Bash Parameter Expansion - To set default value to a variable in sh

parameter-expansion.sh

### variable doesn't exist
echo $variable1

variable1="${variable1:-default value}"
echo $variable1
# return default value

### variable exists
variable2="a"
echo $variable2
# reture a

variable2="${variable2:-default value}"
echo variable2
#return a

### using a variable as default
echo $variable3
default="defalut value"

variable3="${variable3:-$default}"
echo $variable3
# return default value