expand to word if the expanded value of param is unset or null, and $param otherwise.
To use negati... ameter:-word}
Provide a default if parameter is unset or null.
Example:
echo ${1:-"default"}
Note:... Assign a value to parameter if it was previously unset or null.
Example:
echo ${HOME:="/home/.nohom... ameter:?word}
Generate an error if parameter is unset or null by printing word to stdout.
Example:
