4.md

Chapter 4: The Conditional Conundrums 🤔

4 - Using conditionals examples

Using Conditionals

In Linux shell scripting, conditions are often used to analyze various scenarios and make decisions based on the results. Here's how you can use conditions to analyze different situations, along with examples demonstrating the commands, options, and syntax used:

1. Analyzing if a Number is Even or Odd

#!/bin/bash

echo "Enter a number:"
read NUM

if (( NUM % 2 == 0 )); then
    echo "$NUM is even."
else
    echo "$NUM is odd."
fi

2. Checking for a Valid Email ID

#!/bin/bash

echo "Enter an email address:"
read EMAIL

if [[ "$EMAIL" =~ ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ]]; then
    echo "$EMAIL is a valid email address."
else
    echo "$EMAIL is not a valid email address."
fi

The regular expression ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$ ensures that the email address follows the standard format:

• Starts with one or more alphanumeric characters, periods, underscores, percentage signs, plus signs, or hyphens ([a-zA-Z0-9._%+-]+).
• Followed by an '@' symbol.
• Followed by one or more alphanumeric characters, periods, or hyphens ([a-zA-Z0-9.-]+).
• Followed by a period ('.').
• Ends with two or more alphanumeric characters (e.g., .com, .org, .edu) ([a-zA-Z]{2,}).

3. Verifying a Valid URL

#!/bin/bash

echo "Enter a URL:"
read URL

if curl --output /dev/null --silent --head --fail "$URL"; then
    echo "$URL is a valid URL."
else
    echo "$URL is not a valid URL."
fi

The curl command is used to send a HEAD request to the specified URL. The options used with curl are:

• --output /dev/null: Redirects the output to /dev/null, suppressing any output.
• --silent: Runs in silent mode, suppressing progress meter and error messages.
• --head: Sends a HEAD request, retrieving only the headers of the URL.
• --fail: Makes curl return a non-zero exit status if the request fails.

If the curl command succeeds (i.e., the URL is reachable), it returns a zero exit status, and the script displays a message indicating that the URL is valid. If the curl command fails (i.e., the URL is unreachable or invalid), it returns a non-zero exit status, and the script displays a message indicating that the URL is not valid.

4. Determining if a Number is Positive or Negative

#!/bin/bash

echo "Enter a number:"
read NUM

if (( NUM > 0 )); then
    echo "$NUM is positive."
elif (( NUM < 0 )); then
    echo "$NUM is negative."
else
    echo "$NUM is zero."
fi

5. Checking if a File is Writable

#!/bin/bash

echo "Enter a file path:"
read FILE

if [[ -w "$FILE" ]]; then
    echo "$FILE is writable."
else
    echo "$FILE is not writable."
fi

6. Checking if a File Exists

#!/bin/bash

echo "Enter a file path:"
read FILE

if [[ -e "$FILE" ]]; then
    echo "$FILE exists."
else
    echo "$FILE does not exist."
fi

7. Checking if a Directory Exists

#!/bin/bash

echo "Enter a directory path:"
read DIR

if [[ -d "$DIR" ]]; then
    echo "$DIR exists."
else
    echo "$DIR does not exist."
fi

Conclusion

These examples demonstrate how to use conditions in Linux shell scripts to analyze different scenarios such as determining if a number is even or odd, validating email addresses and URLs, checking if a number is positive or negative, verifying file and directory existence, and more.

⬅️Previous-----------Next ➡️

Table of contents 🚀