What is Curl? 

Curl is command line utility for transferring data from or to a server designed to work without user interaction. With curl you can download or upload data using one of the supported protocols including HTTP, HTTPS, SCP, SFTP and FTP. Curl provides a number of options allowing you to resume transfers, limit the bandwidth, proxy support, user authentication and much more.

Install Curl 

The curl package is pre-installed on most Linux distributions today.
To check whether the curl package is installed on your system, open up your console, type curl and press enter. If you have curl installed the system will print curl: try 'curl --help' or 'curl --manual' for more information, otherwise you will see something like curl command not found.
If curl is not installed you can easily install it using the package manager of your distro.

Install Curl on Ubuntu and Debian

sudo apt install curl

Install Curl on CentOS and Fedora

sudo yum install curl

Curl Command Syntax
Before going into how to use the curl command, let’s start by reviewing the basic syntax.The curl utility expressions take the following form:
curl [options] [URL...]
  • options - The Curl options starting with one or two dashes.
  • URL - URL of the remote server.

How to Use Curl

In it’s simplest form when used without any option, curl will display the resource specified in the [url] to the standard output.
In the following example we are retrieving the example.com homepage:
 
curl example.com
This command above will print the source-code of the example.com homepage in your terminal window.
If you don’t specify the protocol curl will try to guess the protocol you want to use and it will default to HTTP.

How to Save the Curl Output to a File 

To save the result of the curl command you can use either the -o or -O flags.
Lowercase -o saves the file with a predefined filename, which in the example below is vue-v2.5.16.js:
curl -o vue-v2.5.16.js https://cdn.jsdelivr.net/npm/vue/dist/vue.js
Uppercase -O will save the file with its original filename, which in the example below is vue.js:
curl -O https://cdn.jsdelivr.net/npm/vue/dist/vue.js

How to Download Multiple files with Curl 

To download multiple files at once you can use multiple -O flags followed by the URL to the file you want to download. In the following example we are downloading the Arch Linux and Debian iso files:
curl -O http://mirrors.edge.kernel.org/archlinux/iso/2018.06.01/archlinux-2018.06.01-x86_64.iso  \
     -O https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/debian-9.4.0-amd64-netinst.iso

How to resume a download with Curl 

You can resume a download by using the -C - option. This is useful if your connection drops during a download of a large file and instead of starting the download from scratch you can continue the previous one.
For example, if you are downloading the Ubuntu 18.04 iso file using the following command:
curl -O http://releases.ubuntu.com/18.04/ubuntu-18.04-live-server-amd64.iso
and suddenly your connection drops you can resume the download with:
curl -C - -O http://releases.ubuntu.com/18.04/ubuntu-18.04-live-server-amd64.iso

Get the HTTP Headers of a URL with Curl 

To fetch only the HTTP headers of the specified resource use the -I flag:
curl -I --http2 https://www.ubuntu.com/

How to Test if a Website Supports HTTP/2 with Curl 

To check if a particular URL supports the new HTTP/2 protocol, fetch the HTTP Headers with -I along with the --http2 flag:
curl -I --http2 -s https://linuxize.com/ | grep HTTP
HTTP/2 200
curl -I --http2 -s https://www.ubuntu.com/ | grep HTTP
HTTP/1.1 200 OK
As you can see from the outputs above if the site supports HTTP/2, curl will print HTTP/2.0 200 otherwise it will print HTTP/1.1 200.
If you have curl version 7.47.0 or newer you do not need to use the --http2 flag because HTTP/2 is enabled by default for all HTTPS connections.
In the command above we have also used the -s flag, which tells curl to run in a silent (quiet) and hides the progress meter and error messages.

How to Follow Redirects with cURL 

If you try to retrieve the google.com homepage without www you will notice the following:
curl google.com
 
As you can see from the output above google.com redirects to the www version and because by default curl doesn’t follow the HTTP Location headers you are not getting the source of the Google homepage.
In these cases, you can use the -L flag which instructs curl to follow any redirect until it reaches the final destination:
curl -L google.com

How to Change the Curl User-Agent 

Sometimes when downloading a file, the remote server may be set to block the Curl User-Agent or maybe the page you want to download serves completely different content depending on the visitor device and browser.
In situations like this to emulate a different browser pass the -A flag:
curl -A "Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0" https://getfedora.org/
The command above will emulate Firefox 60 requesting the page from getfedora.org

How to Specify a Maximum Transfer Rate 

To limit the data transfer rate use the --limit-rate flag. The value can be expressed in bytes, kilobytes with the k suffix, megabytes with the m suffix and gigabytes with the g suffix
The following command will download the Go binary and limit the download speed to 1mb:
curl --limit-rate 1m -O https://dl.google.com/go/go1.10.3.linux-amd64.tar.gz
This option is useful when you don’t want curl to consume all the available bandwidth.

How to Transfer Files via FTP with Curl 

To access a protected FTP server with the curl command you need to pass the -u flag and specify the username and password as show bellow:
curl -u FTP_USERNAME:FTP_PASSWORD ftp://ftp.example.com/
The command above will list all files and directories in the user’s home directory.
You can download a file from the FTP server with:
curl -u FTP_USERNAME:FTP_PASSWORD ftp://ftp.example.com/file.tar.gz
To upload a file to the FTP server use the -T followed by the name of the file you want to upload:
curl -T newfile.tar.gz -u FTP_USERNAME:FTP_PASSWORD ftp://ftp.example.com/

How to Send Cookies with Curl 

When making requests using curl, no cookies are sent or stored by default. Sometimes you may need to make an HTTP request with specific cookies to access a remote resource or to debug an issue.
To send cookies to a server, use the -b switch followed by a filename containing the cookies or a string.
In the following example we are downloading the Oracle Java JDK file jdk-10.0.2_linux-x64_bin.rpm homepage and passing a cookie named oraclelicense with value a.
curl -L -b "oraclelicense=a" -O http://download.oracle.com/otn-pub/java/jdk/10.0.2+13/19aef61b38124481863b1413dce1855f/jdk-10.0.2_linux-x64_bin.rpm

Conclusion 

The examples shown in this tutorial are simple, but demonstrate the most used curl options and are meant to try and help you understand how the curl command work.
If you want to learn more about curl visit the Curl Documentation page.
curl terminal