Re: That DEL key thing...

[Bob Proulx <bob%proulx.com@localhost>]

Michael van Elst wrote:
> pl%szwajn.net@localhost (pl) writes:
> > Why is DEL deletes characters from the left (making BackSpace
> > redundant), while on X (from base) it behaves as expected (removes
> > from right)? How can I make it behave as in X11?

I would want to see the output of "stty -a" and see what the erase
settings are set to in the tty driver.

> > I know there's some historical background ongoing, but so far I
> > know DEL removed chars from right since ever on IBM PC, so logical
> > for me would be making such behaviour default at least on i386 and
> > amd64 ports.

DEL has never removed characters to the right in my experience.
However the Keypad Delete to the right is usually configured that way.

> > If that's important: both DEL and BS generate same control key "^?"

On a standard PC keyboard there is usually a key labeled "Del" which
is the Keypad Delete and distinct from the Backspace key.

> A standard PC keyboard has a ...

Excellent descriptions!

> xterm then(*) interprets the symbols and generates "^H" for "Backspace"
> and "^[[3~" for Delete (unless you have configured "DEL is Delete",
> then it generates a "^?").

My personal opinion is that Xterm does this the wrong way around.

> Other terminal programs use different, even more complex, methods
> to map the key symbols to some character sequence.

Frustratingly true!

Having both Backspace BS ^H and DEL ^? date from paper tape and punch
cards.  If a hole has been punched in paper tape then it cannot be
filled back in again.  But the entire character can be deleted.
Therefore both keys are needed.  If a mistake has been made then
Backspace (^H) until the punch is over the character to be deleted and
then punch all of the holes with DEL (^?) causing the character to be
deleted.  Upon reading in DEL characters are ignored.  Therefore when
dealing with paper BOTH keys are necessary.  When CRTs came along the
choice of which key to use was rather arbitrary and different vendors
made different choices.

My memory of events (which is to say possibly faulty) is that IBM and
therefore others such as HP chose the Backspace key ^H for the stty
erase setting by default while others (fuzzy memory) chose DEL for the
stty erase setting.  Definitely HP-UX as I used it for many years.
Prior to that other characters such as '#' and '@' were used for an
erase character.

Using ^H for erase creates problems of overloading that useful key in
Emacs and Emacs has always had workarounds needed for such
environments.  Emacs on HP-UX always (annoyingly to me) required
workarounds swapping ^H and ^?.

Meanwhile when the Linux kernel came around it chose to use DEL as the
stty erase character.  This makes more sense to me.  Personally I
prefer that configuration.  Everything just works much more nicely
that way.  Because of the huge popularity of the Linux kernel that has
driven default choices in many terminal emulators.  (But not Xterm
which as described has a complicated configuration capability.  I
actually have many more notes about it than already mentioned.)

> Some programs (usually those that do line-editing themselves)
> handle this themselves. They either copy the behaviour of the
> terminal driver, or (like tmux) have a hardcoded behaviour,
> or (like tcsh) accept "^H" and "^?" as equal.

Programs such as tcsh (and bash) which are hard coded to accept ^H and
^? as equal do so out of frustration of the user having things
inconsistently configured.  Which is great.  The shell knows what the
user is trying to do.  But it does lead to people truly not
understanding why things work sometimes and don't work other times.

> The currently most common (but not the original) behaviour is
> to make the "Backarrow" key generate "^H" and use that for the
> erase operation, and since that is handled by the defaults almost
> everywhere, I'm changing only the keyboard map if I have a PS/2
> keyboard.

And for a dissenting opinion I always choose the reverse.  I always
have Backspace produce a DEL character.  I feel that creates the most
rationally consistent configuration.  (Noting again that the Backspace
producing DEL is distinct and separate from the Keypad Delete key.)

Bob