Introduction to Linux Shell and Shell Scripting

Unix architecture

• The software that controls the computer's hardware resources and provides an environment under which programs can run is Kernel.

• The interface to the Kernel is the protection of software called the system calls.
• The shell is a unique application that gives a prompt for running different applications.
• In the long run, an operating system consists of the Kernel and other software that makes a helpful computer and gives it its personality.
• For example, Linux is the Kernel used by the GNU operating system. Generally, It's referred to this combination as the GNU/Linux operating system, but it is more commonly referred to as simply Linux.

Shell Scripting:

The shell script is like the batch file in MS-DOS but has more power than the MS-DOS batch file. A shell script can take input from the user file and output it to the screen. It is Useful to create commands that can save us lots of time and automate some tasks.

Variables in Linux

Sometimes, variables must be kept in a computer's RAM to process our data. RAM is divided into remote locations, and each area has a distinct value called memory address, and it is used to store the data. Programmers can give a specific name to this memory location/address called memory variable.

In Linux, the Variables are of mainly two types:

1. System variables: This type of variable is defined in CAPITAL LETTERS.
2. User-defined variables: Created and maintained by the User. This type of variable is defined in lowercase LETTERS.

Examples of System Variables

Some of the above configurations can be different on your device. You can use any of the above to print variables contained as follows.

Defining User-defined variables

variable name=value 

NOTE: Here' value' is assigned to given 'variable name' and value has to be on right side = sign 

For Example:

Writing Shell Script

We will display our first script as "Each word has its value" on screen. Write a shell script you can use in Linux's text editor such as vi or credit, or even use the cat command. Here we are using the "cat" command. You may use any of the above text editors—first type following cat command and rest of text as it is.

Press Ctrl + D to save. Now our script is ready. To execute it, type the command

The first screen will be clear, and then "Each word has its value." is printed on the screen. To print a message of variables containing we user echo command, the general form of echo command is as follows

echo "Message"

echo "Message variable1, variable2....variable N"

Running Shell Script

Because of the security of files, in Linux, the creator of Shell Script does not get execution permission by default. So if we wish to run a shell script, we have to do two things as follows

1. Use the chmod command as follows to give execution permission to our script.

Syntax: chmod +x  shell-script-name 

OR Syntax: chmod 777 shell-script-name

1. Run our script as

Syntax: ./your-shell-program-name

For e.g.

Note that to run the script, you need to have it in the same directory where you created your script.

Frequently Asked Questions

Q1. What is shell scripting?

A shell scripting is a text file that contains a sequence of commands for a UNIX-based operating system. In Linux, shells like bash and Korn support programming constructs saved as scripts. These scripts become shell commands, and hence many Linux commands are scripts.

Q2. What are the applications of shell scripting?

Shell scripting is easy and effective. It allows us to program commands in chains and have the system execute them as a scripted event, just like batch files. It creates customized datasets on the fly and wraps programs over which you have no control inside an environment that you can control.

Q3.What is Input/Output Redirection?

When you execute a command, the command is read from the keyboard (the standard input), and the result of the command(output) appears on the screen (the standard output). If you want the command output to appear in a different place, or if you want to send the output to another command for processing, then we need to change the output with a process called Input/output redirection.

Key Takeaways

We learned about shell programming and its different types in brief. We understood the basics of Unix architecture.e, Kernel, system calls, and library routine. We understood the concept of Linux variables and their types, i.e., user-defined variables and system variables. We also learned how to write and run Shell Scripts in Linux Environment with examples.

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