Conditional substitution in the Bourne shell

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Normally, the shell replaces the expression $Variable with the string value assigned to the Variable variable, if there is one. However, there is a special notation that allows conditional substitution, depending on whether the variable is set or not null, or both.

By definition, a variable is set if it has ever been assigned a value. The value of a variable can be the null string, which you can assign to a variable in any one of the following ways:

Item	Description
`A=`
`bcd=""`
`Efg=''`	Assigns the null string to the `A`, `bcd`, and `Efg`.
`set '' ""`	Sets the first and second positional parameters to the null string and unsets all other positional parameters.

The following is a list of the available expressions you can use to perform conditional substitution:

Item	Description
${`Variable`- `String`}	If the variable is set, substitute the `Variable` value in place of this expression. Otherwise, replace this expression with the `String` value.
${`Variable`:-`String`}	If the variable is set and not null, substitute the `Variable` value in place of this expression. Otherwise, replace this expression with the `String` value.
${`Variable`=`String`}	If the variable is set, substitute the `Variable` value in place of this expression. Otherwise, set the `Variable` value to the `String` value and then substitute the `Variable` value in place of this expression. You cannot assign values to positional parameters in this fashion.
${`Variable`:=`String`}	If the variable is set and not null, substitute the `Variable` value in place of this expression. Otherwise, set the `Variable` value to the `String` value and then substitute the `Variable` value in place of this expression. You cannot assign values to positional parameters in this fashion.
${`Variable`?`String`}	If the variable is set, substitute the `Variable` value in place of this expression. Otherwise, display a message of the following form: `Variable: String` and exit from the current shell (unless the shell is the login shell). If you do not specify a value for the `String` variable, the shell displays the following message: `Variable: parameter null or not set`
${`Variable`:?`String`}	If the variable is set and not null, substitute the Variable value in place of this expression. Otherwise, display a message of the following form: `Variable: String` and exit from the current shell (unless the shell is the login shell). If you do not specify the `String` value, the shell displays the following message: `Variable: parameter null or not set`
${`Variable`+`String`}	If the variable is set, substitute the `String` value in place of this expression. Otherwise, substitute the null string.
${`Variable`:+`String`}	If the variable is set and not null, substitute the `String` value in place of this expression. Otherwise, substitute the null string.

In conditional substitution, the shell does not evaluate the String variable until the shell uses this variable as a substituted string. Thus, in the following example, the shell executes the pwd command only if d is not set or is null:

echo ${d:-`pwd`}