A$= INKEY$(STRING)

INKEY$ checks for key state on the keyboard. It has two modes of operation:

If the string parameter passed is an empty string, it will check if any key is waiting in the keyboard buffer, consuming it from the buffer and returning it, or returning an empty string if no key is available.
If the string parameter passed in is a non-empty single-character string, it will check if that key is currently held, and will not consume it from the buffer. Because this form of the function is not consuming from the buffer you may use this form to concurrently check for multiple depressed keys at the same time. Note that you must still drain the buffer. Use a separate INKEY$("") call to drain any pending key presses so the buffer does not fill. US-ASCII values (A-Z) are considered case-insensitive in the parameter when using this form of INKEY$.
When INKEY$ consumes input from the keyboard buffer by using an empty parameter, if multiple keys have been pressed between this call and the last call to INKEY$ (or an INPUT statement), it will take multiple calls to retrieve them all.

Extended keys

Some non-printing keys return special character codes rather than ASCII. These values can be retrieved by wrapping INKEY$ with ASC().

Key	ASC(INKEY$("")) value
PAGEUP	245
PAGEDOWN	246
DEL	247
INS	248
END	249
UP	250
DOWN	251
LEFT	252
RIGHT	253
HOME	254

Examples

REM Wait for a keypress
PRINT "Press a key..."
REPEAT
    k$ = INKEY$("")
UNTIL k$ > ""
PRINT "You pressed: "; k$

REM Handle arrow keys
k$ = INKEY$("")
IF k$ > "" THEN
    code = ASC(k$)
    IF code = 250 THEN PRINT "Up arrow pressed"
    IF code = 251 THEN PRINT "Down arrow pressed"
ENDIF

REM multiple key example
PRINT "Press both A and Z together to exit"
REPEAT
UNTIL INKEY$("A") <> "" AND INKEY$("Z") <> ""
END

Notes

Printable characters return their usual ASCII values (e.g. "A" → 65).
Extended keys must be checked via ASC().
If no key is pressed, the result is "" (empty string).
If the parameter to INKEY$ contains more than one character, any characters except the first are ignored.

See also: ASC · INPUT