Linux Command Line and Shell Scripting Bible, 4th Edition

CHAPTER 12Using Structured Commands

IN THIS CHAPTER

• Working with the if-then statement
• Nesting if s
• Understanding the test command
• Testing compound conditions
• Using double brackets and parentheses
• Looking at case

In Chapter 11, “Basic Script Building,” the shell processed each individual command in the order in which it appeared within a shell script. Although this works out fine for sequential operations, many programs require some sort of logic flow control between the commands in the script.

There is a whole command class that allows the shell to skip over script sections based on tested conditions and alter the operation flow. These commands are generally referred to as structured commands.

Quite a few structured commands are available in the Bash shell, so we'll break them up into different chapters. In this chapter, we look at if - then and case statements.

Working with the if-then Statement

The most basic type of structured command is the if-then statement. The if-then statement has the following format:

if command
then
    commands
fi

If you're using if-then statements in other programming languages, this format may be somewhat confusing. In other programming languages, the object after the if statement is an equation that is evaluated for a TRUE or FALSE value. That's not how the Bash shell if statement works.

The Bash shell if statement runs the command defined on the if line. If the exit status of the command (see Chapter 11) is zero (the command completed ...