How to Build a Command Line Web Crawler in Go

Table of Contents

1. Introduction
2. Prerequisites
3. Setting Up the Environment
4. Creating the Web Crawler
5. Running the Web Crawler

6. Conclusion

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Introduction

In this tutorial, we will learn how to build a command line web crawler using the Go programming language. A web crawler, also known as a spider or spiderbot, is a program that automatically traverses the web by following links to gather information from websites. By the end of this tutorial, you will be able to develop a basic web crawler that extracts URLs from a given website.

Prerequisites

To follow along with this tutorial, you should have basic knowledge of the Go programming language, including how to set up the Go environment and write simple Go programs. You should also have a text editor and a terminal or command prompt available for running the commands.

Setting Up the Environment

Before we begin, make sure Go is installed on your machine. You can download and install the latest version of Go from the official Go website at https://golang.org/dl.

Verify if Go is installed properly by opening a terminal or command prompt and running the following command:

go version

You should see the installed Go version printed on the screen. If not, double-check your installation.

Creating the Web Crawler

Step 1: Setting up the Project

To start building our web crawler, we need to set up a new Go project. Open your terminal or command prompt and navigate to the directory where you want your project to be located.

Create a new directory for your project and navigate into it:

mkdir webcrawler
cd webcrawler

Step 2: Initializing the Go Module

Go modules are the standard way to manage dependencies in Go projects. We will initialize a new Go module for our web crawler project by running the following command:

go mod init github.com/your-username/webcrawler

Replace your-username with your actual GitHub username or any other appropriate namespace.

Step 3: Creating the Main Package

Create a new file called main.go in the project directory and open it in your text editor. This will be the entry point for our web crawler program.

Add the following code to main.go:

package main

import "fmt"

func main() {
	fmt.Println("Hello, web crawler!")
}

This is a basic Go program that prints “Hello, web crawler!” to the console.

Step 4: Adding Flags

To make our web crawler more flexible, we will add command-line flags to specify the website to crawl and the maximum depth of the traversal.

First, import the flag package by adding the following line at the beginning of the file:

import "flag"

Next, declare variables for the flags we will use:

var (
	website string
	depth   int
)

Then, initialize the flags using the flag.StringVar and flag.IntVar functions:

func init() {
	flag.StringVar(&website, "url", "", "Website URL to crawl (required)")
	flag.IntVar(&depth, "depth", 2, "Maximum depth of traversal (default: 2)")
	flag.Parse()
}

The flag.StringVar function defines a flag that will be bound to the website variable. The flag.IntVar function does the same for the depth variable. The last line, flag.Parse(), parses the command-line arguments and assigns the flag values to the corresponding variables.

Step 5: Implementing the Web Crawler

Now, let’s implement the web crawler functionality.

First, create a new function called Crawl that takes the website URL and the maximum depth as arguments:

func Crawl(website string, depth int) {
	// TODO: Implement web crawler logic here
}

Inside the Crawl function, we will use Go’s net/http package to fetch the webpage content and extract the URLs. Add the following code to the Crawl function:

func Crawl(website string, depth int) {
	if depth <= 0 {
		return
	}

	resp, err := http.Get(website)
	if err != nil {
		fmt.Printf("Failed to crawl website: %v\n", err)
		return
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		fmt.Printf("Failed to read response body: %v\n", err)
		return
	}

	// TODO: Extract URLs from the webpage
}

This code sends an HTTP GET request to the specified website and reads the response body. We will extract the URLs from the webpage in the next step.

Step 6: Extracting URLs

To extract the URLs from the webpage, we will use Go’s regular expression package regexp.

Add the following code to the Crawl function:

func Crawl(website string, depth int) {
	// ...

	re := regexp.MustCompile(`<a\s+(?:[^>]*?\s+)?href="([^"]*)"`)
	matches := re.FindAllStringSubmatch(string(body), -1)

	urls := make([]string, 0, len(matches))
	for _, match := range matches {
		urls = append(urls, match[1])
	}

	fmt.Println("Found URLs:", urls)

	// TODO: Crawl the extracted URLs recursively
}

This code defines a regular expression pattern to match HTML <a> tags with href attributes. It then uses the FindAllStringSubmatch function to find all matches in the webpage body. The URLs are extracted from the matches and stored in the urls slice.

Step 7: Recursive Crawling

Now that we have the extracted URLs, we can recursively crawl each URL up to the specified depth.

Add the following code to the Crawl function:

func Crawl(website string, depth int) {
	// ...

	if depth > 1 {
		for _, url := range urls {
			Crawl(url, depth-1)
		}
	}
}

This code checks if the current depth is greater than 1. If so, it iterates over the extracted URLs and calls the Crawl function recursively with a decreased depth.

Step 8: Calling the Crawler

Update the main function to call the Crawl function with the specified website and depth:

func main() {
	flag.Parse()

	if website == "" {
		fmt.Println("Please provide a website URL to crawl using the -url flag.")
		return
	}

	Crawl(website, depth)
}

This code first parses the command-line flags. Then, it checks if the website flag is empty and exits with an error message if no website is provided. Finally, it calls the Crawl function with the specified website and depth.

Running the Web Crawler

To run the web crawler, open a terminal or command prompt and navigate to the project directory.

Build the Go program by running:

go build

This will generate an executable file with the same name as the project.

Run the program with the desired command-line options. For example:

./webcrawler -url https://example.com -depth 3

Replace https://example.com with the website URL you want to crawl and 3 with the desired depth.

The web crawler will start crawling the website and its subpages up to the specified depth, printing the extracted URLs to the console.

Conclusion

In this tutorial, we learned how to build a command line web crawler in Go. We covered setting up the project, adding command-line flags, implementing the web crawler logic, and recursively crawling the extracted URLs. With this knowledge, you can further enhance the web crawler by implementing features like duplicate URL filtering, parallel crawling, or data storage.

Feel free to explore the Go standard library and third-party packages to extend the functionality of your web crawler. Happy crawling!

Published: 1 August 2019

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