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Mastering Linux Shell Scripting
Mastering Linux Shell Scripting

Mastering Linux Shell Scripting: A practical guide to Linux command-line, Bash scripting, and Shell programming , Second Edition

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Profile Icon Mokhtar Ebrahim Profile Icon Andrew Mallett
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Full star icon Full star icon Full star icon Half star icon Empty star icon 3.8 (23 Ratings)
eBook Apr 2018 284 pages 2nd Edition
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Arrow left icon
Profile Icon Mokhtar Ebrahim Profile Icon Andrew Mallett
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Full star icon Full star icon Full star icon Half star icon Empty star icon 3.8 (23 Ratings)
eBook Apr 2018 284 pages 2nd Edition
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Mastering Linux Shell Scripting

Creating Interactive Scripts

In Chapter 1, The What and Why of Scripting with Bash, we learned how to create a script and use some of its basic elements. These included optional parameters that we can pass through to the script when it is executed. In this chapter, we will extend this by using the shell's built-in read command to allow for interactive scripts. Interactive scripts are scripts that prompt for information during the script's execution.

In this chapter, we will cover the following topics:

  • Using echo with options
  • Basic script using read
  • Script comments
  • Enhancing read scripts with read prompts
  • Limiting the number of entered characters
  • Controlling the visibility of the entered text
  • Passing options
  • Read options values
  • Try to be standard
  • Enhancing learning with simple scripts

Technical requirements

Using echo with options

So far, in this book we have been able to see that the echo command is very useful and is going to be used in many of our scripts, if not all of them. When running the echo command, the built-in command will be used unless we state the full path to the file. We can test this with the following command:

$ which echo

To gain help on the built-in command, we can use man bash and search for echo; however, the echo command is identical to the internal command, so we recommend that you use man echo in most cases in order to display the command options.

The basic use of echo that we have seen so far will produce a text output and a new line. This is often the desired response, so we don't need to be concerned that the next prompt will append to the end of the echoed text. The new line separates the script output from the next shell prompt. If we do not supply...

Basic script using read

When used as part of a script that prompts for user input, the suppression of the line feed is exactly what we want. We will begin by copying the existing hello2.sh script to hello3.sh and build an interactive script. Initially, we will use echo as the prompt mechanism, but, as we gradually enhance the script, we will generate the prompt directly from the shell built-in read command:

$ cp $HOME/bin/hello2.sh $HOME/bin/hello3.sh
$ chmod +x $HOME/bin/hello3.sh

Edit the $HOME/bin/hello3.sh script so that it reads as follows:

#!/bin/bash
echo -n "Hello $(basename $0)! May I ask your name: "
read
echo "Hello $REPLY"
exit 0

As we execute the script, we will be greeted and prompted with whatever is typed. This is echoed using the $REPLY variable in the echo statement. As we have not yet supplied a variable name to the read built-in command, the...

Script comments

We should always introduce commenting scripts early in the piece. A script comment is prefaced with a # symbol. Anything after the # symbol is a comment and is not evaluated by the script. The shebang, #!/bin/bash, is primarily a comment and, as such, is not evaluated by the shell. The shell running the script reads the whole shebang, so it knows which command interpreter to hand the script over to. A comment may be at the start of a line or partway into the line. Shell scripting does not have the notion of multi-line comments.

If you are not already familiar with comments, then please note that they are added to the script to describe who wrote the script, when it was written and last updated, and what the script does. They are the metadata of the script.

The following is an example of comments in scripts:

#!/bin/bash
# Welcome to bash scripting
# Author: Mokhtar...

Enhancing scripts with read prompts

We have seen how we can use the built-in read to populate a variable. So far, we have used echo to produce the prompt, but this can be passed to read itself using the -p option. The read command will surpass the additional linefeed, so we reduce both the line count and the complexity to some degree.

We can test this at the command line itself. Try typing the following command to see read in action:

$ read -p "Enter your name: " name

We use the read command with the -p option. The argument that follows the option is the text that appears in the prompt. Normally, we would make sure that there is a trailing space at the end of the text to ensure that we can clearly see what we type. The last argument supplied here is the variable we want to populate; we simply call it name. Variables are case-sensitive too. Even if we do not supply the...

Limiting the number of entered characters

We have not needed this functionality in the scripts we have used so far, but we may need to ask users to hit any key to continue. At the moment, we have set it up in such a way that the variable is not populated until we hit the Enter key. Users have to hit Enter to continue. If we use the -n option followed by an integer, we can specify the number of characters to accept before continuing; we will set 1 in this case. Take a look at the following code extract:

#!/bin/bash
read -p "May I ask your name: " name
echo "Hello $name"
read -n1 -p "Press any key to exit"
echo
exit 0

Now the script will pause after displaying the name until we press any key; we can literally press any key before continuing, as we accept just 1 key stroke, whereas earlier we were required to leave the default behavior in place, as we could...

Controlling the visibility of the entered text

Even though we have limited the input to a single character, we do get to see the text on the screen. In the same way, if we type the name, we get to see the entered text before we hit Enter. In this case, it is just untidy, but if we are entering sensitive data, such as a PIN or a password, we should hide the text. We can use the silent option, or -s, to achieve this. A simple edit in the script will set this in place:

#!/bin/bash
read -p "May I ask your name: " name
echo "Hello $name"
read -sn1 -p "Press any key to exit"
echo
exit 0

Now, when we use a key to continue, it will not be displayed on the screen. We can see the behavior of the script in the following screenshot:

Passing options

So far, we have seen in the first chapter how to read parameters from the user. Also, you can pass options. So, what are options? And how are they different from parameters?

Options are characters with a single dash before them.

Check out this example:

$ ./script1.sh -a

The -a is an option. You can check from your script if the user entered this option; if so, then your script can behave in some manner.

You can pass multiple options:

$ ./script1.sh -a -b -c

To print these options, you can use the $1, $2, and $3 variables:

#!/bin/bash
echo $1
echo $2
echo $3

We should check these options, but, since we haven't discussed conditional statements yet, we will keep it simple for now.

Options can be passed with a value, like this:

$ ./script1.sh -a -b 20 -c

Here the -b option is passed with a value of 20.

As you can see, the variable $3=20, which is the passed...

Try to be standard

You may use bash scripts from GitHub, and you may notice that there is a standard option scheme that is followed. It's not required, but it is preferable.

These are some of the commonly used options:

  • -a: List all items
  • -c: Get a count of all items
  • -d: Output directory
  • -e: Expand items
  • -f: Specify a file
  • -h: Show the help page
  • -i: Ignore the character case
  • -l: List a text
  • -o: Send output to a file
  • -q: Keep silent; don't ask the user
  • -r: Process something recursively
  • -s: Use stealth mode
  • -v: Use verbose mode
  • -x: Specify an executable
  • -y: Accept without prompting me

Enhancing learning with simple scripts

Our scripts are still a little trivial, and we have not looked at conditional statements so we can test for correct input, but let's take a look at some simple scripts that we can build with some functionality.

Backing-up with scripts

Now that we have created some scripts, we may want to back these up to a different location. If we create a script to prompt us, we can choose the location and the type of files that we want to backup.

Consider the following script for your first practice. Create the script and name it $HOME/backup.sh:

#!/bin/bash
# Author: @theurbanpenguin
# Web: www.theurbapenguin.com
# Script to prompt to back up files and location
# The files will be search on from...

Summary

Feel proud that you now have your I can read badge for shell scripting. We have developed our scripts to be interactive and to prompt users for input during the script execution. These prompts can be used to simplify user operations on the command line. In this way, they do not need to remember the command-line options or have passwords that end up stored in the command-line history. When using passwords, we can simply store the value using the read -sp options.

Also, we saw how to pass options with and without values, and how to identify values correctly. We saw how to pass options and parameters at the same time, thanks to the double dash.

In the next chapter, we will take our time to look at the conditional statements in bash.

Questions

  1. How many comments are in the following code?
#!/bin/bash
# Welcome to shell scripting
# Author: Mokhtar
  1. If we have the following code:
#!/bin/bash
echo $1
echo $2
echo $3

And we run the script with these options:

$ ./script1.sh -a -b50 -c

What is the result of running this code?

  1. Check the following code:
#!/bin/bash
shift
echo $#

If we run it with these options:

$ ./script1.sh Mokhtar -n -a 35 -p
    1. What is the result?
    2. What is the dropped parameter?
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Key benefits

  • •Identify high-level steps such as verifying user input
  • •Using the command line and conditional statements in creating/executing simple shell scripts
  • •Create and edit dynamic shell scripts to manage complex and repetitive tasks
  • •Leverage the command-line to bypass GUI and automate common tasks

Description

In this book, you’ll discover everything you need to know to master shell scripting and make informed choices about the elements you employ. Grab your favorite editor and start writing your best Bash scripts step by step. Get to grips with the fundamentals of creating and running a script in normal mode, and in debug mode. Learn about various conditional statements' code snippets, and realize the power of repetition and loops in your shell script. You will also learn to write complex shell scripts. This book will also deep dive into file system administration, directories, and system administration like networking, process management, user authentications, and package installation and regular expressions. Towards the end of the book, you will learn how to use Python as a BASH Scripting alternative. By the end of this book, you will know shell scripts at the snap of your fingers and will be able to automate and communicate with your system with keyboard expressions.

Who is this book for?

If you are a Linux administrator or a system administrator and are interested in automating tasks in your daily lives, saving time and effort, this book is for you. Basic shell scripting and command-line experience will be required. Familiarity with the tasks you need to automate will be helpful.

What you will learn

  • •Make, execute, and debug your first Bash script
  • •Create interactive scripts that prompt for user input
  • •Foster menu structures for operators with little command-line experience
  • •Develop scripts that dynamically edit web configuration files to produce a new virtual host
  • •Write scripts that use AWK to search and reports on log files
  • •Draft effective scripts using functions as building blocks, reducing maintenance and build time
  • •Make informed choices by comparing different script languages such as Python with BASH

Product Details

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Publication date, Length, Edition, Language, ISBN-13
Publication date : Apr 19, 2018
Length: 284 pages
Edition : 2nd
Language : English
ISBN-13 : 9781788990158
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Table of Contents

16 Chapters
The What and Why of Scripting with Bash Chevron down icon Chevron up icon
Creating Interactive Scripts Chevron down icon Chevron up icon
Conditions Attached Chevron down icon Chevron up icon
Creating Code Snippets Chevron down icon Chevron up icon
Alternative Syntax Chevron down icon Chevron up icon
Iterating with Loops Chevron down icon Chevron up icon
Creating Building Blocks with Functions Chevron down icon Chevron up icon
Introducing the Stream Editor Chevron down icon Chevron up icon
Automating Apache Virtual Hosts Chevron down icon Chevron up icon
AWK Fundamentals Chevron down icon Chevron up icon
Regular Expressions Chevron down icon Chevron up icon
Summarizing Logs with AWK Chevron down icon Chevron up icon
A Better lastlog with AWK Chevron down icon Chevron up icon
Using Python as a Bash Scripting Alternative Chevron down icon Chevron up icon
Assessments Chevron down icon Chevron up icon
Other Books You May Enjoy Chevron down icon Chevron up icon

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This book is a must have for those that want to get right at it putting good & meaningful scripts together. Plenty of great examples and explains in such a easy to follow format.
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This is book is really worth having. As with any knowledge you use, the stuff you learned and don't use tends to get swapped out of memory (your head). You end up using the same ways and tools over and over, even though they might not be the best way or may not even get the job done.The author here has really captured a style that brings a newbie up to speed but also has enough depth to be a mental reference for old pros. I've not seen another that does that, and I think it's his get-to-the-point style and non-trivial examples that do it. I find I go back to it for review often. I don't have to wade through pages of silly newbie talk to find some help, and the depth taught me some new things and new uses!Most other books seem to fill up pages with a copy of the man pages, template style wording, and stuff you can find repeated everywhere. This book doesn't do that. Instead, his uses give enough to get the sense of how to use a tool like sed or awk or any of the others while also giving examples that "as is" can come in handy - because it seems to draw from real-world uses and needs. So in that sense, it even has a second benefit to offer.I'm very happy with the book. It's by a real programmer who is good at what he does, and doesn't talk 'down' to the reader. It's also enjoyable to read, without the pedantic approach most others take. I waited a long while to review just to see if it was "love at first sight" or something more. It has proven itself many times to this day, so I am comfortable giving it some praise and a recommendation!
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Precise and well understandable, good amount of knowledge for its length
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