The main codes tetr and lev00, as well as some others, have a special tex-based interface (Text User Interface, TUI) that reminds menus. All menus work in a very simple and straightforward fashion: every menu item has either a number or a symbol. At each point in time you see on the screen the whole menu.
To execute a particular option you press the number (symbol) corresponding to the option required paying special attention to small/capital characters if present, and then press Enter. Normally this would cause some kind of calculation and the original menu would reappear (it may be different now!), so that something else can be done. In some other cases choosing an option invokes a different menu (sub-menu) or may change some settings - you should notice it when the original menu reappears!
Many menus contain several parts. Usually, the upper part of the menus contain the executable bits, the middle - some information options, while at the bottom of the menu one can find various options that control the other options of the menu (the settings). An example of a typical menu is given below (it is, in fact, the main menu of tetr):
/--------------------------\\
|.................... MAIN MENU:.....................|
\\--------------------------/
>>>>>>>>>>>>> Create unit cell from scratch <<<<<<<<
[current geometry will be lost]
B. Generate supercell geometry from scratch
Pc. Generate primitive cell from space group symbol
>>>>>>>>>>>>>>>>> Cluster construction <<<<<<<<<<<<
Cl. Build up a cluster from the lattice
>>>>>>>>>>>>> Geometry file already exists <<<<<<<<<<
M. Complex cell modifications
KP. k-points generation
>>>>>>>>>>>>>>>>> Coulomb potential <<<<<<<<<<<<<<<
Ew. Coulomb potential inside the unit cell
>>>>>>>>>>>>>>> Calculation of phonons <<<<<<<<<<<<<
Vm. Vibrations of molecules => preparation
Vc. Vibrations of crystals (point group) => preparation
Vs. Vibrations of crystals (space group) => preparation
V2. Vibrations => calculation [normal modes]
V3. Vibrations => application [DOS, masses]
>>>>>>>>>>>> Representation/visualisation <<<<<<<<<<<
Cm. Compare geometry with the one in another file
D. Distances between atoms
Ch. Check specified point in the cell
Sb. Save in a box running across adjacent cells
>>>>>>>>>>>>>> Potential energy surface <<<<<<<<<<<<<
PE. PES: move atoms keeping the symmetry
----- g e n e r a l s e t t i n g s ------
XY. Format of <geom.xyz> is set to Xmol
R. Read geometry from another file
S. Save
Q. Quit.
Specify the character and press ENTER ---->
We shall be using a bold font for the menu options in what follows.