Linux Command Line and Shell Scripting Bible, 5th Edition

Chapter 13More Structured Commands

In This Chapter

• Looking at the for command
• Trying the C-style for command
• Exploring the while command
• Using the until command
• Nesting loops
• Looping on file data
• Controlling the loop
• Processing the output of a loop

In the previous chapter, you saw how to manipulate the flow of a shell script program by checking the output of commands and the values of variables. This chapter continues to look at structured commands that control the flow of your shell scripts. You’ll see how you can perform repeating processes, commands that can loop through a set of commands until an indicated condition has been met. This chapter discusses and demonstrates the for, while, and until Bash shell looping commands.

Looking at the for Command

Iterating through a series of commands is a common programming practice. Often, you need to repeat a set of commands until a specific condition has been met, such as processing all the files in a directory, all the users on a system, or all the lines in a text file.

The Bash shell provides the for command to allow you to create a loop that iterates through a series of values. Each iteration performs a defined set of commands using one of the values in the series. Here’s the basic format of the Bash shell for command:

for var in list
do
   commands
done

You supply the series of values used in the iterations in the list parameter. You can specify the values in the list in several ways.

In each iteration, the var variable ...