Unix Shell Scripting Tutorial: Commands and Examples

What is Shell?

Shell is a user program that provides a Command Line Interpreter (C.L.I.) that carries the user-typed, human-readable input commands from the terminal and converts them into instructions the kernel can understand. The shell examines the commands and communicates the actions for each user logged in to the kernel.

What is a Unix shell?

The shell used in Unix-based operating systems such as Linux, macOS, and others is called Unix Shell. However, the term "shell" can describe any command line interface type, and "Unix shell" refers exclusively to the shells used in Unix-based operating systems.

These Unix shells include several implementations with syntax and features such as command execution, file manipulation, process management, and input/output redirection.

There are several Unix shells available with unique features for different user needs, with the most common ones being:

• Bourne Shell (sh): It is the first Unix shell that was introduced in 1970. It offers basic functionality and scripting capabilities like list files, read inputs, variables, and control flow statements.
• Bash (Bourne Again Shell): It is the advanced version of Bourne Shell that offers additional features like command line editing and improved scripting capabilities.
• C Shell (csh): It is known for its C-like syntax and interactive features such as history substitution.
• Korn Shell (ksh): It was developed by David Korn as an extension of the Bourne Shell, which provides advanced scripting capabilities and is backward compatible with Bourne Shell(sh).
• Z Shell (zsh): It is highly customizable with advanced features like tab completion, spelling correction, and extensive configuration options.
• Fish (Friendly Interactive Shell): fish (stylized in lowercase) is a Unix shell focusing more on interactivity and usability with features like syntax highlighting, auto suggestions, and an intuitive scripting language.

Variables and Control Flow Statements in Shell Scripting

We have seen the basic shell scripting commands; let's unlock the power of automation. Bash scripts can handle repetitive tasks, saving time and effort. Here is a breakdown of key features that make this possible:

Variables and User Input

In shell scripting, variables are the named containers for storing information in the form of string, boolean, or numerical values. We can write the variable name and assign a value to it to define a variable in Unix shell scripting. We can use the read command to take variable values from the user input. To access the value of any variable in a Unix shell we can use the "$" symbol with the variable name.

Here is an example:

#!/bin/bash
printf "What is your name? \n"
read name
printf "\n Hello, $name! \n"

In this script, the user's name is stored in the name variable and then used in the greeting message.

Conditionals (if/else)

In shell scripting, conditional statements provide decision-making control. The 'if/else' statements evaluate conditions and execute commands accordingly.

Below is the syntax for constructing an 'if-else' condition in Unix shell script:

#!/bin/bash
printf "Enter a number: "
read number
if [[ "$number" -gt 10 ]]
then
 printf "The number is greater than 10.\n"
else
 printf "The number is not greater than 10.\n"
fi

This script checks if the entered number is greater than 10, displaying different messages accordingly.

Loops (for/while)

Loops handle the repetitive part of a task. The for loop runs once per item in a list, which suits a known set of files or servers. The while loop keeps going until its condition turns false, which suits work whose length you cannot predict, such as reading a file until it ends.

Here is a Unix shell script for-loop example:

#!/bin/bash


for i in {1..5}
do
 echo "Number: $i"
done

This script prints the numbers 1 to 5 using a for loop that iterates 5 times.

Functions

Functions in shell scripting are the reusable blocks of code that can be defined once and used multiple times by calling them. The syntax for defining and calling a function in Unix shell scripts is as follows:

#!/bin/bash


# Define a function to greet someone
welcome() {
 echo "Hello, $1!"
}


# Call the greet function with an argument
welcome "World"

This script defines a welcome function that takes a name and displays a greeting. The function is then called with the argument "World".

How to Run Your First Unix Shell Script

Let's proceed to execute our first Unix shell script. We will start by creating a basic script to print "Hello World!". We can run this Unix shell script in two ways, as stated below.

Run scripts directly from the shell:

To run the Unix shell script directly, let's open the terminal/shell on a Unix based operating system like MacOS or Linux and write the command in it as shown in the image below:

printf 'Hello World'

Output:

Run the script using a script file:

In this method, we can create a Unix shell script file that usually ends with .sh for Bourne shell scripts, .bash for Bash scripts, .zsh for Z Shell, .ksh for Korn Shell, and .csh for C Shell.

In our example, we will create a Bash shell script file. The detailed instructions below are provided on creating and running an executable bash script.

Step 1: Create a new empty file named "helloworld.sh" using the below command in your terminal.

touch helloworld.sh

touch command followed by file name creates a new empty file in the terminal's working directory.

Step 2: Open the created file in a text editor and write the Unix shell script to print "Hello World!". In this example, we will use the nano text editor. We can write the command below in the terminal to open the file in the nano editor.

nano helloworld.sh

Executing this command will open a text editor in your terminal. Now, write the below command in the text editor as shown below:

#!/bin/bash

# Print a greeting and end the line cleanly
printf "Hello World\n"

Next, press "(ctrl + X)" to exit the editor. While exiting you will be prompted with an option whether to save the file or not. Click on 'Yes' to save the file.

Step 3: The next step is to make the Unix shell script file executable. By default, the created file does not have permission to execute as a program; hence we need to modify the permission for the file. To do so, we need to use the command "chmod +x <filename>" which adds permission to execute the specific file, enabling it to be run as a Unix script. To execute the command below in your terminal, go to the directory where the file is located and run "chmod +x <filename>"".

chmod +x helloworld.sh

Step 4: The final step is to run the created script using the below command in the terminal.

./helloworld.sh

Output:

Those four steps, create the file, write the script, make it executable, run it, are the same for every script you write from here on. Only the contents of the file change.