Understanding Variables in Shell Scripting

In shell scripting, variables are fundamental constructs that store data for use in scripts. They allow for dynamic content manipulation and can simplify the tasks performed by scripts. This article provides three diverse examples of variables in shell scripting, showcasing different contexts and use cases.

Example 1: Storing User Input

In this example, we will create a simple script that prompts the user for their name and stores it in a variable. This is useful for personalizing outputs in scripts.

#!/bin/bash

# Prompt the user for their name
read -p "Enter your name: " USER_NAME

# Greet the user using the variable
echo "Hello, $USER_NAME! Welcome to our script."

This script uses the read command to take input from the user and stores it in the USER_NAME variable. The variable is then used in the echo command to display a personalized message.

Notes

• Ensure that the script has executable permissions using chmod +x script.sh before running it.
• The variable can be used later in the script for further customization.

Example 2: Using Environment Variables

Environment variables can be set and accessed within shell scripts. This example demonstrates how to use an environment variable to define a directory path that can be reused throughout the script.

#!/bin/bash

# Set an environment variable for the directory
export PROJECT_DIR="/home/user/projects"

# Use the variable to navigate to the directory and list files
cd $PROJECT_DIR
ls -l

In this script, we set an environment variable named PROJECT_DIR to a specific directory path. This variable is then used to change the current directory with cd and list the files within it.

Notes

• Environment variables are accessible in child processes spawned by the script, making them very versatile.
• You can unset an environment variable using unset VARIABLE_NAME if needed.

Example 3: Performing Arithmetic Operations

Shell scripting allows for arithmetic operations directly using variables. This example illustrates how to declare numeric variables, perform calculations, and display results.

#!/bin/bash

# Declare two numeric variables
NUM1=10
NUM2=5

# Perform arithmetic operations
SUM=$((NUM1 + NUM2))
DIFF=$((NUM1 - NUM2))
PRODUCT=$((NUM1 * NUM2))
QUOTIENT=$((NUM1 / NUM2))

# Display the results
echo "Sum: $SUM"
echo "Difference: $DIFF"
echo "Product: $PRODUCT"
echo "Quotient: $QUOTIENT"

This script defines two numeric variables, NUM1 and NUM2, and calculates their sum, difference, product, and quotient using the $(( )) syntax. The results are then printed using the echo command.

Notes

• Be mindful of integer division in shell scripting; if both numbers are integers, the result will also be an integer.
• You can also use bc for handling floating-point arithmetic if necessary.

By understanding and utilizing these examples of variables in shell scripting, you can enhance your scripting capabilities and introduce more dynamic functionality into your scripts.