Parameter substitution in Linux shell

 Following the  table of parameter substitution in shell based on if parameter is set / empty (‘null’) / or unset:

+--------------------+----------------------+-----------------+-----------------+

|                    |         Set          |   Empty('null') |      Unset      |

+--------------------+----------------------+-----------------+-----------------+

| ${parameter:-word} | substitute parameter | substitute word | substitute word |

| ${parameter-word}  | substitute parameter | substitute null | substitute word |

| ${parameter:=word} | substitute parameter | assign word     | assign word     |

| ${parameter=word}  | substitute parameter | substitute null | assign word     |

| ${parameter:?word} | substitute parameter | error, exit     | error, exit     |

| ${parameter?word}  | substitute parameter | substitute null | error, exit     |

| ${parameter:+word} | substitute word      | substitute null | substitute null |

| ${parameter+word}  | substitute word      | substitute word | substitute null |

+--------------------+----------------------+-----------------+-----------------+

${parameter-substitute_value} --> Use substitute only if parameter is 'not set' 

${parameter:-substitute_value} --> Use substitute when parameter is 'not set' and also when parameter is set but empty ('null')