How to Set or Change User Agent with curl

Curl is a valuable tool for web development and testing that every developer and IT professional should know about. Its adaptability to numerous protocols and ability to be used in scripts or directly from the command line make it an essential component of many processes. Among its many features, one of the most powerful is the ability to modify or update the User Agent.

In this article, we will discuss the definition of Curl and User Agents and how to manipulate them. Furthermore, we will cover advanced techniques and best practices while handling User Agents with Curl.

What is CURL

curl stands for Client URL. It’s a command-line tool used to transfer data to or from a server using various protocols, including HTTP, HTTPS, FTP, and more. curl is not just limited to fetching web pages it can upload files, post forms, and even test APIs.

What is a User Agent

A User Agent is a string that browsers and other web clients send to web servers to identify themselves. This string contains information about the client, including the browser name, version, operating system, and sometimes the device. User Agents help servers deliver content adapted to the specific client. For example, a typical User Agent string might look like this:

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36

Why Change the User Agent

Changing the User Agent can be useful in various scenarios, such as:

1. Ensuring that your web application behaves correctly across different browsers and devices.
2. Avoid detection by websites that block known scraping tools or non-browser User Agents.
3. Some websites may restrict content based on the User Agent string.

Setting User Agent with curl

To change user agent Curl, the basic techniques with examples are described as follows:

Basic Syntax

To change the User Agent in curl, you use the -A or --user-agent flag followed by the User Agent string. Here’s the basic syntax:

curl -A "UserAgentString" URL

Example Usage

Let’s look at an example. Suppose we want to fetch a web page pretending to be a Googlebot. We would use the following command:

curl -A "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" https://www.example.com

Depending on your operating system open your command-line interface. Enter the curl command, the -A flag, and the desired User Agent string. Add the URL of the website you want to access.

Specify User Agent in Script

If you are creating a bash script that uses curl, you can include the User Agent string as part of your command. Here’s an example in a shell script:

#!/bin/bash
URL="https://www.example.com"
USER_AGENT="Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36"
curl -A "$USER_AGENT" "$URL"

Using Configuration File

curl allows you to use a configuration file to store your options, including the User Agent string. Here’s how you can do it:

1. Create a .curlrc File: This file can be in your home directory.
2. Add User Agent String: Use the user-agent option in the file.

user-agent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36"

Automate User Agent Switching

In some cases, you may want to rotate User Agents automatically, such as during web scraping. Here’s a simple example using a bash script:

#!/bin/bash
URL="https://www.example.com"
USER_AGENTS=("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36" 
             "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0.3 Safari/605.1.15" 
             "Mozilla/5.0 (iPhone; CPU iPhone OS 14_3 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.0 Mobile/15E148 Safari/604.1")
for USER_AGENT in "${USER_AGENTS[@]}"; do
  echo "Using User Agent: $USER_AGENT"
  curl -A "$USER_AGENT" "$URL"
  sleep 5
done

This script rotates through an array of User Agents, fetching the URL with each one and pausing for 5 seconds between requests.

Advanced Usage

Following are the advanced usage on how to set user agent with Curl:

Setting Multiple Headers

Sometimes, changing the User Agent alone may not be sufficient. You might need to set multiple headers to mimic a browser more accurately. Here’s an example:

curl -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36" \
     -H "Accept-Language: en-US,en;q=0.9" \
     -H "Accept-Encoding: gzip, deflate, br" \
     -H "Connection: keep-alive" \
     https://www.example.com

Handling Cookies

If you need to manage cookies alongside changing the User Agent, you can use the -b and -c flags to read and write cookies. Here’s an example:

curl -A "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36" \
     -b cookies.txt -c new_cookies.txt \
     https://www.example.com

Best Practices

Following are the best practices while handling the User Agent with the help of Curl:

• Avoid using fake or random User Agents that might raise suspicion.
• For web scraping, rotate User Agents to avoid detection and blocking.
• Always adhere to the website’s terms of service and robots.txt file.

Conclusion

Changing the User Agent with curl is a powerful technique for web development, testing, and scraping. Whether you are mimicking different browsers, bypassing restrictions, or automating tasks, understanding how to set and manipulate the User Agent can enhance your control and flexibility. Always remember to use this capability ethically and responsibly.

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