Linux® Command Line and Shell Scripting Bible, Second Edition

Chapter 11

Using Structured Commands

In This Chapter

• Working with the if-then statement
• The if-then-else statement
• Nesting ifs
• The test command
• Compound condition testing
• Advanced if-then features
• The code command
• Managing user accounts

In the shell scripts presented in Chapter 10, the shell processed each individual command in the shell script in the order it appeared. This works out fine for sequential operations, where you want all of the commands to process in the proper order. However, this isn't how all programs operate.

Many programs require some sort of logic flow control between the commands in the script. This means that the shell executes certain commands given one set of circumstances, but it has the ability to execute other commands given a different set of circumstances. There is a whole class of commands that allows the script to skip over or loop through commands based on conditions of variable values or the result of other commands. These commands are generally referred to as structured commands.

The structured commands allow you to alter the flow of operation of a program, executing some commands under some conditions while skipping others under other conditions. There are quite a few structured commands available in the bash shell, so we'll look at them individually. In this chapter, we look at the if-then statement.

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 ...