[ast-developers] [BUG] ${var+set} does not work within loops

[Martijn Dekker]

I was bitten today by yet another bizarre bug in ksh93.

In the current release version of ksh93 as well as the beta, expansions
like ${var+set} and ${var+:nonempty) do not work correctly when used
within a 'for', 'while' or 'until' loop.


Test script 1:

unset -v var
for i in 1 2 3 4 5; do
case ${var+s} in
( s ) echo set; unset -v var;;
( '' ) echo unset; var=;;
esac
done

Actual output:
unset
unset
unset
unset
unset

Expected output:
unset
set
unset
set
unset


Test script 2:
var=
for i in 1 2 3 4 5; do
case ${var+s} in
( s ) echo set; unset -v var;;
( '' ) echo unset; var=;;
esac
done

Actual output:
set
set
set
set
set

Expected output:
set
unset
set
unset
set


It looks like the expansion remains static within the loop, though it
should change with the variable's set/unset status.

Bizarrely, this bug only applies to ${var+set} and ${var:+nonempty}...
something like ${var-unset} and ${var:-empty} works fine:

unset -v var
for i in 1 2 3 4 5; do
case ${var-u} in
( x ) echo set; unset -v var;;
( u ) echo unset; var=x;;
esac
done

Output (OK):
unset
set
unset
set
unset


Thanks,

- Martijn