question about disk_var.sh

[Thomas Neumann]

Hello

I was always assuming that disk_var.sh just contains simple variable 
definitions like:

SWAPLIST="/dev/sda2"
BOOT_DEVICE="/dev/sda"
ROOT_PARTITION=/dev/sda1
BOOT_PARTITION=/dev/sda1

but either that's not the case or there is a bug in my configuration. The 
actual content is:

SWAPLIST=${SWAPLIST:-"/dev/sda2"}
BOOT_DEVICE=${BOOT_DEVICE:-"/dev/sda"}
ROOT_PARTITION=${ROOT_PARTITION:-/dev/sda1}
BOOT_PARTITION=${BOOT_PARTITION:-/dev/sda1}

(Same output for setup_storage 1.5 and 1.6.)

Sourcing the file and printing the variables does indeed yield the expected 
values.

a) can someone confirm his setup_storage generates the same strange values?
b) can someone explain what kind of bash trickery is going on here?

bye
thomas