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The Fiveth interactive shell

Notes for running under Linux

The shell assumes that it's running in something like a serial terminal and has access to a terminal in "raw mode".

make qemu and make run sets this up for you. It mostly just boils down to:

stty raw -echo
build/fiveth
stty sane

Since the shell takes control of the terminal, if it gets stuck you might have to kill it from another shell session. Sorry.

The prompt

The shell starts with a prompt like this

0 >

The number is simply the current number of natives in the data stack, or stack depth. If you open a list without closing it, the > changes to a | indicating line continuation:

0 > "hello" [
0 |   "Hello, world!" s.
0 | ] define  ;
0 >

Line editing

  • CR (Return or Enter key): Commit current line
  • BS or DEL (Backspace): Delete last character
  • Ctrl-C: Cancels current input
  • Ctrl-D: Exits the shell

Known issues:

  • Other control characters are ignored. This means, for example, arrow keys, which are encoded as \x1a [ <some-letter>, generate weird input.
  • Non-ASCII characters are currently not handled correctly if you start editing a line containing them. Sorry.

The output comment

When the shell finishes processing a section of the input, either normally or abnormally, it will write an output comment starting with " ; " (two spaces, a semicolon, one space).

  • If a line has been cancelled, the comment is " ; ///", indicating that the line was not executed

    0 > I don't need this line anymore  ; ///
0 >

  • If a line has a parse error, the comment is " ; ???". Error messages comes in the next few lines.

    0 > 10 [ nonexistent ] count  ; ???
! Undefined word
! 10 [ >>>nonexistent<<< ] count
0 >

  • If a line is executed, the comment is " ; ", then output of the code

    0 > 1 2 + .  ; 3
0 > "test" s.  ; test
0 >