What is (what does) a shell?

• a shell is a macro processor that executes commands
• a shell provides internal commands
• a shell provides control structures to combine programs
• a shell works in the following manner:
  1. Read the input
  2. Break the input into words and operators
  3. Parse the tokens into simple and compound commands
  4. Perform the various shell expansions
  5. Perform any necessary redirections
  6. Execute the command
  7. Optionally wait for the command to complete and collect its exit status