Use sqlite in cloud9 (development environment). Heroku uses PostgreSQL. It is recommended to use the same SQL for the development environment and the production environment.

I wonder why they are using it for learning. (´-ω-`) I don't know.

AWS Free Tier on Amazon RDS For Amazon RDS Single-AZ db.t2.micro instances that use MySQL, MariaDB, PostgreSQL, Oracle BYOL, or SQL Server (SQL Server Express Edition), enough to run DB instances consecutively each month. 750 hours of usage time for Use of the Oracle BYOL db.t3.micro Single-AZ instance is included in the Amazon RDS free tier. If you run both instances of db.t2.micro Single-AZ and db.t3.micro Single-AZ on Oracle BYOL, their usage is totaled across the instance classes. 20 GB general purpose (SSD) DB storage * 20 GB backup storage for automatic database backups and any DB snapshot by the user *

I can use PostgreSQL ... why? .. ..

Rails installation related

Install Rails Use the gem command provided by RubyGems.
gem install rails -v 6.0.3
gem install [App] -v [Version specification]

#Empty template creation command in Rails
rails _6.0.3_ new hello_app
rails _[version]_ new [app name]

Gemfile.Is the library to install(parts)File that describes

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.6.3'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 6.0.3'
# Use sqlite3 as the database for Active Record
gem 'sqlite3', '~> 1.4'
# Use Puma as the app server
gem 'puma', '~> 3.11'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '~> 4.0'
# Turbolinks makes navigating your web application faster.
# Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.7'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use Active Model has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Active Storage variant
# gem 'image_processing', '~> 1.2'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.4.2', require: false

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a
  # debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
end

group :development do
  # Access an interactive console on exception pages or by calling 'console'
  # anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the
  # background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :test do
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '>= 2.15'
  gem 'selenium-webdriver'
  # Easy installation and use of web drivers to run system tests with browsers
  gem 'webdrivers'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

#Command to install all libraries listed in Gemfile
bundle install

#Commands to use if there is a dependency problem with the installed library
bundle update

#Rails server launch command
rails server
rails s　#(Abbreviation)← This one is used more often

rb　config/environments/development.rb

Rails.application.configure do
  .
  .
  .
  #Allow connection to Cloud9
  config.hosts.clear ← If you do not add this, it will be played by accessing the Rails server
end

Git related

Git is a convenient app for managing source code Github is a service that provides a place to store source code on the Internet

#Name settings and email address settings used in Git
#* Please note that the name and email address set in Git will be released to the public on the repository in the future.

$ git config --global user.name "name"
$ git config --global user.email email address

#Allow the checkout command to be omitted for co.
$ git config --global alias.co checkout

#Change the time required for a password when doing git push Reduces the hassle of typing a password every time
$ git config --global credential.helper "cache --timeout=86400"

#Repository(Storage box)Command to make
$ git init

#Command to put files in temporary storage box-A changes(New / Add / Delete)Means to add all the files that were
$ git add -A

#Command to add a memo to the temporary storage box and move it to the storage box-m "message"You can write notes with
$ git commit -m "Initialize repository"

#Register your source code storage on the Internet with the name origin
$ git remote add origin https://github.com/<Your GitHub account name>/hello_app.git

#My upload
$ git push -u origin master

#You can see if there are any changes
$ git status

#Check what you have committed in the past
$ git log

source 'https://rubygems.org'　
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

gem 'rails',      '6.0.3'
gem 'puma',       '4.3.4'
gem 'sass-rails', '5.1.0'
gem 'webpacker',  '4.0.7'
gem 'turbolinks', '5.2.0'
gem 'jbuilder',   '2.9.1'
gem 'bootsnap',   '1.4.5', require: false

#I think that everything from here onwards will be installed in common('ω')No

#Install only development environment and test environment
group :development, :test do　
  gem 'sqlite3', '1.4.1'
  gem 'byebug',  '11.0.1', platforms: [:mri, :mingw, :x64_mingw]
end

#Install only development environment
group :development do　
  gem 'web-console',           '4.0.1'
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.1.0'
  gem 'spring-watcher-listen', '2.0.1'
end

#Install test environment only
group :test do　
  gem 'capybara',           '3.28.0'
  gem 'selenium-webdriver', '3.142.4'
  gem 'webdrivers',         '4.1.2'
end

#Install only in production environment
group :production do　
  gem 'pg', '1.1.4'　
end

#On Windows tzinfo for timezone information-Must include data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

#Install non-production gems
$ bundle install --without production

#install heroku command
source <(curl -sL https://cdn.learnenough.com/heroku_install)

This is the command inside that is being executed!
curl -OL https://cli-assets.heroku.com/heroku-linux-x64.tar.gz
tar zxf heroku-linux-x64.tar.gz && rm -f heroku-linux-x64.tar.gz
sudo mv heroku /usr/local
echo 'PATH=/usr/local/heroku/bin:$PATH' >> $HOME/.profile
source $HOME/.profile > /dev/null

#Log in to heroku(--You can log in without using a browser using the interactive option)
$ heroku login --interactive

#Create a new application on Heroku(Analyze the git source code and prepare an appropriate server)
$ heroku create　

#Upload the source code. Now the service you created will be published on the net.
$ git push heroku master