Shell Scripting

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Shell scripting is a way of automating and scripting tasks on a Unix or Unix-like operating system, such as Linux or macOS, using shell commands and scripting constructs. A shell script is a series of commands and instructions written in a plain text file, which can be executed by the system's shell interpreter. The shell interpreter reads and executes the commands in the script one by one, allowing you to automate repetitive tasks, perform system administration and more.

Note that for you to run a script, you have the give the file executable permissions using the command shown below:

sudo chmod +x <filename>

Shell Scripting Syntax Elements

1. Variables

Bash allows you to define and work with variables. Variables can store data of various types such as numbers, strings and arrays. You can assign values to variables using the = operator and access their values using the variable name preceded bu a $ sign.

Assigning value to a variable.

name="John"

Retrieving value from a variable.

echo $name

2. Control Flow

Bash provides control flow statements like if-else, for loops, while loops and case statements to control the flow of execution in your scripts. These statements allow you to make decisions, iterate over lists and execute different different commands based on conditions.

Using if-else to execute script.

• Create a file named control_flow.sh using the command shown below:

touch control_flow.sh

• Give the file executable permissions in order to run the script using the command shown below:

sudo chmod +x control_flow.sh

• Use the command shown below to open the file for you to input data.

vi control_flow.sh

• Copy and paste the code shown below into the file.

#!/bin/bash

# Example script to check if a number is positive, negative, or zero

read -p "Enter a number: " num

if [ $num -gt 0 ]; then
    echo "The number is positive."
elif [ $num -lt 0 ]; then
    echo "The number is negative."
else
    echo "The number is zero."
fi

• Save and quit the file by using the esc key and typing :wq!

This piece of code prompts you to type a number and prints a statement stating the number is positive or negative.

• Run the script and enter a positive integer 2 in the prompt.

• Run the script and enter a negative interger -2 in the prompt.

• Run the script and enter 0 in the prompt.

Iterating through a list using a for loop.

• Create a file named test2.sh using the command shown below:

touch test2.sh

• Give the file executable permissions by using the command shown below:

sudo chmod +x test2.sh

• Run the following command in order to open the script.

vi test2.sh

• Copy and paste the code shown below.

#!/bin/bash

# Example script to print numbers from 1 to 5 using a for loop

for (( i=1; i<=5; i++ ))
do
    echo $i
done

The result is shown below:

3. Command Substitution

Command substitution allows you to capture the output of a command and use it as a value within your script. You can use the backtick or the $()syntax for command substitition.

Using backtick for command substitution.

• Run the following command to declare the variable for current_date:

current_date=`date +%Y-%m-%d`

• Run the following command to display the variable stored:

echo $current_date

Using %() syntax for command substitution.

• Run the following command to declare the variable for current_date:

current_date=$(date +%Y-%d-%m)

• Run the following command to display the new variable stored.

echo $current_date

4. Input and Output

Bash provides various ways to handle input and output. You can use the read command to accept user input and output text to the console using the echo command. Additionally, you can redirect input and output using operators like > (output to a file), < (input from a file) and | (pipe the output of one command as input to another).

• Accept user input.

echo "Enter your name:"
read name

• Output text to the terminal.

echo "Hello World"

• Output the result of a command into a file.

echo "hello world" > index.txt

• Pass the content of a file as input to a command.

grep "hello" < index.txt

• Pass the result of a command as input to another command.

echo "hello world" | grep "world"

5. Functions

Bash allows you to define and use functions to group related commands together. Functions provide a way to modularize your code and make it more reusable. You can define functions using the function keyword or simply by declaring the function name followed by parentheses.

• Create a file named function.sh using the following command:

touch function.sh

• Give the file executable permissions using the following command:

sudo chmod +x function.sh

• Run the following command to open the file.

vi function.sh

• Copy and paste the code below into the file.

#!/bin/bash

# Define a function to greet the user
greet() {
    echo "Hello, $1! Nice to meet you."
}

# Call the greet function and pass the name as an argument
greet "John"

• Run the script using the command below:

./ function.sh

Writing Your First Shell Script

Step 1: On your terminal, create and open a folder named shell-scripting using the commands shown below:

mkdir shell-scripting

cd shell-scripting

This will hold all the script we will write.

Step 2: Create a filed named user-input.sh using the command shown below:

touch user-input.sh

Step 3: Use the vi user-input.sh command to open the file then copy and paste the block of code below:

#!/bin/bash

# Prompt the user for their name
echo "Enter your name:"
read name

# Display a greeting with the entered name
echo "Hello, $name! Nice to meet you."

A little bit about the code block. The script prompts for your name. When you type your name, it displays the text hello! Nice to meet you. Also #!/bin/bash helps you specify the type of bash interpreter used to execute the script.

Step 4: Save your file using the esc key and :wq!.

Step 5: Run the command below to make the file executable.

sudo chmod +x user-input.sh

Step 6: Run the script using the command shown below:

./user-input.sh

Directory Manipulation and Navigation

We will be writing a simple shell script as a way of practicing what we learnt. This script will display the current directory, create a new directory called "my_directory", change to that directory, create two files inside it, list the files, move back one level up, remove the "my_directory" and its contents and finally list the files in the current directory again.

Proceed by following the steps below:

Step 1: Create and a file named navigating-linux-filesystem.sh using the following command:

vi navigating-linux-filesystem.sh

Step 2: Paste the code block below into the file.

#!/bin/bash

# Display current directory
echo "Current directory: $PWD"

# Create a new directory
echo "Creating a new directory..."
mkdir my_directory
echo "New directory created."

# Change to the new directory
echo "Changing to the new directory..."
cd my_directory
echo "Current directory: $PWD"

# Create some files
echo "Creating files..."
touch file1.txt
touch file2.txt
echo "Files created."

# List the files in the current directory
echo "Files in the current directory:"
ls

# Move one level up
echo "Moving one level up..."
cd ..
echo "Current directory: $PWD"

# Remove the new directory and its contents
echo "Removing the new directory..."
rm -rf my_directory
echo "Directory removed."

# List the files in the current directory again
echo "Files in the current directory:"
ls

Step 3: Run the command below to give executable permission on the file.

sudo chmod +x navigating-linux-filesystem.sh

Step 4: Run your script using the command below:

./navigating-linux-filesystem.sh

File Operations and Sorting

Here, we will be writing a simple shell that focuses on File Operations and Sorting. This scripts creates three files (file1.txt, file2.txt and file3.txt), displays the files in their current order, sorts them alphabetically, saves the sorted files.txt, displays the sorted files, removes the original files, renames the sorted_files_sorted_alphabetically.txt and finally displays the contents of the final sorted file.

Lets proceed using the steps below:

Step 1: Open your terminal and create a file named sorting.sh using the command shown below:

vi sorting.sh

Step 2: Copy and paste the code block below into the file.

#!/bin/bash

# Create three files
echo "Creating files..."
echo "This is file3." > file3.txt
echo "This is file1." > file1.txt
echo "This is file2." > file2.txt
echo "Files created."

# Display the files in their current order
echo "Files in their current order:"
ls

# Sort the files alphabetically
echo "Sorting files alphabetically..."
ls | sort > sorted_files.txt
echo "Files sorted."

# Display the sorted files
echo "Sorted files:"
cat sorted_files.txt

# Remove the original files
echo "Removing original files..."
rm file1.txt file2.txt file3.txt
echo "Original files removed."

# Rename the sorted file to a more descriptive name
echo "Renaming sorted file..."
mv sorted_files.txt sorted_files_sorted_alphabetically.txt
echo "File renamed."

# Display the final sorted file
echo "Final sorted file:"
cat sorted_files_sorted_alphabetically.txt

Step 3: Set execute permission on sorting.sh using the command shown below.

sudo chmod +x sorting.sh

Step 4: Run your script using the command shown below:

./sorting.sh

Working With Numbers and Calculations

This script defines two variables num1 and num2 with numeric values, performs basic arithmetic operations (addition, subtraction, multiplication, division and modulus) and displays the results. It also performs more complex calculations such as raising num1 to the power of 2 and calculating the square root of num2 and displays those results as well.

Lets proceed by following the steps below: Step 1: On your terminal, create a file named calculations.sh using the command below:

vi calculations.sh

Step 2: Copy and paste the code block below into the file:

#!/bin/bash

# Define two variables with numeric values
num1=10
num2=5

# Perform basic arithmetic operations
sum=$((num1 + num2))
difference=$((num1 - num2))
product=$((num1 * num2))
quotient=$((num1 / num2))
remainder=$((num1 % num2))

# Display the results
echo "Number 1: $num1"
echo "Number 2: $num2"
echo "Sum: $sum"
echo "Difference: $difference"
echo "Product: $product"
echo "Quotient: $quotient"
echo "Remainder: $remainder"

# Perform some more complex calculations
power_of_2=$((num1 ** 2))
square_root=$(awk "BEGIN {print sqrt($num2)}")

# Display the results
echo "Number 1 raised to the power of 2: $power_of_2"
echo "Square root of number 2: $square_root"

Step 3: Set execute permission on calculations.sh using the command shown below:

sudo chmod +x calculations.sh

Step 4: Run your script using this command:

./calculations.sh

File Backup and Timestamping

As a DevOps Engineer backing up databases and other storage devices is one of the most common task you to carryout.

This script defines the source directory paths. It then creates a timestamp using the current date and time, and creates a backup directory using the cp command with the -r option for recursive copying.

Finally, it displays a message indicating the completion of the backup process and shows the path of the backup directory with the timestamp.

Proceed by following the steps below:

Step 1: On your terminal, create a file named backup.sh using the command shown below:

vi backup.sh

Step 2: Copy and paste the code below into the file.

#!/bin/bash

# Define the source directory and backup directory

# Define the source directory using your preferred directory
#   source_dir="/path/to/source_directory"
source_dir="/home/vagrant/shell-scripting/test"

# Define the backup directory using your preferred directory
#   backup_dir="/path/to/backup_directory"
backup_dir="/home/vagrant/shell-scripting/backup"

# Create a timestamp with the current date and time
timestamp=$(date +"%Y%m%d%H%M%S")

# Create a backup directory with the timestamp
backup_dir_with_timestamp="$backup_dir/backup_$timestamp"

# Create the source directory
mkdir -p "$source_dir"

# Create the backup directory
mkdir -p "$backup_dir_with_timestamp"

# Copy all files from the source directory to the backup directory
cp -r "$source_dir" "$backup_dir_with_timestamp"

# Display a message indicating the backup process is complete
echo "Backup completed. Files copied to: $backup_dir_with_timestamp"

Step 3: Set execute permission on backup.sh using the command shown below:

sudo chmod +x backup.sh

Step 4: Run your script using the command shown below:

./backup.sh