Curl download file and rename

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cURL is both a library and a command line utility written to handle the transfer of data using many different protocols. It is scriptable and extremely versatile but this makes it quite complicated.

If you are looking for a utility to download a file then please see wget. We would recommend reading our wget tutorial first and checking out before using curl as wget is more user-friendly in most cases. However, for more complex operations you cannot beat cURL. It has over different command line options many of which can be used in combinations. It is very powerful and can even handle cookies, forms and ssl. However it can also be used for some very simple tasks that you will find useful. In this tutorial we will concentrate on the things that cURL can do that wget can’t.

The Basics

At its most basic you can use cURL to download a file from a remote server. To download the homepage of you would use . cURL can use many different protocols but defaults to HTTP if none is provided. It will, however, try other protocols as well and it can intelligently guess which protocol to use if hints are given. For instance, if you use it will automatically try the FTP:// protocol.

If however you want to help cURL to choose the right protocol then prefix the url with the protocol such as or .

Setting the output file

If you want to give the downloaded file a different name you would use the -o option. For example.

If you don’t set the output file it will display it to the console.

Viewing the complete request and response

Quite often when learning curl you will either get an unexpected output or no output at all. The option is very useful in these situations. The option displays all the information in the request sent to the remote server and the response it receives.

Saving a redirected file

If a site has WordPress® installed for example and they are using redirects you will by default download the redirect response only. To ensure you follow the redirects and get the final file you will need to use the -L option. If you try you will just get the redirect page, if you now try you will get the page you were after.

Those are the basics of cURL. We will now move on to the intermediate levels of cURL usage.

Viewing only the response headers for debugging

When you are writing a script using cURL sometimes you will want to view the response headers only without seeing the data or the request. Having a clean view of what is happening, without all the data to obscure things, can be helpful with debugging. To do this you would use the option. For instance in the previous example with Google® we could use

Skipping SSL checks

When connecting to a remote server that has a self signed certificate you will want to skip the ssl checks. To do this use the option. For example

Setting the user agent

The option allows you to set the user agent. For example .

Rate Limiting

To avoid hitting the remote server hard you can limit the download rate you will use. The command to do this is and use like this .

FTP Login

To set the username and password you can use the –user username:password option.

FTP upload and download

To download you just need to use the basic curl command but add your username and password like this .

To upload you need to use both the –user option and the -T option as follows.

To delete a file from the remote server.

For details on the option please see the next section.

Sending POST requests and different FTP Commands

The -X command allows you to send custom commands to the receiving server. For HTTP this could be a POST request or WebDAV’s copy or move. For FTP you can use the option to send other commands instead of the default file LIST, like in the previous section's example of using to send the DELE command to an FTP server. However, this can also be used to send full POST data to an HTTP server.

If the page you wanted to POST to had a form like this:

you could submit the form request using .

Using Cookies

If the server you are connecting to requires a cookie then you can send it using

Sending custom headers

If you need to send a custom header to the server you would use the option like this
. Which would set the content type to text/html.

Verifying an SSL Certificate

If you want to verify that your SSL cert is valid without using your browser and run into potential caching issues then use . This is also useful if you need to validate the connection to ensure that you are connecting to the right server.

The certificate must be in PEM format. If the optional password is not specified, it will be queried for on the terminal. Note that this option assumes a “certificate” file that is the private key and the private certificate concatenated.

Limiting the connection timeout time

Sometimes you will want the connection to time out quickly if it can’t make the connection within a certain time frame. The option to use to do this is –connect-timeout. This only affects the connection time so once you are connected it no longer applies.

Finding out more

As stated in the introduction there are over command line options for cURL so we have just covered some of the most commonly used ones in our experiences. cURL can do a lot more than described above and is a good place to start to find out more. If you prefer a web-based reference then rushbrookrathbone.co.uk is the authoritative one.

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