el vs rhel

el (enterprise linux) e.g. centos, rocky linux, alma linux

rhel (red hat enterprise linux)

The main difference is that Red Hat provides technical support to the company/project if needs on RHEL. Also, there are various subscription options for being able to use RHEL for the companies at a cost.

A basic git exercise

e.g. an example with basic git commands 
source: develop branch 
target: private branch (not exist on the git local/remote repository at the begin)

1st step:
$ git clone http://blabla --branch develop
-- cloning develop branch data on the local machine

2nd step:
$ git branch private
-- defining a new branch as "private" on the local repository

3rd step:
$ git checkout private
-- switching the active/using branch to "private" on the local repository

4th step:
make your changes on the files/folders

5th step:
$ git status
-- see the summary that what you changed on the local repository

6th step:
$ git diff
-- see the differences between local repo and remote repo

7th step:
$ git add --all
-- add all modified files from (local) working directory to git staging area

8th step:
$ git commit -m "my latest changes applied on the code"
-- save your changes to the local repository to be ready for git push

9th step:
$ git push --set-upstream origin private
-- push all changes to the (remote) git repository by defining a new remote branch as the same local branch name; origin private

That's all!

Difficulty level: Low

How to resolve "[Errno 13] Permission denied"?

Action

$ ansible-playbook -i /etc/ansible/hosts /etc/ansible/example.yml -v

Error
ERROR! Unexpected Exception, this is probably a bug: [Errno 13] Permission denied

Analysis

ansible user does not have the owner permissions on "/dev/shm"

Possible Reason

Running "umount -a" or "umount /dev/shm"

Solution
$ sudo chown $USER /dev/shm

unzip vs gunzip

e.g. using unzip and gunzip tools for extracting the zip files

* example.txt file compressed into example.zip file

-> Listing contents of the zip file
$ unzip -l example.zip
$ gunzip -l example.zip

-> Extracting the zip file
$ unzip example.zip
$ gunzip -S .zip -c example.zip > example.txt

-> Extracting the zip file to specified directory
$ unzip example.zip -d /home/alper
$ gunzip -S .zip -c example.zip > /home/alper/example.txt

How to reset mariadb root password on using Galera Cluster system?

e.g. reset root password
two machines in a cluster

1st step:
-> Activate "safe_to_bootstrap" parameter on first machine
# vi /var/lib/mysql/grastate.dat
safe_to_bootstrap: 1

2nd step on first/second machine:
# systemctl stop mariadb

3rd step:
-> Run database in safe mode on first machine
# /usr/bin/mysqld_safe --skip-grant-tables --wsrep-new-cluster "${@:2}" > /dev/null 2>&1 & 

4th step:
-> Note the process id of mysql service
# ps -ef | grep mysql

5th step:
-> Connect mysql terminal then run following commands on first machine
# mysql -u root
 use mysql;
 update user SET PASSWORD=PASSWORD("password") WHERE USER='root';
 flush privileges;
 exit

6th step:
-> Try to connect with new password on first machine
# mysql -u root -ppassword

7th step:
-> kill the odd service on first machine
# kill -9 1111

8th step:
-> Start new cluster on first machine
# galera_new_cluster

9th step:
-> Start mariadb on second machine
$ systemctl start mariadb

Done...

Ref:
https://robbinespu.github.io/eng/2018/03/29/Reset_mariadb_root_password.html
https://blog.toonormal.com/2015/11/23/mariadb-cluster-install-notes/

e.g. preparing multiple virtual machines in local network

Hostnames: VM node-A, VM node-B, VM node-C

Adapter 1: Host-only Adapter (VirtualBox-Host-Only Ethernet Adapter)
Type -> Paravirtualized network adapter (virtio-net) - Deny Mode

Adapter 2: Bridged Adapter (Intel Ethernet Connection)
Type -> Paravirtualized network adapter (virtio-net) - Deny Mode

Test
Try to connect one of VMs such as "node-A" by SSH

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