Assigning default values to shell variables with a single command in bash

Very close to what you posted, actually.

To get the assigned value, or default if it's missing:

FOO="${VARIABLE:-default}"  # If variable not set or null, use default.

Or to assign default to VARIABLE at the same time:

FOO="${VARIABLE:=default}"  # If variable not set or null, set it to default.

For command line arguments:

VARIABLE="${1:-$DEFAULTVALUE}"

which assigns to VARIABLE the value of the 1st argument passed to the script or the value of DEFAULTVALUE if no such argument was passed. Qouting prevents globbing and word splitting.

If the variable is same, then

: "${VARIABLE:=DEFAULT_VALUE}"

assigns DEFAULT_VALUE to VARIABLE if not defined. The double quotes prevent globbing and word splitting.

Also see Section 3.5.3, Shell Parameter Expansion, in the Bash manual.