`reset` command makes backspace unusable

Polytropon

2018-05-31 07:11:29 UTC

Post by å´ç (CUI Hao)
Hi, new to FreeBSD. On Linux, I often use `reset` command to recover
an broken terminal. On FreeBSD, `reset` seems to render backspace
unusable in some CLI programs.
1. SSH login FreeBSD server from a Linux terminal emulator. TERM is
set to `xterm-256color`.
I tried Konsole / Termite. Both reproducible.
But xterm/uxterm doesn't trigger the problem.
2. Executing `reset` on FreeBSD.
3. Then pressing backspace will yields `^?` instead of backward
delete in most CLI programs including bash/vim/cat, but not tcsh and
sh.
Erase is backspace.

This indicates that backspace's ^H has been altered to ^?, which
usually is delete, but it only works when the "support chain" does
react correctly to this code (the C shell sometimes doesn't, which
is why you see the "replacement text" instead of the shell per-
forming the correct edit action).

Post by å´ç (CUI Hao)
I googled about the problem. `stty erase ^?` after `reset`, or `reset
-e ^?` fix it. But I still wonder why `reset` doesn't work like on
1. terminal line settings (`stty -a`)
I checked the output of `stty -a`. On Linux terminal (Termite), I saw
`erase = ^?`. For full output, see https://cfp.vim-cn.com/cbff6

Here is some relevant output to compare:

% stty -a
[...]
cchars: discard = ^O; dsusp = ^Y; eof = ^D; eol = <undef>;
eol2 = <undef>; erase = ^H; erase2 = ^H; intr = ^C; kill = ^U;
lnext = ^V; min = 1; quit = ^\; reprint = ^R; start = ^Q;
status = ^T; stop = ^S; susp = ^Z; time = 0; werase = ^W;

Note that backspace is called "erase" here, and correctly set to ^H,
while there is no mentioning of delete (see further notes).

Post by å´ç (CUI Hao)
After SSH login FreeBSD, `stty -a` shows `erase = ^?; erase2 = ^H`,
which seem to be consistent with Linux. See
https://cfp.vim-cn.com/cbff7

This makes the DEL key the backspace key, if I read it correctly.

Post by å´ç (CUI Hao)
But after `reset`, the settings changed and becomes `erase = ^H;
erase2 = ^H`. see https://cfp.vim-cn.com/cbff8

Now ^H equals backspace (here: "erase"), which is what should
happen when the terminal capabilities are reset according to
the database.

Post by å´ç (CUI Hao)
2. termcap
It seems default terminal line settings are determined by termcap db.
I checked `/etc/termcap` and found the entry of `xterm-256color`,
which inherits settings of `xterm-basic` entry. `xterm-basic` sets
`kb=^H`.

Yes, that again is "key backspace" with the correct setting.

Post by å´ç (CUI Hao)
On Linux, I get the termcap settings by `infocmp -Cr`. Terminal
`xterm-256color` sets `kb=^H`, while `xterm-termite` sets `kb=\177`.

Key code 177 (0x7F or 127) is a problematic key code because
it sometimes means backspace, sometimes delete.

Post by å´ç (CUI Hao)
xterm-256color|xterm alias 3:\
:Co#256:pa#32767:\
:kb=\177:\
:AB=\E[48;5;%dm:AF=\E[38;5;%dm:tc=xterm-new
Above settings indeed works and `reset` won't alter erase setting.
However, Linux `xterm-256color` termcap does set `kb=^H` like FreeBSD,
but it doesn't cause the problem on Linux.

This kind of modification should not be needed. However, you
can do certain customization to your C shell's settings (and
let's assume for now you're using the C shell as your dialog
shell, simply because this is FreeBSD's default).

Add the following settings to /etc/csh.cshrc (for global effect)
or to ~/.cshrc (for user-local effect):

if ($?prompt) then
# ... stuff omitted ...
bindkey ^? delete-char # delete for console
bindkey ^[[3~ delete-char # delete for xterm
# ... more stuff ...
endif

You can also experiment with the "ESC sequence spelling" of the
key codes (using "\e" instead of "^["), like this:

bindkey "\e[3~" delete-char # delete
bindkey "\e[1~" beginning-of-line # home
bindkey "\e[4~" end-of-line # end

Note that backspace will be reset to ^H which is the correct
setting for this key, so there should be no need to add it.

You can still _always_ simply press Ctrl+H to check if it does
correctly perform the backspace operation on the current line.

Post by å´ç (CUI Hao)
Although I have found several workaround to fix backspace problem, may
anyone explain why terminal settings inconsistent on Linux and
FreeBSD? Is this a bug or just some compatibility issues? Are there
other compatibility issues with Linux terminal emulator?

This is probably because FreeBSD's console subsystem currently
still is in a flow, and Linux is always in a flow, so there is
no real agreement about how things should be solved in a cross-
compatible way... ;-)

--
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...