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 command. 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. Examples: % 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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