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								        What is a shell? What are shell variables?What is a shell?
  - A shell is an interface to the user of any operating system.
  - It gathers input from you and executes program based on that input.
  - The users instruct an operation to the computer by typing commands as text for the execution.
  - It is an environment in which we can run our commands, programs and shell scripts.
  Shell variables:
  - Shell variables are special variables. 
  - They are name and value pairs created and maintained by the concerned shell. 
  - Shell variables are contained exclusively within the shell in which they were set or defined.
  - It is a set of shell and is required by the shell in order to function correctly.
  Following are the different shell variables:
  1. PATH: A list of directories that the system will check when looking for commands.
  2. HOME: The current user's home directory.
  3. MAIL: The path to the current user's mailbox.
  4. TERM: It specifies the type of terminal to emulate when running the shell.
  5. USER: The current logged user.
  6. PWD: The current working directory.
								          
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