Tech Chorus


written by Sudheer S on 2016-10-02

Rakesh: Golix, what's lucky for you?

Golix: Lucky, I learned probability.

PostgreSQL Cheatsheet

written by Sudheer Satyanarayana on 2016-07-25

Install PostgreSQL Server

Fedora 30:

sudo dnf install postgresql-server

CentOS 7:

sudo yum install postgresql-server

Ubuntu 18.04:

sudo apt install postgresql

New Server Initialization

On CentOS 7/Fedora 30:

sudo postgresql-setup initdb

Upgrading From An Older Version

sudo postgresql-setup --upgrade

Administering The Database Server

Managing The postgresql Daemon

Starting PostgreSQL server

sudo systemctl start postgresql

Checking PostgreSQL Server Status:

sudo systemctl status postgresql

Enabling PostgreSQL Server Systemd Unit/Enabling PostgreSQL Server On Boot:

sudo systemctl enable postgresql

Allowing Password Based Login From localhost

Edit /var/lib/pgsql/data/pg_hba.conf as privileged user(root) and add this line:

host    all     all    md5

Allowing Password Based Login From A Particular Network

Edit /var/lib/pgsql/data/pg_hba.conf as privileged user(root) and add this line:

host    all             all             md5

In this example password based login will be allowed from the network

After making changes to the pg_hba.conf file, restart PostgreSQL server.

Restarting The PostgreSQL server:

sudo systemctl restart postgresql

Listening On A Particular IP Address:

Edit the file /var/lib/pgsql/data/postgresql.conf and add:

listen_addresses = ''

Save the file and restart PostgreSQL server. This configuration will allow PostgreSQL to listen on the IP address

Listening On All IP addresses:

Edit the file /var/lib/pgsql/data/postgresql.conf and add:

listen_addresses = '*'

Save the file and restart PostgreSQL server.

Becoming postgres :

The root user of postgresql is postgres.

sudo su - postgres

Launching The PostgreSQL Console:


From the PostgreSQL console, you can perform administrative tasks such as creating and deleting users, databases, etc.

Creating A PostgreSQL User

 CREATE USER username login PASSWORD 'password';

Creating A PostgreSQL Database

 CREATE DATABASE databasename WITH OWNER = username;

Listing Databases

Launch the PostgreSQL console:

sudo su - postgres

From the PostgreSQL console, execute \d


Listing Tables Of A Database

From the PostgreSQL console:


Replace DATABASE_NAME with the actual database name.

Backing Up A Database:


For example:

pg_dump -Uusername -h databasename > my-pg-databasename-backup.sql

Restoring A Database From A Dump File

Copy the file to the host where the dump has to be restored.

sudo su - postgres
psql databasename < /path/to/dump/file

Generating The CREATE TABLE Statement For A Particular Table

pg_dump -U username -h dbname -t tablename --schema-only

Using Sass Version Of Bootstrap With Webpack

written by Sudheer Satyanarayana on 2016-07-21

You must have gone through the webpack tutorial to understand this blog post. If you have done that, using Sass version of Bootstrap is a breeze.

In your project directory install these packages using npm.

npm install bootstrap-loader css-loader node-sass resolve-url-loader sass-loader style-loader url-loader file-loader jquery imports-loader bootstrap-sass --save-dev

Create the file webpack.config.js

'use strict';

var path = require('path');

module.exports = {
    entry: ['bootstrap-loader', './index.js'],
    output: {
        path: 'output',
        filename: 'bundle.bootstrap-sass.js'
    devtool: 'inline-source-map',
    module: {
        loaders: [
                test: /bootstrap-sass\/assets\/javascripts\//, 
                loader: 'imports?jQuery=jquery'
                test: /\.woff2?$|\.ttf$|\.eot$|\.svg$/,
                loader: 'url'
                test: /\.scss$/,
                loader: 'style-loader!css-loader!'
        resolve: {
            extensions: ['', '.js', '.jsx', '.css', '.scss'],
            modulesDirectories: [

index.js: your project entry file.

/* Put whatever you want in your entry JS file */

index.html: Minimalist HTML file to check whether Bootstrap styles are loaded.

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <title>Minimalistic webpage</title>

    <script src="output/bundle.bootstrap-sass.js"></script>

  <body role="document">
     <button type="button" class="btn btn-primary" aria-label="Left Align"><span class="glyphicon glyphicon-align-left" aria-hidden="true"></span>Some text</button>

Generate the build


Open index.html in your browser and you should see the web page with Bootstrap styles loaded.

Firewalld Cheatsheet

written by Sudheer Satyanarayana on 2016-07-12

List supported zones

firewall-cmd --get-zones

List active zones

firewall-cmd --get-active-zones

List zones with enabled features

firewall-cmd --list-all-zones

Permanently add source IP address to a zone

firewall-cmd --permanent --zone=<zone_name> --add-source=<ip_address>

Permanently remove source IP address from a zone

firewall-cmd --permanent --zone=<zone_name> --remove-source=<ip_address>

Reload firewall

firewall-cmd --reload

Permanently add a service to zone

firewall-cmd --permanent --zone=<zone_name> --add-service=<service_name>

Permanently add an interface to a zone

firewall-cmd --permanent --zone=<zone_name> --add-interface=<interface_name>

Permanently remove an interface from a zone

firewall-cmd --permanent --zone=<zone_name> --remove-interface=<interface_name>

Adding Custom Service

Copy from a template.

cp /usr/lib/firewalld/services/https.xml /etc/firewalld/services/myservice.xml

and edit the values. If you have an interface alias, this method might suit your needs:


How To Interchange Values And Keys Of A Dictionary In Python?

written by Sudheer Satyanarayana on 2015-12-09

Here's our sample dictionary:

my_dict = {'game': 'cricket', 'planet': 'earth'} The values() method of the dictionary returns a list of values.

>>> my_dict.values()
['earth', 'cricket']

The keys() method of the dictionary returns a list of keys.

>>> my_dict.keys()
['planet', 'game']

Notice the 1-1 correspondence between the items of the returned lists. This correspondence remains unchanged if the dictionary is unchanged.

To interchange the keys and values all you have to do is zip and generate another dictionary. Let's complete the remaining steps.

zip(my_dict.values(), my_dict.keys())
[('earth', 'planet'), ('cricket', 'game')]

Now, we have a list of tuples. Each tuple contains the value and key pairs. The last step is to create a dictionary using this list. Pass the above list to the built-in function dict().

>>> dict(zip(my_dict.values(), my_dict.keys()))
{'earth': 'planet', 'cricket': 'game'}

Here's a one-liner to summarize the concept.

dict(zip(my_dict.values(), my_dict.keys()))

Github For Mercurial User

written by Sudheer Satyanarayana on 2015-03-27

You are a Mercurial user and have little experience with Github. You are required to publish your Mercurial repository on Github. The good news is, you can do it without parting with Mercurial. There are bridges between Git and Mercurial. The one we are going to talk about today, is hg-git.

Let's start demonstrating our solution.

Step 1: Create a mercurial repository somewhere

I do it on Bitbucket. We'll call this our canonical repository. Clone the repository.

hg clone ssh://

Step 2:Add some contents to the cloned repository

cd mycoolrepo/
echo "Sudheer Satyanarayana" >> contributors.txt
hg add contributors.txt
hg commit -m "Initial commit with contributors"

Step 3: Push the changes to the canonical Mercurial repository

hg push ssh://

Step 4: Create the repository on Github

There are several ways to setup the mirror. One way to do it is importing the Mercurial repository to Gitbut. But in this article, we'll directly pull and push to Github repository. While creating the Git repository on Github, I chose the option - "Initialize this repository with a README".

Step 5: Enable the hg-git extension

Edit .hg/hgrc file in your repository and add these:

hggit =

Step 6: Pull from the newly created Git repository

hg pull git+ssh://

Pulling for the first time from the newly created Git repository will create another head in your Mercurial repository. For example, running 'hg heads' shows:

changeset:   1:5b467a77b006
bookmark:    master
tag:         tip
parent:      -1:000000000000
user:        Sudheer Satyanarayana <>
date:        Fri Mar 27 22:14:32 2015 +0530
summary:     Initial commit

changeset:   0:688bd09fe790
user:        sudheer
date:        Fri Mar 27 22:07:26 2015 +0530
summary:     Initial commit with contributors
The changeset 1:5b467a77b006 is bookmarked as 'master'.

Step 7: Merge the heads

hg merge master
hg commit -m "merging master"

Step 8: Push the changeset to the canonical Mercurial repository

hg push

Move the bookmark 'master' to default and push to Githib.

hg bookmark -r default master
hg push git+ssh://

Now, you can adopt this method to work on the repository and keep pushing to both the repositories - The canonical Mercurial repository and the Github Git repository.

Work on the repository. Make some commits. Push to the canonical repository Move the bookmark 'master' to default and push to Githib Git repository If you do not move the bookmark, pushing to the Git repository makes no sense because hg-git will attempt to push the changeset the bookmark points to. In other words, the bookmark 'master' needs to be updated every time you want to push to the Git repository. You can automate the process of moving the bookmark to your latest line of work by activating the bookmark.

hg up master

That's it. Now your working method is:

Work on the repository. Make some commits. Push to the canonical repository Push to Githib Git repository Once the bookmark is active, Mercurial moves the bookmark to your latest line of work automatically.

Short repository name: For your convenience, you can give a short name to your Github repository URL. In your .hg/hgrc update the paths secion:

default = ssh://
github = git+ssh://

Notes: Make sure not to add '.git' directory. Github will not accept the changeset if there is a '.git' directory.

Access Dictionary Keys As Object Attributes

written by Sudheer Satyanarayana on 2015-02-24

You access Python dictionary keys using the syntax:


For example:

>>> my_dict = {'food': 'idly'}
>>> my_dict['food']

Sometimes, you might want to access the dictionary keys using:


syntax. If you do this is what happens:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'food'

How can you solve this? Easy.

 pip install attrdict

How do you use the newly installed package?

>>> from attrdict import AttrDict
>>> my_dict = AttrDict({'food': 'idly'})

How to get Fedora to suspend laptop when lid is closed?

written by Sudheer Satyanarayana on 2015-02-24

Edit the file:


Look for the 'HandleLidSwitch' entry. Add the entry if it is not there already. If the entry is already there, change to:


Then restart systemd-logind:

systemctl restart systemd-logind.service

This is tested on Fedora 21.


written by Sudheer Satyanarayana on 2014-10-21

Pani version 0.1 has been released.

Pani is a glue tool that provides a web interface atop hg-ssh. Through the web interface, you can add, modify and remove packages and assign access to users.

The documentation is also available.

How To Change Hostname In CentOS 7

written by Sudheer Satyanarayana on 2014-10-07

CentOS 7 comes with the hostnamectl utility to read and write hostname.

To view the hostname simply run the command:


[root@c7p1 ~]# hostnamectl
Static hostname: localhost.localdomain
Icon name: computer
Chassis: n/a
Machine ID: cda43e8062fb4d5881491f41e06a0c0d
Boot ID: 7cb835d3dd64468392e3399d771051ac
Virtualization: kvm
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-123.el7.x86_64
Architecture: x86_64

To set a hostname:

hostnamectl set-hostname mynewhostname


hostnamectl set-hostname