GRASS can be run in 3 modes (you can combine them any time)
command line:r.slope.aspect myelevation slope=myslope aspect=myaspectGRASS
interactive parser: r.slope.aspect followed by dialogue
Start GRASS by typing:
(See the GRASS5 startup man page for details about start-up options.)
You will then be asked to provide the database directory (only the first time you start GRASS) and either choose from the availabe locations and mapsets or create a new location (see below on how to do this).
Once at the GRASS prompt you can type any grass or unix commands, and, if it hasn't started up automatically, you can start the TclTkGRASS-GUI by typing tcltkgrass&.