The Best Kept Secrets About cURL File Upload

cURL has a lot of power, but most of it is concealed behind its deliberate non-verbosity and its sophisticated multiple flags, configurations, and options. Once you understand it, however, it is easy to see that cURL has a wide range of powerful features. So let’s explore the best-kept secrets about the cURL upload file.

In order to get you up to speed on the best-kept secrets about cURL file uploads, let’s go over some of its key features. While we are at it we can explore the underlying power of this prominent open-source file uploader

What exactly is cURL, and what is a cURL command?

cURL is an open-source project. It is split into two parts — cURL and libcurl. cURL is a command-line program that uses URL syntax to transfer or receive files. In addition, to its basic functionality, cURL offers several capabilities and flags that allow it to do so in various ways. libcurl, on the other hand, is a free client-side URL transfer library that can also perform many of these tasks. 

cURL is often called a “Swiss Army Knife solution”. This is because it comes with many dynamic flags and options dynamic flags and options. It also creates a variety of cool effects. In addition to all that, it is portable, backward compatible, and has millions of construction variations that provide a very fluid set of functions. All in all, cURL supports 24 protocols and has 226 command-line arguments. 

One thing to keep in mind about cURL is that it is minimalist by default. It accomplishes only what you want it to –or what it understands you need. For example, when you first configure cURL, a basic request returns basic headers and nothing else.

You control cURL with a set of levers and toggles. You can switch these on and off individually, and, in combination, these levers and toggles are very powerful.

But before we get into the interesting hidden powers of cURL, let’s have a look at a typical cURL request. If you’re interested in learning more about cURL file uploads, consider exploring a “Curl upload file tutorial” to delve deeper into this powerful tool’s capabilities.

What do cURL Standard Requests look like?

In its most basic form, a cURL request looks like this:


There are no response headers for this basic call (remember, cURL is, by design, minimal). However, when you add the -i flag, cURL acquires response headers:

curl -i

You can also take it a step further and feed the result into a rendering engine to get more complicated responses:

curl | jq

What do cURL Complex Requests look like?

To this point, cURL produces some rather simple outputs. That is because we sent simple requests even though we were able to see the JSON result and render it understandable using jq. For a more sophisticated use case, we can look at redirects. Because redirects are so ubiquitous in HTTP APIs, dealing with them is crucial to our flow. 

We can easily issue the following (somewhat more sophisticated) request to follow a redirect:

curl -I -L

What do cURL Hybrid Requests  look like?

Hybrid queries are an intriguing application of the request system. Using cURL, you can join several requests can from the same command prompt:

curl -d user=daniel

It’s worth noting that there is no practical limit to the number of URLs you can use with this method.  You can theoretically make a large number of queries to a large number of API endpoints with a single CLI line. This is particularly useful when you need to make several requests to consolidate content. In this scenario, you use the hybrid technique and then pipe the contents to a filename for combinatory attempts.

How to Boost Your Verbosity?

Requests are useful, but understanding how to make them and how to comprehend the process itself can be challenging. cURL, thankfully, has a lot of options for transparent cURL queries.

Creating a more verbose conversation record is one of the simplest ways to learn more about what cURL does. To accomplish this, use the -v switch to create more verbosity:

curl -v -o /dev/null

This request will return a much larger quantity of data, Even that data, however, is confined to the command prompt itself. That said, there are two extremely powerful methods for further investigating this behavior.

The first and probably most dense method is using the trace-acai function to print everything that happens in cURL. This function takes everything that happens during the request process and turns it into an ASCII output you can see and parse. You can do this using the following request:

curl -d sendthisdata --trace-ASCII

Another approach is the SSLKEYLOGFILE environmental variable. With this variable, you can save encrypted traffic activity to a specific file, making the data available for analysis using tools like Wireshark. 

To do this, first set your variable:

export SSLKEYLOGFILE=$HOME/tmp/tlskey

You can send any cURL request you want from here. After making the request, notify Wireshark where the credentials are stored. Do this by pointing its settings file to the TLS key file.

Now Wireshark can read everything that happened during the submission process.

How to be More Adaptable?

cURL allows you versatility in how you make requests. For example, cURL lets you pass custom HTTP headers. You use this to hide a particular header like user-agent:

curl -H "User-agent: removed"

Headers aren’t the only thing you can alter. You can even modify the method you are using for your unique interaction at a very fine level. For example, to make a more specialized type of PUT, you can use cURL’s SWOOSH method:

curl -T localfile -X SWOOSH -o save

Of course, there are times when your surroundings require flexibility. Testing servers within your test environment is a prime example. If you have a server on, a certificate error can block your request. In this case, you can exploit cURL’s extensive flexibility and tell the system to expect a certificate mismatch.

curl --resolve

With this flexibility in your requests and methodologies, you can also alter protocols. For example, depending on your current command request, you can set multiple HTTP versions:

curl --http1.0

curl --http2

Are You Ready to take your cURL file uploads to the next level?

cURL is a tremendously powerful tool for modern web developers. This is because it offers something for almost everyone. cURL’s seeming simplicity lies in its ability to perform complicated interactions, manage cookies in fascinating and useful ways, and even expose its underlying capabilities in great detail, as we’ve seen today.

But if cURL seems difficult to navigate to you, then we can help. Every month, Filestack processes billions of files uploads, transformations, and downloads, to provide a safe and secure system that allows users to manage their files quickly and without problems—and we can’t help you do that as well. 

Filestack is a great place to upload and store files using cURL file uploads.

Related: How to Upload Files to an Amazon S3 Bucket Using Python

Related: Export Files to Zip Folders Using Filestack

FAQs about cURL file upload

How do I save a file in cURL?

To download the output of a cURL request to a file, write down the “o” or “–output” option and then your desired file name.

Where are cURL files saved?

You can see saved cURL files in your current working directory. You can change the save location by adding the full path of your preferred directory after the “o” or “–output” option.

How can I download a file from cURL command?

There are two ways. The first refers to using the “-O” or “–remote-name” option to download a file and retain its original name. The second refers to specifying a custom name by appending “-o” or “output” followed by your desired file name in the cURL command.

Can cURL transfer files?

You can transfer files in cURL. You can use various protocols, including HTTP, HTTPS, and more.


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