Section D.

                           INTRODUCTION TO UNIX

                               The Shell

       The user interface to Unix is "the shell", also called the command
       interpreter, and is a program itself.  It executes automatically
       at login and the environment is initially configured by the system
       administrator, but is easily customized (using aliases and setting
       environmental variables).  Several flavors of command interpreters
       exist (the C-shell [csh], the Bourne shell [sh]) and have built-in
       languages of their own: loop control, if-then-else constructs and
       arithmetic operations capability.  User control is via commands,
       and optionally switches (entered on the command-line usually
       preceded by a "-"), and other parameters as required by particular
       commands.  The standard format for Unix commands is:

       % command -switches arg1 arg2 ...

       Unless a command specifically involves output to the screen, no
       news is good news in Unix.  When a command is entered and executes
       successfully, nothing will be reported; the prompt will re-appear.
       Only if some kind of error occurs will anything be displayed,
       reporting the error that occurred or the proper syntax of the

       Two sources of on-line help are accessed by the commands help, and
       man.  The apropos command (on some systems) is also a useful tool
       to search for appropriate commands to accomplish particular tasks.


       % man mail

       The on-line manual pages for the command mail will be displayed.

       % apropos mail

       Header lines from on-line man pages containing the string mail
       will be displayed.

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