On the behaviour of the checkwinsize shopt from within a subshell

[Kerin Millar]

I am writing to report that the documented behaviour of the checkwinsize shopt 
can be impeded by the use of the subshell.

Firstly, some preliminaries.

$ declare -p BASH_VERSION
declare -- BASH_VERSION="5.2.15(1)-release"
$ shopt checkwinsize
checkwinsize    on
$ sz() { /bin/true; declare -p COLUMNS LINES; }

In both of the following two cases, I manually resized the terminal window 
during the time that bash was waiting for sleep to exit.

$ sz; sleep 3; sz
declare -- COLUMNS="86"
declare -- LINES="29"
declare -- COLUMNS="95"
declare -- LINES="34"

In this case, the values of the variables were updated as expected.

$ ( sz; sleep 3; sz )
declare -- COLUMNS="95"
declare -- LINES="34"
declare -- COLUMNS="95"
declare -- LINES="34"
$ declare -p COLUMNS LINES
declare -- COLUMNS="85"
declare -- LINES="31"

In this case, the values of the variables were not updated from within the 
subshell, only the initial shell. I consider this to be surprising because the 
manual does not mention any such limitation. Consequently, the utility of the 
checkwinsize feature suffers in terms of being a potential alternative to the 
invocation of stty(1) with the "size" operand (an operand which will eventually 
be standard, but isn't yet).

-- 
Kerin Millar