Hi all,

In the constext of the Basta project https://www.kylheku.com/cgit/basta/
it came to my attention that the background interrupt was clobbering the
occasionally useful $_ variable which holds the last argument of the
previous command.

This was introduced in the Korn shell; Bash has it.

After some failed experiments, I worked out a stable trick for
protecting it.

You cannot simply save $_ into a local variable on entry into your trap
handler, and restore it at the end because Bash clobbers it afterward,
when the trap command finishes executing. $_ ends up with the last
argument of the trap command.

The trap setup for catching the SIGWINCH and SIGALRM signals now looks
like this:

trap 'basta_uln_save=$_; basta.update_status; : "$basta_uln_save"' ALRM WINCH

instead of just

trap basta.update_status ALRM WINCH

The first command of the sequence saves $_ into a global basta_uln_save.

The last command is the null command

: "$basta_uln_save"

You can see how that works. Bash executes the null command, expanding
the quoted parameter to form the rightmost argument, and that argument
becomes the value of $_. Mission accomplished!

I now see a stable value of $_ at the prompt, in spite of the
periodic interrupt that refreshes the status line.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

On 10.04.2024 07:29, Kaz Kylheku wrote:
> [...]
>
> The trap setup for catching the SIGWINCH and SIGALRM signals now looks
> like this:
>
> trap 'basta_uln_save=$_; basta.update_status; : "$basta_uln_save"' ALRM WINCH
>
> instead of just
>
> trap basta.update_status ALRM WINCH

I know that this looks like a hack, but since 'update_status' needs no
arguments you could also more simply just write...?

trap 'basta.update_status "$_"' ALRM WINCH

(Not sure whether introducing a global variable and supplementary code
or this hack is "better". Just mentioning it for a possible variant.)

Janis

> [...]

On 2024-04-10, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
> On 10.04.2024 07:29, Kaz Kylheku wrote:
>> [...]
>>
>> The trap setup for catching the SIGWINCH and SIGALRM signals now looks
>> like this:
>>
>> trap 'basta_uln_save=$_; basta.update_status; : "$basta_uln_save"' ALRM WINCH
>>
>> instead of just
>>
>> trap basta.update_status ALRM WINCH
>
> I know that this looks like a hack, but since 'update_status' needs no
> arguments you could also more simply just write...?
>
> trap 'basta.update_status "$_"' ALRM WINCH
>
> (Not sure whether introducing a global variable and supplementary code
> or this hack is "better". Just mentioning it for a possible variant.)

Nice streamlining; I changed to this.

One command, and no extra variable is better.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca