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Arithmetic in Shell Programs

Although the original Bourne shell has no built-in arithmetic handlers, arithmetic can still be done using command substitution along with the expr command. For example:

var3=`expr var1 + var2`

This adds the values contained in var1 and var2 together and then stores the result in var3. Note that the arguments and the operator must be separated by whitespace. expr var1+var2 will not have the intended result.

The expr command can do only very simple arithmetic—it can handle only integer arithmetic. Entering floating point numbers will cause an error. In addition, changing the order of operations with parentheses is not supported. Division that does not return an integer value will have the decimal portion dropped. ...