Expansions are required to deliver a value to script. We will discuss arithmetic expansion and parameter expansion here. Placing '$' prior to a variable expands its value.
Arithmetic expansion is not part ofportable shell syntax, but was added as aPOSIX extension. Arithmetic expansion is achieved using a $((operation)) construct. Parameter expansion is achieved by using a ${parameter} construct.
Let's give an example of arithmetic expansion:
one=1$(($one+1))
$(($one+1)) will have the value of 2.
Let's try to do some examples for parameter expansion. Let's say we want to remove a string 'Hello' from 'HelloBob'. The following will do the trick:
string='HelloBob' ${string%Bob}${string%Bob} portion cuts out 'Bob' from end of variable string, so we are left with Hello as its output.
If variable string had value of 'HelloBobBob', ${string%%Bob*} would do the same trick. The '%%' portion is required because it signifies to remove the longest pattern matching 'Bob*'. So, we are also left with 'Hello'.
If '%' is replaced with '#' and '%%' replaced with '##', this signifies the same meaning but this time from beginning of parameter, not from end. For example, the following code would remove 'Hello' from 'HelloBobBob':
${string#Hello}Following subsections represent a syntax for handling of situations, where variable may be undefined.
${param-value}Use 'value' if param's original value is null, otherwise value ofparam variable is taken.
${param?value}Use param if defined, otherwise print value to output – it effectively aborts the command.
${param+value}Use 'value' if param is defined (otherwise an empty string).
${param=value}Use 'value' and set param to 'value' if param is undefined.
Besides the above-mentioned syntax, there is also a syntax with a colon preceding the-/?/+/=, eg.:
${param:?Parameter param must be set and non-empty}Which for all four operators above causes thatempty variables are handled in the same way as undefined ones in the above-mentioned definition.