Source : Free On-Line Dictionary of Computing

shell variable
     
         One of the {variables}
        accessible to a {Unix} {shell} process, including {environment
        variables} (e.g. $HOME), {command line arguments} (e.g. $1)
        and local variables (e.g. $input_file).  Other {operating
        systems} have similar variables.
     
        (1999-01-26)