How To Use Curl Command.

In this tutorial, we will show you how to use the curl tool through practical examples and detailed explanations of the most common curl options.

Installing Curl.

The curl package is pre-installed on most Linux distributions today.

If curl is not installed you can easily install it using the package manager of your distribution.

Install Curl on Ubuntu and Debian

sudo apt updatesudo apt install curl

Install Curl on CentOS and Fedora

sudo yum install curl

How to Use Curl

The syntax for the curl command is as follows:

curl [options] [URL...]

In its simplest form, when invoked without any option, curl displays the specified resource to the standard output.

For example, to retrieve the example.com homepage you would run:

curl nexonhost.com

The command will print the source code of the nexonhost.com homepage in your terminal window.

If no protocol is specified, curl tries to guess the protocol you want to use, and it will default to HTTP.

Save the Output to a File

To save the result of the curl command, use either the -o or -O option.

Lowercase -o saves the file with a predefined filename, which in the example below is vue-v2.6.10.js:

curl -o vue-v2.6.10.js https://cdn.jsdelivr.net/npm/vue/dist/vue.js

Uppercase -O saves the file with its original filename:

curl -O https://cdn.jsdelivr.net/npm/vue/dist/vue.js

Download Multiple files

To download multiple files at once, use multiple -O options, 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

Resume a Download

You can resume a download by using the -C – option. This is useful if your connection drops during the 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

HTTP headers are colon-separated key-value pairs containing information such as user agent, content type, and encoding. Headers are passed between the client and the server with the request or the response.

Use the -I option to fetch only the HTTP headers of the specified resource:

curl -I --http2 https://www.ubuntu.com/

Test if a Website Supports HTTP/2

To check whether a particular URL supports the new HTTP/2 protocol , fetch the HTTP Headers with -I along with the –http2 option:

curl -I --http2 -s https://nexonhost.com/ | grep HTTP

The -s option tells curl to run in a silent (quiet) and hide the progress meter and error messages.

If the remote server supports HTTP/2, curl prints HTTP/2.0 200:

HTTP/2 200

Otherwise, the response is HTTP/1.1 200:

HTTP/1.1 200 OK

If you have curl version 7.47.0 or newer, you do not need to use the –http2 option because HTTP/2 is enabled by default for all HTTPS connections.

Follow Redirects

By default, curl doesn’t follow the HTTP Location headers.

If you try to retrieve the non-www version of google.com, you will notice that instead of getting the source of the page you’ll be redirected to the www version:

curl nexonhost.com

The -L option instructs curl to follow any redirect until it reaches the final destination:

curl -L nexonhost.com

Change the User-Agent

Sometimes when downloading a file, the remote server may be set to block the Curl User-Agent or to return different contents depending on the visitor device and browser.

In situations like this to emulate a different browser, use the -A option.

For example to emulates Firefox 60 you would use:

curl -A "Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0" https://getfedora.org/

Specify a Maximum Transfer Rate

The –limit-rate option allows you to limit the data transfer rate. The value can be expressed in bytes, kilobytes with the k suffix, megabytes with the m suffix, and gigabytes with the g suffix.

In the following example curl will download the Go binary and limit the download speed to 1 mb:

curl --limit-rate 1m -O https://dl.google.com/go/go1.10.3.linux-amd64.tar.gz

This option is useful to prevent curl consuming all the available bandwidth.

Transfer Files via FTP

To access a protected FTP server with curl, use the -u option and specify the username and password as shown below:

curl -u FTP_USERNAME:FTP_PASSWORD ftp://ftp.example.com/

Once logged in, the command lists all files and directories in the user’s home directory.

You can download a single file from the FTP server using the following syntax:

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/

Send Cookies

Sometimes you may need to make an HTTP request with specific cookies to access a remote resource or to debug an issue.

By default, when requesting a resource with curl, no cookies are sent or stored.

To send cookies to the server, use the -b switch followed by a filename containing the cookies or a string.

For example, to download the Oracle Java JDK rpm file jdk-10.0.2_linux-x64_bin.rpm you’ll need to pass 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

curl is a command-line tool that allows you to transfer data from or to a remote host. It is useful for troubleshooting issues, downloading files, and more.