Ksh93 (or bash) doesn’t have such expressions, so it’s better
to make it explicit. But you can bundle multiple variables
(with their initial values) in a single export phrase:
export A=1 B=2 C=3
Testing:
$ (export A=1 B=2 C=3 && ksh -c 'echo A=$A B=$B C=$C D=$D')
A=1 B=2 C=3 D=
Awkward alternatives
There is no C-like shortcut, unless you want this ugly thing:
A=${B:=${C:=1}}; echo $A $B $C
1 1 1
… which does not work with export, nor does it work when B or C are empty or non-existent.
Arithmetic notation
Ksh93 arithmetic notation does actually support C-style chained assignments, but for obvious reasons, this only works with numbers, and you’ll then have to do the export separately:
$ ((a=b=c=d=1234))
$ echo $a $b $c $d
1234 1234 1234 1234
$ export a b d
$ ksh -c 'echo a=$a b=$b c=$c d=$d' # Single quotes prevent immediate substitution
a=1234 b=1234 c= d=d1234 # so new ksh instance has no value for $c
Note how we do not export c, and its value in the child shell is indeed empty.