Book Image

Linux Command Line and Shell Scripting Bible - Third Edition

By : Richard Blum, Christine Bresnahan
Book Image

Linux Command Line and Shell Scripting Bible - Third Edition

By: Richard Blum, Christine Bresnahan

Overview of this book

The Linux command line enables you to type specific shell commands directly into the system to manipulate files and query system resources. Command line statements can be combined into short programs called shell scripts, a practice increasing in popularity due to its usefulness in automation. Linux is a robust system with tremendous potential, and Linux Command Line and Shell Scripting Bible opens the door to new possibilities. Linux Command Line and Shell Scripting Bible is your essential Linux guide. It contains new functional examples that are fully updated to align with the latest Linux features. Beginning with command line fundamentals, the book moves into shell scripting and shows you the practical application of commands in automating frequently performed functions. This book is a complete guide providing detailed instruction and expert advice working within this aspect of Linux. Whether used as a tutorial or as a quick reference, this book contains information that every Linux user should know.
Table of Contents (34 chapters)
Part I: The Linux Command Line
Part II: Shell Scripting Basics
Part III: Advanced Shell Scripting
Part IV: Creating Practical Scripts
End User License Agreement

The while Command

The while command is somewhat of a cross between the if-then statement and the for loop. The while command allows you to define a command to test and then loop through a set of commands for as long as the defined test command returns a zero exit status. It tests the test command at the start of each iteration. When the test command returns a non-zero exit status, the while command stops executing the set of commands.

Basic while format

Here's fhe format of the while command:

 while test command
  other commands

The test command defined in the while command is the exact same format as in if-then statements (see Chapter 12). As in the if-then statement, you can use any normal bash shell command, or you can use the test command to test for conditions, such as variable values.

The key to the while command is that the exit status of the test command specified must change, based on the commands run during the loop. If the exit status never changes, the while...