Re: [ptxdist] Backspace not working in terminal (using ssh)

[Ian Abbott <abbotti@mev.co.uk>]

On 20/03/2019 09:15, Dold, Wolfram wrote:
> On Tue, 2019-03-19 at 17:42 +0000, Ian Abbott wrote:
>> On 19/03/2019 12:33, Dold, Wolfram wrote:
>>> Hi Ian,
>>> On Tue, 2019-03-19 at 12:06 +0000, Ian Abbott wrote:
>>>> On 19/03/2019 09:17, Dold, Wolfram wrote:
>>>>> Hi there,
>>>>> when we connect via ssh to our embedded device, the backspace key does not work.
>>>>> During the analysis we found out that the passage
>>>>>
>>>>> # This fixes the backspace when telnetting in.
>>>>> if [ "$TERM" != "linux" ]; then
>>>>>            stty erase ^H
>>>>> fi
>>>>>
>>>>> from the file '/etc/profile' contained in ptxdist.
>>>>>
>>>>> Now I want to ask, how best to fix this behavior?
>>>>> Can the passage simply be omitted? (At least the comment lets me assume that, because there 'telnet' is
>>>>> mentioned
>>>>> and
>>>>> that is no longer necessarily state of the art).
>>>>> Or does omitting it lead to other unwanted side effects?
>>>>>
>>>>> Any help would be appreciated.
>>>>>
>>>>
>>>> Code can interpret input characters in various ways independently of the
>>>> stty settings.  For example, if the "Command Line Editing" feature is
>>>> enabled in the Busybox configuration, the Busybox "ash" and "hush"
>>>> shells' interactive line editing will handle both ASCII DEL and ASCII BS
>>>>     as a "backspace" operation.  This can be enabled in the PTXdist
>>>> menuconfig via:
>>>>
>>>> Shell & Console Tools --->
>>>>      -*- busybox --->
>>>>          Settings --->
>>>>            [*] Command Line Editing
>>>>
>>>> This sets "PTXCONF_BUSYBOX_FEATURE_EDITING=y" in the ptxconfig file.
>>>>
>>>
>>> The feature is already enabled.
>>> But it does not work for me. It only works when I delete the stty command from /etc/profile,
>>>
>>>> If you are using Bash or some other other program with READLINE support,
>>>> I think it handles ASCII BS and ASCII DEL similarly to Busybox's command
>>>> line editing feature.
>>>
>>> We do not use bash, we use the busybox shell.
>>
>> Just for comparison, I'm using the busybox "ash" shell.  (There are two
>> shells in Busybox - "ash" and "hush".)
>>
>> My SSH client is set to send ASCII DEL when the backspace is pressed,
>> and sets TERM="xterm" or TERM="xterm-256color".  The stty command on my
>> target system shows "erase ^H".
>>
>> Both the backspace key (which sends ASCII DEL) and the CTRL-H sequence
>> (which sends ASCII BS) result in a backspace operation on the ash
>> command line.
>>
>> If I run a command that doesn't do any fancy line editing, such as:
>>
>>     od -c
>>
>> then the ASCII DEL characters get passed through to standard input and
>> the ASCII BS characters erase the previous characters (as per the stty
>> erase ^H setting).
>>
>> If I change the stty setting:
>>
>>     stty erase '^?'
>>
>> then both ASCII DEL and ASCII BS result in a backspace operation on the
>> ash command line, but for commands that do not do any fancy line
>> editing, the ASCII BS is passed through to standard input and the ASCII
>> DEL erases the previous character as per the stty erase '^?' setting.
>>
>> So in summary, the PTXCONF_BUSYBOX_FEATURE_EDITING option seems to work
>> for me, at least in the "ash" shell.  (I haven't tried the "hush" shell.)
>>
>> I am using BusyBox 1.29.3 from PTXdist 2019.03.0.
>>
> Sorry, I didn't describe the problem detailed enough:
> When I type in the shell, everything works as you described it.
> The problem that the backspace key doesn't work occurs when we
> use the busybox-read function from a shell script.
> 
> We are using ash from busybox.
> We are using BusyBox v1.29.3 from ptxdist 2019.03.1
> 
> Sorry again for the miscommunication.

I understand your problem now, the BusyBox ash "read" built-in does not 
have line editing support, so only the kernel-level, n_tty 
line-discipline editing features can be used.  (Bash's "read" built-in 
supports the "-e" option to allow line-editing when the input is from a 
tty.)

I've no idea why that bit of the /etc/profile file is there, but I guess 
it must have suited one of the PTXdist developers at the time.  A "git 
blame" only revealed that the whole file was added in 2006, so I don't 
know how relevant that part of it is today.

Still, it's easy enough to replace the default file with a modified one 
in your project since it uses PTXdist's "install_alternative" to install 
it in the root file system.  See:

https://www.ptxdist.org/doc/ref_manual.html#install-alternative

This does mean you might need to keep track of future changes to the 
default copy of the file in future PTXdist releases and modify your 
local copy if relevant.

-- 
-=( Ian Abbott <abbotti@mev.co.uk> || Web: www.mev.co.uk )=-
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