The Power of cURL

Photo by Markus Spiske on Unsplash.

Advanced Rest Client, Postman and SoapUI are top API clients you may have used while building or testing API integrations. These applications are useful. However there are cases where invoking the API from the command line is advantageous and is made possible through cURL.

What is cURL?

cURL is a tool used to transfer files. The cURL command can be used inside scripts or from the command line. cURL provides support for common protocols like HTTP, HTTPS, FTP and much more.

This article will focus on using the cURL command to invoke integrations that use HTTP. The cURL commands provided in this article will be demonstrated by invoking a sample API built in Mule 4, running on my local machine. Let’s get started!

How do I start?

Open your command prompt (Windows) or terminal (Mac/Linux). Run

curl 

Look for the following output:

curl: try 'curl --help' for more information

This is a good indicator that cURL is installed on your local machine. If not, you will need to do a fresh install of cURL. Download cURL here.

cURL commands typically use the following syntax:

curl [options] [url]

Need help?

Use the --help option to provide a list of options.

curl --help 

Use the --manual option to reference the “man” page (or manual).

curl --manual

NOTE: Since I am using a Windows machine, I often use the command line from a Git Bash terminal.

Demo Time

The remainder of this article will focus on HTTP. All demonstrated cURL command examples will be HTTP requests (unless otherwise specified) and invoked against a simple Mule API running on my local machine. The API takes in the HTTP request and provides a breakdown of the request in the response.

REQUEST OPTION: To create an HTTP/HTTPS request. Use the --request option.

SYNTAX:

curl --request <method> '<url>'

NOTE: URL will include the protocol (http/https).

GET

GET requests can be written in two formats:

1) Short-hand

SYNTAX:

curl '<url>'

EX:

curl 'http://localhost:8081/curl/helloget'

2) Long-hand

SYNTAX:

curl --request GET '<url>'

EX:

curl --request GET 'http://localhost:8081/curl/helloget'

QUERY PARAMS

SYNTAX:

curl --request <method> '<url>?<queryParam1>=<value1>...&<queryParamN>=<valueN>'

EX:

curl --request GET 'http://localhost:8081/curl/helloget?firstname=Daffy&lastname=Duck'

URI PARAMS

SYNTAX:

 curl --request <method> '<url>/<uriParam>'

EX:

 curl --request GET 'http://localhost:8081/curl/helloget/1'

HEADERS

SYNTAX:

curl --request <method> '<url>'  \
    --header '<key>:<value>'

Single Header

EX:

curl --request GET 'http://localhost:8081/curl/helloget' \
    --header 'Authorization: Bearer xybaneoovganfZbe'

Multiple Headers

EX:

curl --request GET 'http://localhost:8081/curl/helloget' \
    --header 'client_id: bond' \
    --header 'client_secret: 0007'

POST

SYNTAX:

curl --request <method> '<url>' \
    --header 'Content-Type: <content-type>' \
    --data-raw '<data>'

EX:

curl --request POST 'http://localhost:8081/curl/hellopost' \
    --header 'Content-Type: application/json' \
    --data-raw '{ "RQ" : {"message" : "hello prostdev readers"}}'

Print Output

Interested in saving the output of your API invocation? Use the --output option.

SYNTAX:

curl --request <method> <url> --output output.txt

EX:

curl --request GET 'http://localhost:8081/curl/helloget'\ 
    --output output.txt

Lessons Learned

1. Although quotes are not required around the URL, when sending requests with query parameters make sure you wrap requests around quotes. Without the quotes, I noticed that cURL dropped the query parameters.

2. Wrap your request URL in “double-quotes” if using command prompt and ‘single-quotes’ if using command line terminal. I found issues with ‘’ and “” between command prompt and command line.

3. Multi-line in command prompt is different from multi-line on command line.

   1. Add “\” at the end of the line to enter a new line on a multi-line request from the command line.

   2. Add “^” at the end of the line to enter a new line on a multi-line request from the command prompt.

Conclusion

This post invoked an API through HTTP. However, most of the time we invoke APIs over HTTPS. If you are interested in an HTTPS version of this post, please let us know through a comment.

If you would like to learn more about cURL, check out the cURL documentation here.

Lastly, I am Whitney Akinola. I’m a fellow MuleSoft Developer and content creator. Feel free to check out my technical content here.

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