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 }}"

How to know last login info on the system?

-> show all records
$ last

-> show all records page by page
$ last | more

-> show first 10 records
$ last | head

-> show first 100 records
$ last | head -100

-> show last 10 records
$ last | tail -10

-> show last 100 records
$ last | tail -100

How to change service name?

e.g.
current service name: postgresql.service
desired service name: postgresql-9.1.service

Run following commands to change service name;

Step 1: Stop current service
# systemctl stop postgresql.service
# systemctl daemon-reload

Step 2: Change service name
# cd /usr/lib/systemd/system
# mv postgresql.service postgresql-9.1.service

Step 3: Start and enable new service
# systemctl start postgresql-9.1.service
# systemctl enable postgresql-9.1.service

Step 4: Check service status:
# systemctl status postgresql-9.1.service

Step 5: Make sure that service runs properly
# journalctl -u postgresql-9.1.service

How to solve remote connection error cause of iptables/firewalld service?

Case
Machine A(192.168.1.2) wants to connect on Machine B(192.168.1.5) by database port.

Action
In Machine A;
# su - postgres
$ psql -h 192.168.1.5 -d alper -U alper -p 5433

Error
psql: could not connect to server: No route to host
        Is the server running on host "192.168.1.5" and accepting
        TCP/IP connections on port 5433?

Solution
In Machine B;
For iptables service:
# iptables -I INPUT -p tcp --dport 5433 -j ACCEPT
# iptables-save

For firewalld service:
# firewall-cmd --zone=public --add-port=5433/tcp --permanent

# firewall-cmd --reload

Check
In Machine A;

# nc -vz 192.168.1.5 5433

Ncat: Connected to 192.168.1.5:5433

How to solve database module error?

Action
# ansible-playbook -i myhosts example.yml -v

Error
TASK [example role : example task] *******************
fatal: [local]: FAILED! => {"changed": false, "msg": "The MySQL-python module is required."} 

Solution
# yum install MySQL-python

The other solutions related to database tasks:
# yum install python-redis (Redisdb)
# yum install python-psycopg2 (PostgreSQL)

Check it for up to date list of database modules:
https://docs.ansible.com/ansible/latest/modules/list_of_database_modules.html

How to prevent sleep mode on laptop when lid closes?

Press Window Key then type "Power Options" and click on it -> Click "Choose what closing the lid does" -> Set "When I close the lid" to "Do nothing" -> Click "Save changes" -> Done!

How to run a SQL query/statement in a task?

You are able to run a SQL query/statement with "echo" command by shell module for the ansible task.

# an example task for sql query/statement
- name: set default schema for user
  shell: echo "ALTER ROLE alper SET SEARCH_PATH TO myschema" | psql
  become: yes
  become_user: postgres

benefit of "gather_facts: no" setting

If you do not need any facts for your variables, you can run your playbook more fast via gather_facts setting.

For this, just add following setting in your playbook:

# example playbook
- name: Run the role for me 
  hosts: my_nodes
  gather_facts: no
  roles:
  - example-role

"database "example" is being accessed by other users" solution

Action
postgres=# drop database example;

Error
ERROR:  database "example" is being accessed by other users
DETAIL:  There is 1 other session using the database.

Solution
Run following sql code;

postgres=# 
SELECT pg_terminate_backend(pg_stat_activity.pid) FROM pg_stat_activity WHERE pg_stat_activity.datname = 'example' AND pid <> pg_backend_pid();

All sessions should be terminated where belong to "example" database except backend sessions.

Lastly, try to drop it again.

How to backup database in XE?

It is very simple to backup your database in Oracle XE with already prepared .bat file quickly.

Steps;
Press Windows Key -> Type "Backup Database" -> Click "Backup Database" link -> .bat file "Backup.bat" runs -> Follow the steps -> Done

Finally, check the backup path;
...\oracle\fast_recovery_area\XE

e.g. truncate table cascade

Preparation of Test Data
test=# 
create table employees (id integer, name varchar, department_id integer);

test=# 
create table departments (id int, department_name varchar);

test=# 
ALTER TABLE DEPARTMENTS
ADD CONSTRAINT DEPT_PK 
PRIMARY KEY (ID);

test=# 
ALTER TABLE EMPLOYEES
ADD CONSTRAINT EMP_DEPT_FK 
FOREIGN KEY (DEPARTMENT_ID)
REFERENCES DEPARTMENTS(ID);

test=# 
insert into departments
values (1,'IT');

test=# 
insert into departments
values (2,'Sales');

test=# 
insert into departments
values (3,'Marketing');

test=# 
insert into employees
values (1,'John',1);

test=# 
insert into employees
values (2,'Alex',1);

test=# 
insert into employees
values (3,'Guru',2);

test=# 
insert into employees
values (4,'Maximus',3);

test=# 
commit;

Data Check
-- select all rows of parent table 
test=# 
select * from departments;

-- select all rows of child table
test=# 
select * from employees;

Action
-- by using truncate table cascade, parent table and child table data will be removed. 
test=# 
truncate table departments cascade;

Last Data Check
-- select all rows of parent table 
test=# 
select * from departments;

-- select all rows of child table
test=# 
select * from employees;

Info: In Oracle 12c, you are able to use "cascade" clause with "truncate table" statement as well.

e.g. fetching JSON data

Preparation of Test Data
test=# 
create table T_JSON_DATA (id INTEGER, body JSON);

test=#
insert into T_JSON_DATA
values (1, '{ "name":"John", "age":30, "city":"New York" }');

test=#
insert into T_JSON_DATA
values (2, '{ "name":"Alex", "age":25, "city":"China" }');

test=#
insert into T_JSON_DATA
values (3, '{ "name":"Guru", "age":21, "city":"Russia" }');

test=#
insert into T_JSON_DATA
values (4, '{ "name":"Maximus", "age":25, "city":"Somewhere" }');

test=# commit;

Examples for populating test data
test=#
select * from T_JSON_DATA;

-- to retrieve specific field data
test=#
select body->>'name' from T_JSON_DATA;

-- with aliases
test=#
select body->'name' AS NAME, body->'city' AS CITY from T_JSON_DATA;

-- use with where clause
test=#
select body->'name' AS NAME, body->'city' AS CITY from T_JSON_DATA where id = 1;

ORA-02292 solution

Preparation
-- Insert test data into DEPARTMENT and EMPLOYEES tables.
SQL>
INSERT INTO DEPARTMENTS
VALUES (280,'New Department',100,1700);

SQL>
INSERT INTO EMPLOYEES
VALUES (207,'John','Mathew','JMATHEW','111.111.111',TO_DATE('25.04.2018','DD.MM.YYYY'),'IT_PROG',10000,0.5,100,280);

SQL>
INSERT INTO EMPLOYEES
VALUES (208,'Jim','Parker','JPARKER','111.111.111',TO_DATE('25.05.2018','DD.MM.YYYY'),'IT_PROG',20000,0.6,100,280);

SQL>
INSERT INTO EMPLOYEES
VALUES (209,'Sophia','Ran','SRAN','111.111.111',TO_DATE('25.06.2018','DD.MM.YYYY'),'IT_PROG',30000,0.7,100,280);

SQL>
INSERT INTO EMPLOYEES
VALUES (210,'Wendi','Blake','WBLAKE','111.111.111',TO_DATE('25.07.2018','DD.MM.YYYY'),'IT_PROG',40000,0.8,100,280);

SQL>
COMMIT;

Action
SQL>
DELETE FROM DEPARTMENTS
WHERE DEPARTMENT_ID = 280;

Error
ORA-02292: integrity constraint (HR.EMP_DEPT_FK) violated - child record found

Solution
-- Find referencial integrity constraint between DEPARTMENT and EMPLOYEES tables for this error.
SQL>
SELECT * 
FROM ALL_CONSTRAINTS 
WHERE TABLE_NAME IN ('EMPLOYEES', 'DEPARTMENTS') AND CONSTRAINT_TYPE = 'R';

-- Drop constraint
SQL>
ALTER TABLE EMPLOYEES DROP CONSTRAINT EMP_DEPT_FK;

-- Re-define constraint with "ON DELETE CASCADE" option
SQL>
ALTER TABLE EMPLOYEES
ADD CONSTRAINT EMP_DEPT_FK 
FOREIGN KEY (DEPARTMENT_ID)
REFERENCES DEPARTMENTS(DEPARTMENT_ID)
ON DELETE CASCADE;

-- Try again
SQL>
DELETE FROM DEPARTMENTS
WHERE DEPARTMENT_ID = 280;

SQL>
COMMIT;

e.g. count and distinct aggregate functions

examples; count (1) - count(*) - count(column_name) - count(distinct (column_name))

e.g. count aggregate function
SQL>
SELECT
    COUNT(*)
FROM
    employees;

  COUNT(*)
----------
       107

SQL>
SELECT
    COUNT(1)
FROM
    employees;

  COUNT(*)
----------
       107

SQL>
SELECT
    COUNT(commission_pct)
FROM
    employees;

COUNT(COMMISSION_PCT)
---------------------
                   35

Info: COUNT(1) and COUNT(*) calculate all rows even all columns data are NULL. However, COUNT(COLUMN_NAME) ignores NULL data.

e.g. distinct aggregate function
SQL>
SELECT
    distinct(commission_pct)
FROM
    employees;

COMMISSION_PCT
--------------
          NULL
           ,15
           ,35
            ,4
            ,3
            ,2
           ,25
            ,1

8 rows selected.

Info: DISTINCT(COLUMN_NAME) function uses not to see duplicated rows and also it shows NULL column if it exists.

e.g. both of count and distinct aggregate functions
SQL>
SELECT
    COUNT(DISTINCT(commıssıon_pct) )
FROM
    employees;

COUNT(DISTINCT(COMMISSION_PCT))
-------------------------------
                              7

Info: count(column_name) ignores NULL values, even if distinct function is used.

ORA-02449 solution

Preparation
SQL>
CREATE TABLESPACE EXAMPLE_TS DATAFILE
  'C:\ORACLEXE_XE\APP\ORACLE\ORADATA\XE\example_01.dbf' SIZE 512K REUSE
LOGGING
ONLINE
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;

SQL>
CREATE TABLESPACE EXAMPLE_TS_2 DATAFILE
  'C:\ORACLEXE_XE\APP\ORACLE\ORADATA\XE\example_02.dbf' SIZE 512K REUSE
LOGGING
ONLINE
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;

SQL>
CREATE TABLE T_EXAMPLE (ID NUMBER)
TABLESPACE EXAMPLE_TS;

SQL>
CREATE TABLE T_EXAMPLE_CHILD (ID NUMBER, FID NUMBER)
TABLESPACE EXAMPLE_TS_2;

SQL>
ALTER TABLE T_EXAMPLE
ADD CONSTRAINT ID_EXAMPLE_PK
PRIMARY KEY (ID);

SQL>
ALTER TABLE T_EXAMPLE_CHILD
ADD CONSTRAINT FID_EXAMPLE_FK
FOREIGN KEY (FID)
REFERENCES T_EXAMPLE (ID);

Action
SQL>
DROP TABLESPACE EXAMPLE_TS;

Error
ORA-01549: tablespace not empty, use INCLUDING CONTENTS option

Next Action
SQL>
DROP TABLESPACE EXAMPLE_TS INCLUDING CONTENTS AND DATAFILES;

Next Error
SQL>
ORA-02449: unique/primary keys in table referenced by foreign keys

Solid Solution
SQL>
DROP TABLESPACE EXAMPLE_TS INCLUDING CONTENTS AND DATAFILES CASCADE CONSTRAINTS;

ORA-01549 solution

Preparation
SQL>
CREATE TABLESPACE EXAMPLE_TS DATAFILE
  'C:\ORACLE_XE\APP\ORACLE\ORADATA\XE\example_01.dbf' SIZE 512K REUSE
LOGGING
ONLINE
EXTENT MANAGEMENT LOCAL AUTOALLOCATE
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;

SQL>
CREATE TABLE T_EXAMPLE (ID NUMBER)
TABLESPACE EXAMPLE_TS;

Action
SQL>
DROP TABLESPACE EXAMPLE_TS;

Error
ORA-01549: tablespace not empty, use INCLUDING CONTENTS option

Solution
SQL>
DROP TABLESPACE EXAMPLE_TS INCLUDING CONTENTS AND DATAFILES;

e.g. pivot clause

Preparation 
* Define column names for pivot table
SQL>
SELECT LISTAGG (
          '''' || JOB_ID || '''' || ' AS ' || '"' || JOB_TITLE || '"' || ', ')
       WITHIN GROUP (ORDER BY JOB_TITLE)
  FROM JOBS;

Pivot example
* Show that how many jobs are set to persons according to department names.
SQL>    
SELECT *
    FROM (SELECT JOB_ID, DEPARTMENT_NAME
            FROM EMPLOYEES A, DEPARTMENTS B
           WHERE A.DEPARTMENT_ID = B.DEPARTMENT_ID) PIVOT (
COUNT (JOB_ID)
            FOR JOB_ID
            IN  ('AD_VP' AS "ADMIN",
                'FI_ACCOUNT' AS "Accountant",
                'AC_MGR' AS "Accounting Manager",
                'AD_ASST' AS "Administration Assistant",
                'FI_MGR' AS "Finance Manager",
                'HR_REP' AS "Human Resources Representative",
                'MK_MAN' AS "Marketing Manager",
                'MK_REP' AS "Marketing Representative",
                'AD_PRES' AS "Presidentt",
                'IT_PROG' AS "Programmer",
                'AC_ACCOUNT' AS "Public Accountant",
                'PR_REP' AS "Public Relations Represent.",
                'PU_CLERK' AS "Purchasing Clerk",
                'PU_MAN' AS "Purchasing Manager",
                'SA_MAN' AS "Sales Manager",
                'SA_REP' AS "Sales Representative",
                'SH_CLERK' AS "Shipping Clerk",
                'ST_CLERK' AS "Stock Clerk",
                'ST_MAN' AS "Stock Manager"))
ORDER BY DEPARTMENT_NAME DESC;