Commands in shell script in linux

Shell Scripting for Beginners – How to Write Bash Scripts in Linux

Shell scripting is an important part of process automation in Linux. Scripting helps you write a sequence of commands in a file and then execute them.

This saves you time because you don’t have to write certain commands again and again. You can perform daily tasks efficiently and even schedule them for automatic execution.

You can also set certain scripts to execute on startup such as showing a particular message on launching a new session or setting certain environment variables.

The applications and uses of scripting are numerous, so let’s dive in.

In this article, you will learn:

1. What is a bash shell?
2. What is a bash script and how do you identify it?
3. How to create your first bash script and execute it.
4. The basic syntax of shell scripting.
5. How to see a system’s scheduled scripts.
6. How to automate scripts by scheduling via cron jobs.

The best way to learn is by practicing. I highly encourage you to follow along using Replit. You can access a running Linux shell within minutes.

Introduction to the Bash Shell

The Linux command line is provided by a program called the shell. Over the years, the shell program has evolved to cater to various options.

Different users can be configured to use different shells. But most users prefer to stick with the current default shell. The default shell for many Linux distros is the GNU Bourne-Again Shell (bash). Bash is succeeded by Bourne shell ( sh ).

When you first launch the shell, it uses a startup script located in the .bashrc or .bash_profile file which allows you to customize the behavior of the shell.

When a shell is used interactively, it displays a $ when it is waiting for a command from the user. This is called the shell prompt.

If shell is running as root, the prompt is changed to # . The superuser shell prompt looks like this:

Bash is very powerful as it can simplify certain operations that are hard to accomplish efficiently with a GUI. Remember that most servers do not have a GUI, and it is best to learn to use the powers of a command line interface (CLI).

What is a Bash Script?

A bash script is a series of commands written in a file. These are read and executed by the bash program. The program executes line by line.

For example, you can navigate to a certain path, create a folder and spawn a process inside it using the command line.

You can do the same sequence of steps by saving the commands in a bash script and running it. You can run the script any number of times.

How Do You Identify a Bash Script?

File extension of .sh .

By naming conventions, bash scripts end with a .sh . However, bash scripts can run perfectly fine without the sh extension.

Scripts start with a bash bang.

Scripts are also identified with a shebang . Shebang is a combination of bash # and bang ! followed the the bash shell path. This is the first line of the script. Shebang tells the shell to execute it via bash shell. Shebang is simply an absolute path to the bash interpreter.

Below is an example of the shebang statement.

The path of the bash program can vary. We will see later how to identify it.

Execution rights

Scripts have execution rights for the user executing them.

An execution right is represented by x . In the example below, my user has the rwx (read, write, execute) rights for the file test_script.sh

File colour

Executable scripts appear in a different colour from rest of the files and folders.

In my case, the scripts with execution rights appear as green.

How to Create Your First Bash Script

Let’s create a simple script in bash that outputs Hello World .

Create a file named hello_world.sh

Find the path to your bash shell.

In my case, the path is /usr/bin/bash and I will include this in the shebang.

Write the command.

We will echo «hello world» to the console.

Our script will look something like this:

Edit the file hello_world.sh using a text editor of your choice and add the above lines in it.

Provide execution rights to your user.

Modify the file permissions and allow execution of the script by using the command below:

chmod modifies the existing rights of a file for a particular user. We are adding +x to user u .

Run the script.

You can run the script in the following ways:

Here’s the output:

Two ways to run scripts

The Basic Syntax of Bash Scripting

Just like any other programming language, bash scripting follows a set of rules to create programs understandable by the computer. In this section, we will study the syntax of bash scripting.

How to define variables

We can define a variable by using the syntax variable_name=value . To get the value of the variable, add $ before the variable.

Tux is also the name of the Linux mascot, the penguin.

Hi, I am Tux.

Arithmetic Expressions

Below are the operators supported by bash for mathematical calculations:

Operator	Usage
+	addition
—	subtraction
*	multiplication
/	division
**	exponentiation
%	modulus

Let’s run a few examples.

Note the spaces, these are part of the syntax

Numerical expressions can also be calculated and stored in a variable using the syntax below:

Let’s try an example.

Fractions are not correctly calculated using the above methods and truncated.

For decimal calculations, we can use bc command to get the output to a particular number of decimal places. bc (Bash Calculator) is a command line calculator that supports calculation up to a certain number of decimal points.

echo «scale=2;22/7» | bc

Where scale defines the number of decimal places required in the output.

Getting output to 2 decimal places

How to read user input

Sometimes you’ll need to gather user input and perform relevant operations.

In bash, we can take user input using the read command.

To prompt the user with a custom message, use the -p flag.

read -p «Enter your age» variable_name

Example:

Numeric Comparison logical operators

Comparison is used to check if statements evaluate to true or false . We can use the below shown operators to compare two statements:

Operation	Syntax	Explanation
Equality	num1 -eq num2	is num1 equal to num2
Greater than equal to	num1 -ge num2	is num1 greater than equal to num2
Greater than	num1 -gt num2	is num1 greater than num2
Less than equal to	num1 -le num2	is num1 less than equal to num2
Less than	num1 -lt num2	is num1 less than num2
Not Equal to	num1 -ne num2	is num1 not equal to num2

Syntax:

Example:

Let’s compare two numbers and find their relationship:

Conditional Statements (Decision Making)

Conditions are expressions that evaluate to a boolean expression ( true or false ). To check conditions, we can use if , if-else , if-elif-else and nested conditionals.

The structure of conditional statements is as follows:

• if. then. fi statements
• if. then. else. fi statements
• if..elif..else..fi
• if..then..else..if..then..fi..fi.. (Nested Conditionals)

Syntax:

To create meaningful comparisons, we can use AND -a and OR -o as well.

The below statement translates to: If a is greater than 40 and b is less than 6.

if [ $a -gt 40 -a $b -lt 6 ]

Example: Let’s find the triangle type by reading the lengths of its sides.

Output:

Looping and skipping

For loops allow you to execute statements a specific number of times.

Looping with numbers:

In the example below, the loop will iterate 5 times.

Looping with strings:

We can loop through strings as well.

While loop

While loops check for a condition and loop until the condition remains true . We need to provide a counter statement that increments the counter to control loop execution.

In the example below, (( i += 1 )) is the counter statement that increments the value of i .

Example:

Reading files

Suppose we have a file sample_file.txt as shown below:

We can read the file line by line and print the output on the screen.

Output:

Lines with line number printed

How to execute commands with back ticks

If you need to include the output of a complex command in your script, you can write the statement inside back ticks.

Syntax:

Example: Suppose we want to get the output of a list of mountpoints with tmpfs in their name. We can craft a statement like this: df -h | grep tmpfs .

To include it in the bash script, we can enclose it in back ticks.

How to get arguments for scripts from the command line

It is possible to give arguments to the script on execution.

$@ represents the position of the parameters, starting from one.

Run it like this:

./script arg1 arg2

How to Automate Scripts by Scheduling via cron Jobs

Cron is a job scheduling utility present in Unix like systems. You can schedule jobs to execute daily, weekly, monthly or in a specific time of the day. Automation in Linux heavily relies on cron jobs.

Below is the syntax to schedule crons:

Here, * represent represents minute(s) hour(s) day(s) month(s) weekday(s), respectively.

Below are some examples of scheduling cron jobs.

SCHEDULE	SCHEDULED VALUE
5 0 * 8 *	At 00:05 in August.
5 4 * * 6	At 04:05 on Sunday.
0 22 * * 1-5	At 22:00 on every day-of-week from Monday through Friday.

You can learn about cron in detail in this blog post.

How to Check Existing Scripts in a System

Using crontab

crontab -l lists the already scheduled scripts for a particular user.

My scheduled scripts

Using the find command

The find command helps to locate files based on certain patterns. As most of the scripts end with .sh , we can use the find script like this:

• . represents the current directory. You can change the path accordingly.
• -type f indicates that the file type we are looking for is a text based file.
• *.sh tells to match all files ending with .sh .

If you are interested to read about the find command in detail, check my other post.

Wrapping up

In this tutorial we learned the basics of shell scripting. We looked into examples and syntax which can help us write meaningful programs.

What’s your favorite thing you learned from this tutorial? Let me know on Twitter!

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Shell Scripting Tutorial: How to Create Shell Script in Linux/Unix

Updated October 29, 2022

Shell Scripting

Shell Scripting is an open-source computer program designed to be run by the Unix/Linux shell. Shell Scripting is a program to write a series of commands for the shell to execute. It can combine lengthy and repetitive sequences of commands into a single and simple script that can be stored and executed anytime which, reduces programming efforts.

This Shell Scripting tutorial helps to learn a basic understanding of the Linux/Unix shell scripting program to advanced concepts of Shell Scripting. This Shell Script tutorial designed for beginners and professionals who want to learn What is Shell Scripting? How shell scripting works, types of shell, and more.

What is Shell?

Shell is a UNIX term for an interface between a user and an operating system service. Shell provides users with an interface and accepts human-readable commands into the system and executes those commands which can run automatically and give the program’s output in a shell script.

An Operating is made of many components, but its two prime components are –

• Kernel
• Shell

Components of Shell Program

A Kernel is at the nucleus of a computer. It makes the communication between the hardware and software possible. While the Kernel is the innermost part of an operating system, a shell is the outermost one.

A shell in a Linux operating system takes input from you in the form of commands, processes it, and then gives an output. It is the interface through which a user works on the programs, commands, and scripts. A shell is accessed by a terminal which runs it.

When you run the terminal, the Shell issues a command prompt (usually $), where you can type your input, which is then executed when you hit the Enter key. The output or the result is thereafter displayed on the terminal.

The Shell wraps around the delicate interior of an Operating system protecting it from accidental damage. Hence the name Shell.

This Unix/Linux Shell Script tutorial helps understand shell scripting basics to advanced levels.

In this Shell Script tutorial, you will learn-

Click here if the video is not accessible

Types of Shell

There are two main shells in Linux:

1. The Bourne Shell: The prompt for this shell is $ and its derivatives are listed below:

• POSIX shell also is known as sh
• Korn Shell also knew as sh
• Bourne Again SHell also knew as bash (most popular)

2. The C shell: The prompt for this shell is %, and its subcategories are:

• C shell also is known as csh
• Tops C shell also is known as tcsh

We will discuss bash shell based shell scripting in this tutorial.

How to Write Shell Script in Linux/Unix

Shell Scripts are written using text editors. On your Linux system, open a text editor program, open a new file to begin typing a shell script or shell programming, then give the shell permission to execute your shell script and put your script at the location from where the shell can find it.

Let us understand the steps in creating a Shell Script:

1. Create a fileusing a vi editor(or any other editor). Name script file with extension .sh
2. Start the script with #! /bin/sh
3. Write some code.
4. Save the script file as filename.sh
5. For executing the script type bash filename.sh

“#!” is an operator called shebang which directs the script to the interpreter location. So, if we use”#! /bin/sh” the script gets directed to the bourne-shell.

Let’s create a small script –

Let’s see the steps to create Shell Script Programs in Linux/Unix –

Steps to Create Shell Script in Linux/Unix

Command ‘ls’ is executed when we execute the scrip sample.sh file.

Adding shell comments

Commenting is important in any program. In Shell programming, the syntax to add a comment is

Let understand this with an example.

What are Shell Variables?

As discussed earlier, Variables store data in the form of characters and numbers. Similarly, Shell variables are used to store information and they can by the shell only.

For example, the following creates a shell variable and then prints it:

Below is a small script which will use a variable.

Let’s understand, the steps to create and execute the script

As you see, the program picked the value of the variable ‘name’ as Joy and ‘remark’ as excellent.

This is a simple script. You can develop advanced scripts which contain conditional statements, loops, and functions. Shell scripting will make your life easy and Linux administration a breeze.

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