Book Image

Learning Linux Shell Scripting

By : Ganesh Sanjiv Naik
Book Image

Learning Linux Shell Scripting

By: Ganesh Sanjiv Naik

Overview of this book

Linux is the one of the most powerful and universally adopted OSes. Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell can read this file and act on the commands as if they were typed on the keyboard. Shell scripting is used to automate day-to-day administration, and for testing or product development tasks. This book covers Bash, GNU Bourne Again SHell, preparing you to work in the exciting world of Linux shell scripting. We start with an introduction to the Shell environment and explain basic commands used in Shell. Next we move on to check, kill, and control the execution of processes in Linux OS. Further, we teach you about the filter tools available in Linux and explain standard output and standard errors devices. Then we will ensure you understand Shell’s interpretation of commands and get a firmer grasp so you use them in practice. Next, you’ll experience some real-world essentials such as debugging and perform Shell arithmetic fluently. Then you’ll take a step ahead and learn new and advanced topics in Shell scripting, such as starting up a system and customizing a Linux system. Finally, you’ll get to understand the capabilities of scripting and learn about Grep, Stream Editor, and Awk.
Table of Contents (20 chapters)
Learning Linux Shell Scripting
Credits
About the Author
Acknowledgments
About the Reviewers
www.PacktPub.com
Preface
Index

Learning shell interpretation of commands


When we log in, the $ sign will be visible in the shell terminal (# prompt if logged in as root or administrator). The Bash shell runs scripts as interpreter. Whenever we type a command, the BASH shell will read them as series of words (tokens). Each word is separated by a space ( ), semi colon (;), or any other command delimiter. We terminate the command by pressing the Enter key. This will insert a new line character at the end of the command. The first word is taken as a command, then consecutive words are treated as options or parameters.

The shell processes the command line as follows:

  • If applicable, substitution of history commands

  • Converting command line into tokens and words

  • Updating history

  • Processing of quotes

  • Defining functions and substitution of alias

  • Setting up of pipes, redirection, and background

  • Substitution of variables (such as $name and $user) is performed

  • Command substitution (echo `cal` and echo `date`) is performed

  • Globing is performed...