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Arithmetic Expansion
The shell allows arithmetic to be performed by expansion. This allows us to use the shell prompt as a calculator:
[me@linuxbox ~]$ echo $((2 + 2))
4
[me@linuxbox ~]$ echo $((2 + 2))
4
Arithmetic expansion uses the form:
$((expression))
where expression is an arithmetic expression consisting of values and arithmetic opera- tors.
Arithmetic expansion only supports integers (whole numbers, no decimals), but can per- form quite a number of different operations. Here are a few of the supported operators:
Table 7-1: Arithmetic Operators
Operator Description
+ Addition
- Subtraction
* Multiplication
/ Division (but remember, since expansion only supports integer arithmetic, results are integers).
% Modulo, which simply means, “ remainder.”
** Exponentiation
Spaces are not significant in arithmetic expressions and expressions may be nested. For example, to multiply 5 squared by 3:
[me@linuxbox ~]$ echo $(($((5**2)) * 3))
75
[me@linuxbox ~]$ echo $(($((5**2)) * 3))
75
Single parentheses may be used to group multiple subexpressions. With this technique, we can rewrite the example above and get the same result using a single expansion in- stead of two:
[me@linuxbox ~]$ echo $(((5**2) * 3))
75
[me@linuxbox ~]$ echo $(((5**2) * 3))
75
Here is an example using the division and remainder operators. Notice the effect of inte- ger division:
[me@linuxbox ~]$ echo Five divided by two equals $((5/2))
Five divided by two equals 2
[me@linuxbox ~]$ echo with $((5%2)) left over.
with 1 left over.
[me@linuxbox ~]$ echo Five divided by two equals $((5/2))
Five divided by two equals 2
[me@linuxbox ~]$ echo with $((5%2)) left over.
with 1 left over.
Arithmetic expansion is covered in greater detail in Chapter 34.