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
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