See online information about Mathematica; ``basics''.
Type mathematica for an interactive, Notebook Interface X-Windows session of Mathematica.
You would run ``math'' instead of ``mathematica'' if you want to run Mathematica in text mode, that is, without X Windows or the fancy notebook interface. This is necessary if you are logged on remotely via modem, or from a computer without X Windows.
Use mathbg to run a Mathematica script in the background.
Usage: mathematica [options] [file1 file2 ...] Options: -appHelper : Same as using the options -querySingleLaunch and -forceSaveAs. -backgroundTaskDelay t : Wait t milliseconds before resuming background tasks. -best : Use an optimal visual. -cleanStart : Ignore cache information and preferences. -copyright : Display copyright information from the command line. -font f : Use font f in menus, buttons, etc. -forceSaveAs : Force Save dialog box when closing any command line file. -help : Print a listing of all available command-line options. -lmverbose : Print license manager stderr diagnostic information. -mathlink : Run in MathLink mode. -noSplashScreen : Prevent the splash screen from being displayed on startup. -nostderrWindow : Send stderr to terminal instead of popup window. -nowatchCLIPBOARD : Don't watch the CLIPBOARD property for change in ownership. -preferencesDirectory d : Specify the location d where preferences are stored. -primaryModifierMask m : Use modifier mask m as the command key. -printCommand c : Use command c to send file to printer. -pureKeys : Disable X server specific key bindings for modifier keys. -pwfile f : Get its password from the specified file f. -pwpath p : Search along the specified path p for a password file. -querySingleLaunch : -singleLaunch plus ask if open command line files. -screenXResolution x : Specify horizontal screen resolution x in dpi. -screenYResolution y : Specify vertical screen resolution y in dpi. -secondaryModifierMask m : Use modifier mask m as option key. -singleLaunch : Allow only one copy of the front end per display. -topDirectory d : Specify the directory d where Mathematica is installed. -version : Print the version number of the front end from the command line. -visualChooser : Enable the user to choose the visual the front end uses. -watchCLIPBOARD : Watch the CLIPBOARD property for change in ownership. -wwwBrowser c : Use command c to open a URL link from the front end.