Use a default value #

${parameter:-word} - If parameter is empty/unset results in word, else results in parameter

Assign a default value #

${parameter:=word} - If parameter is empty/unset results in word and word is assigned to parameter, else results in parameter.

Note: Positional and other special parameters cannot be assigned this way.

Display error if null or unset #

${parameter:?word} - If parameter is empty/unset script exits and word sent to stderr, else results in parameter.

Use an alternate value #

${parameter:+word} - If parameter is empty/unset expansion results in nothing, else the value of word is substituted for parameter, but parameter is not changed.