ref
--
https://www.tldp.org/LDP/abs/html/parameter-substitution.html

${parameter} same as $parameter; In certain contexts, only the less ambiguous ${parameter} form works.

$a=100
$ echo ${a}; echo $a
100
100

id=${USER}-on-${HOSTNAME}
echo $id

${parameter-default}, ${parameter:-default}	# If parameter not set, use default. 

var1=1
var2=2
# var3 is unset.

echo ${var1-$var2}   # 1
echo ${var3-$var2}   # 2

$ default_weight=65
$ john=70
$ w=${john-$default_weight}	# 70
$ w=${jack-$default_weight}	# 65

${parameter-default} and ${parameter:-default} are almost equivalent. 
variable=		# variable has been declared, but is set to null.

echo "${variable-0}"    # (no output)
echo "${variable:-1}"   # 1

unset variable
echo "${variable-2}"    # 2
echo "${variable:-3}"   # 3