Two of the fundamental types of operations in any programming language are the comparison and the test. They are the building blocks of conditional statements (
case) and iteration statements (
while). They give you the ability to examine relationships between data items, such as files, variables, strings, and numeric values, and to take action based on the result. You can also carry out actions based on the attributes of a particular item, such as a file's type, whether a variable has been assigned a value, and the return code of a command.
The examples in this chapter illustrate various ways to compare items or to check their properties, and use the results. I have used all of these techniques throughout ...