hangs
# systemctl stop mariadb.service

-> uuid and seqno (wsrep_last_committed) should be same with node-1
# vi /var/lib/mysql/grastate.dat
uuid: 0000
seqno: 100

-> start mariadb service
# systemctl start mariadb.service

How to find/follow postgresql server logs?

-> Check log level from config file
# cat /var/lib/pgsql/data/postgresql.conf
e.g.
log_min_messages= 'ERROR' (default value)
(options; DEBUG5, DEBUG4, DEBUG3, DEBUG2, DEBUG1, INFO, NOTICE, WARNING, ERROR, LOG, FATAL, and PANIC)

-> Determine log directory name (via postgres user)
$ psql
postgres=# show log_directory;
e.g. pg_log

-> Determine data directory name (via postgres user)
$ psql
postgres=# show data_directory;
e.g. /var/lib/pgsql/data

-> Determine log file name (via postgres user)
$ psql
postgres=# show log_filename;
e.g. postgresql-%a.log (%a will be "day" of the week)
e.g. postgresql-%a-%H.log (%H will be "hour" of the day)

-> Check log rotation age of the log files
postgres=# show log_rotation_age;
e.g. 1d (1d means one day)
* The database system generate log files daily
e.g. 1h (1h means one hour)
* The database system generate log files hourly

Eventually, the log path could be like this;
e.g. 
# ls /var/lib/pgsql/data/pg_log/postgresql-Mon.log
# ls /var/lib/pgsql/data/pg_log/postgresql-Mon-07.log

-> To follow the logs by tail
e.g.
# tail -100f /var/lib/pgsql/data/pg_log/postgresql-Mon.log
# tail -100f /var/lib/pgsql/data/pg_log/postgresql-Mon-07.log

Ref: https://www.postgresql.org/docs/9.5/static/runtime-config-logging.html

How to setup/configure/use mailx for Office365 account?

-> Update the OS
# yum -y update
or (y -> yes)
# yum update

-> Install mailx
# yum install -y mailx

-> Define soft link to use "email" the command
# ln -s /bin/mailx /bin/email

-> Prepare config file
# vi /etc/mail.rc
set smtp=outlook.office365.com
set smtp-auth-user=alper@alper.com
set smtp-auth-password=alper123
set ssl-verify=ignore
set nss-config-dir=/etc/pki/nssdb/
set smtp-use-starttls
set from="alper@alper.com(Alper Ozsoyler)"

-> Try to send an email to yourself
# echo "Your message is my message!" | email -v -s "Message Subject is Bla Bla" alper@alper.com

Ref: https://gist.github.com/ilkereroglu/aa6c868153d1c5d57cd8

How to solve "restarting/pending pod repeatedly" problem?

-> Check the pods status
e.g.
# kubectl get pods --all-namespaces
NAMESPACE     NAME                            READY     STATUS
kube-system   coredns-78fcdf6894-6ggkk        0/1       Pending
kube-system   coredns-78fcdf6894-tm52r        0/1       Pending 

-> Check the deployments
# kubectl get deployment --all-namespaces
NAMESPACE     NAME      DESIRED   CURRENT   UP-TO-DATE   AVAILABLE
kube-system   coredns   2         2         2            0

-> Needs to remove the deployments related to pod and service
e.g.
# kubectl delete deployment coredns -n kube-system
deployment.extensions "coredns" deleted

How to enable Hyper-V in Windows 10?

Press Windows key -> Type "Turn Windows features on or off" then click it -> Select "Hyper-V" check box then click "OK" -> Restart PC -> Done!

How to install/configure/enable NTP service?

-> Install ntp service
# yum install ntp

-> Once NTP server pool zones are taking
e.g. check from https://www.pool.ntp.org/zone/europe

-> Add specific pool zones into config file if they are not existed
# vi /etc/ntp.conf
server 0.europe.pool.ntp.org

-> Enable service
# systemctl enable ntpd

-> Run service
# systemctl start ntpd 

-> Lastly, check the service and the settings

# timedatectl 

How to check linux operating system info?

-> show kernel version and other info
# uname -a

-> show machine info
# hostnamectl

-> show specific release version
# rpm --query centos-release 

How to check available yum packages?

e.g. install/update ansible by yum

-> Go to repo path
# cd /etc/yum.repos.d/

-> Add source paths into repo file to be able to install ansible
# vi epel.repo
[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-7&arch=$basearch
failovermethod=priority
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
#baseurl=http://download.fedoraproject.org/pub/epel/7/$basearch/debug
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-debug-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
#baseurl=http://download.fedoraproject.org/pub/epel/7/SRPMS
metalink=https://mirrors.fedoraproject.org/metalink?repo=epel-source-7&arch=$basearch
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=1

-> Search yum package
# yum search ansible

-> Check available yum package
# yum list available | grep ansible

-> Check available all yum packages with its duplicates
# yum list available --showduplicates | grep ansible

-> Install ansible 
# yum install ansible

-> Download only yum package
# yum install ansible --downloadonly --downloaddir=/root

-> Check available yum packages for upgrade
# yum check-updates | grep ansible

-> Update ansible
# yum update ansible

-> Check yum history
# yum history list

How to install/configure/run ansible?

-> Set hostname
# hostnamectl set-hostname ansible_host

-> Add remote hosts into /etc/hosts file
# vi /etc/hosts
10.10.10.2 remote_host_2
10.10.10.3 remote_host_3

-> Generate ssh key without passphase
# ssh-keygen

-> Install ssh key into remote servers
# ssh-copy-id remote_host_2
# ssh-copy-id remote_host_3

-> Install ansible rpm packages
# yum install ansible

-> Add remote hosts as nodes into /etc/ansible/hosts file
# vi /etc/ansible/hosts
[remote_hosts]
remote_host_2
remote_host_3

-> Prepare an ansible playbook in /etc/ansible/
# vi my_very_first_playbook.yml
# example playbook
- name: Run the role for me 
  hosts: remote_hosts
  roles:
  - example-role

-> Prepare a role for the playbook in /etc/ansible/roles/example-role/
# vi main.yml
- name: checking Command Scheduler service
  service: 
   name=crond.service
   state=started

-> Run the playbook in /etc/ansible/
# ansible-playbook my_very_first_playbook.yml
or
# ansible-playbook -i hosts my_very_first_playbook.yml
(i -> inventory, its default location is /etc/ansible/hosts)

-> To check the ansible logs
# tail -f /var/log/ansible.log

usage of "run_once: true" option

If you want to run only once a specific task during all playbook execution, you may add "run_once: true" option into your task to do this.

e.g. example task
- name: prompt for directory name
  run_once: true
  pause:
   prompt: "Please enter directory name: "
  register: input_dir_name

- name: show input
  debug: 
   msg="{{ input_dir_name.user_input }}"