What does curl do?

cURL is a project and its primary purpose and focus is to make two products:

• curl, the command-line tool

• libcurl the transfer library with a C API

Both the tool and the library do Internet transfers for resources specified as
URLs using Internet protocols.

Everything and anything that is related to Internet protocol transfers can be
considered curl’s business. Things that are not related to that should be
avoided and be left for other projects and products.

It could be important to also consider that curl and libcurl try to avoid
handling the actual data that is transferred. It has, for example, no knowledge
about HTML or anything else of the content that is popular to transfer over
HTTP, but it knows all about how to transfer such data over HTTP.

Both products are frequently used not only to drive thousands or millions of
scripts and applications for an Internet connected world, but they are also
widely used for server testing, protocol fiddling and trying out new things.

The library is used in every imaginable sort of embedded device where Internet
transfers are needed: car infotainment, televisions, Blu-Ray players, set-top
boxes, printers, routers, game systems, etc.

Command line tool

Running curl from the command line was natural and Daniel never considered
anything else than that it would output data on stdout, to the terminal, by
default. The “everything is a pipe” mantra of standard Unix philosophy was
something Daniel believed in. curl is like ‘cat’ or one of the other Unix tools;
it sends data to stdout to make it easy to chain together with other tools to do
what you want. That’s also why virtually all curl options that allow reading from
a file or writing to a file, also have the ability to select doing it to stdout
or from stdin.

Following that style of what Unix command-line tools worked, it was also never
any question about that it should support multiple URLs on the command line.

The command-line tool is designed to work perfectly from scripts or other
automatic means. It doesn’t feature any other GUI or UI other than mere text
in and text out.

The library

While the command-line tool came first, the network engine was ripped out and
converted into a library during the year 2000 and the concepts we still have
today were introduced with libcurl 7.1 in August 2000. Since then, the command
line tool has been a thin layer of logic to make a tool around the
library that does all the heavy lifting.

libcurl is designed and meant to be available for anyone who wants to add
client-side file transfer capabilities to their software, on any platform, any
architecture and for any purpose. libcurl is also extremely liberally licensed
to avoid that becoming an obstacle.

libcurl is written in traditional and conservative C. Where other languages
are preferred, people have created libcurl bindings for them.