6 essential cURL commands for daily use

cURL is a command line tool for doing all sorts of interesting and essential URL manipulations and data transfers. The original goal of the program was to transfer files programmatically via protocols such as http, ftp, gopher, sftp, ftps, scp, tftp, and many others, via a command line interface. The main benefit of using the command line interface is that you can use the program in your Windows batch file or Linux shell scripts to automate many URL related processes. In this post you will see some essential things you can do using cURL.

1. Reading URLs

Read a plain URL.

curl http://www.google.com

Read a secured URL.

curl https://www.secure-site.com

Get a web page and store it in a file. The following for example will store the index page retrieved to the file savedpage.html

curl -o savedpage.html http://www.example.com/

Get a HTTP Basic authenticated page

curl -u username:password http://www.example.com/

Sometimes a page may redirect to another resource. By default CURL will not follow page redirections. To make CURL follow redirections use the -L option.

curl -L http://www.example.com/

2. Reading URL’s with variable GET parameters

You can also download pages with a variable GET parameter. For e.g take the following url:

http://example.com/pages.php?pageNo=35

The variable here is the pageNo parameter. You can download all the pages by adding a regular expression like parameter in the CURL url as given below.

curl -o pages#1.html http://example.com/pages.php?pageNo=[1-12]

This will download all the pages from page no 1 to page no 12 and save it to a corresponding file.

3. Reading document information

Show document headers only

curl --head http://www.google.com/

You can also use it on any specific resource.

curl --head http://www.google.com/logo_plain.jpg

Dump document headers to a file

curl --dump-header headers.txt http://www.google.com/

4. CURL and FTP

Get a FTP directory listing

curl ftp://username:password@example.com

To get the listing of a different directory append the directory name to the URL.

curl ftp://username:password@example.com/directory/

Upload a file to a remote directory using FTP

curl -T uploadfilename -u username:password ftp://sitename.com/myfile

The ‘uploadfilename’ file will be copied to the remote site and named ‘myfile’. If the destination filename is eliminated the file will be copied with the original name. By default the file will be copied to the root directory. To copy to some other directory specify the directory after the site name;e.g.

curl -T uploadfilename -u username:password 
          ftp://sitename.com/directory/myfile

5. To POST to a page.

You can also process a POST request using CURL. The data will use the application/x-www-form-urlencoded encoding. Lets say you have the following POST form in your page:

<form method="POST" action="process.php">
          <input type=text name="item">
          <input type=text name="category">
          <input type=submit name="submit" value="ok">
</form>

You can use the following CURL command to POST the request.

curl -d "item=bottle&category=consumer&submit=ok" 
           www.example.com/process.php

6. Referer & User Agent

HTTP requests may include a ‘referer’ field, which is used to tell from which URL the client got to this particular page. Many programs/scripts check the referer field of requests to check the source of the request. You can simulate the referer field by the following command.

 curl -e http://some_referring_site.com  http://www.example.com/

All HTTP requests may set the User-Agent field. It names what user agent or client that is being used. Many web applications use this information to decide how to display web pages or use it to track browser usage. You can impersonate a particular browser by the following method:

curl -A "Mozilla/5.0 (compatible; MSIE 7.01; Windows NT 5.0)" 
        http://www.example.com

There are many more options you can use with curl, the ones given above are just some you may require on a regular basis.

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