Environment Variables.

What are environment variables?

• Each user has its own environment.
• Each user can configure its own environment/account by setting preferences.
• These OS configurations should be isolated from other user environments.

Where does OS store all these configurations?

• Environmental Variables -> KEY = value pairs
• Variables store information.
• By convention, names are defined in UPPERCASE. Eg; SHELL=/bin/bash (Default shell program(location) of the user)
• User can change these environment variable values. Eg; SHELL=/bin/zsh
• Variables are variables, which means they can be changed.

List all Environment Variables:-

• printenv -> prints all the environment variables.

Print Specific Environment Variable:-

• printenv <Env. variables> -> prints the provided environment variable's information.

Referencing Environment Variable:-

• Using the $ sign. Eg; $USER

Use cases of Environment Variables:-

• OS stores information about the environment.
• We can create our own environment variables.

Creating Environment Variables:-

• export (env.) variable_name=value -> This is available all over the environment & not like bash variables which are available in the script.

For Eg;

export DB_USERNAME=dbuser
export DB_PASSWORD=secretpwdvalue
export DB_NAME=mydb

Deleting Environment Variables:-

• unset (env.) variable_name=value

For Eg;

unset DB_NAME

Note:-

• Available for the current session(temporary env. variable)
• After exiting the terminal the env. variable created is removed.

Persisting Environment Variables:-

• This is user-specific.

Shell specific configuration file;

• per-user shell-specific configuration files.
• Eg-> if you are using BASH, you can declare the variables in the ~/.bashrc file.
• Variables set in this file are located whenever a bash login shell is entered.

To add these variables, open the .bashrc file & at the last enter:

export DB_USERNAME=dbuser
export DB_PASSWORD=secretpwdvalue
export DB_NAME=mydb

Now, after reopening the terminal, the environment variables will be there as they are saved on the system.

To load the new environment variables into the current shell session;

source ~/.bashrc

Persisting Environment Variables System-wide:-

• To add for all users there is a configuration file in Linux in the home folder.

  /etc/environment
  

PATH environment variable:-

• list of directories to executable files, separated by :(colon)
• tells the shell which directories to search for the executable in response to our executed command. `PATH = $PATH:.....
• you need to provide an absolute location to make available that file.
• adds our custom app/command to available for user(if configured in .bashrc) or for all user(if configured in /etc/environment)

SSH - Secure Shell

It is a network protocol that gives users a secure way to access a computer over the internet. SSH also refers to the suite of utilities that implement that protocol.

Some Use Cases:-

• Copy file to the remote server.
• Install the software on a new server.

2 ways to authenticate with the remote server:-

1) Username & Password.

• Admin creates a user on the remote server
• User can then connect with the username & password.

2) SSH key pair (more secure way).

• The client creates an SSH key pair.

  Key Pair = Private key + Public key
  

• Private Key = Secret key which is stored securely on the client machine.
• Public Key = This can be shared, e.g., with the remote server.
• Client machine for that public key can safely connect.
• Client can "unlock" the public key with his private key.

Note:- If the public key of a person is not registered on the remote server, he/she cannot connect to it.

SSH for services:

Services, like Jenkins, often need to connect to another server via SSH.

• Create a Jenkins user on the application server.
• Create SSH key pair on Jenkins server.
• Add public SSH key to authorized_keys on the application server.

Connect via SSH(Password Authentication):-

ssh username@SSHserver(IP address)

For Eg;

ssh root@159.89.14.94

Generate SSH key pair:-

mkdir .ssh/ -> create a directory for storing keys(if not there)

ssh-keygen -t rsa

• This generates ssh key pairs.
• Here, -t means type & rsa means the method of encryption.

~/.ssh -> .ssh folder under home directory is the default location for your ssh key pair.

id_rsa -> Private key

id_rsa.pub -> Public key

Copy files to the remote server:-

scp(secure copy) -> allows you to securely copy files & directories.

For Eg;

scp test.sh root@159.89.14.94:/root