All the variables that we described in the previous sections can be defined thanks to 2 possible syntaxes which are ${variable-name} or ${variable-name:default-value}.
The first syntax does not define any default value. So if the variable is not set, the value will be ${variable-name} to indicate that it could not be resolved.
The second syntax allows you to define the default value after the semicolon. So if the variable is not set, the value will be the given default value.