unset V; echo "[${V-d}]"
V=; echo "[${V-d}]"
unset V; echo "[${V:-"a b"}]"
unset V; echo "[${V:-'a b'}]"
unset V; echo "${V:-"a b"}"
unset V; echo "${V:-'a b'}"
unset V; echo [${V:-"a b"}]
unset V; echo [${V:-'a b'}]
unset V; echo [${V:-a b}]
unset V; echo "${V:-a b}"
unset V; echo [${V:+"a b"}]
unset V; echo "${V:+"a b"}"
unset V; echo [${V-"a b"}]
unset V; echo "${V-"a b"}"
V=x; echo "[${V:-"a b"}]"
V=x; echo "${V:-"a b"}"
unset V; echo "[${V/#*./X}]"
V=abc.txt; echo "[${V/#*./X}]"
V=abc.txt; echo "[${V//[abc]/X}]"
unset V; printf '[%s]' ${V:-"a b"}
unset V; printf '[%s]' ${V:-a b}
unset V; printf '[%s]' ${V:-'a b'}
unset V; printf '[%s]' "${V:-"a b"}"
unset V; printf '[%s]' "${V:-'a b'}"
