Bash Parameter Expansion

Operator	Parameter Set and Not Null	Parameter Set but Null	Parameter 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

References

• Parameter Expansions - Bash Hackers Wiki
• 10.2. Parameter Substitution
• Bash Parameter Expansions