Bash variable replacement (advanced tutorial)

$ {Var:-value}: If the variable VAR has a value, the value of VaR is referenced. If the value of VaR is null, the value is referenced.

Example: Read-p '... 'Abc; ABC =$ {ABC:-value}

$ {Var: = value}: If the variable VAR has a value, the value of VaR is referenced. If the value of VaR is null, the value is referenced. The value of the variable VAR is set to value.

$ {Var? = Value}: If the variable VAR has a value, reference the value of Var. If the value of the variable VAR is null, print the value in the form of a standard error.

$ {Var: + value}: If the variable VAR has a value, reference the value of value. If the value of the variable VAR is null, reference the value of var.

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