Aaron Swartz was a programmer and political activist, who infamously downloaded an estimated 4.8 million articles from the JSTOR database of academic articles. This led him to be prosecuted by the United States. 

In the documentary "The Internet's Own Boy" about Aaron Swartz, there is a script that is referenced called KeepGrabbing.py. This script is what Aaron used to bulk download PDFs from the JSTOR database from within the MIT network. 

When first saw the documentary, I was fascinate by the idea that a simple computer program could cause such furore. Naturally, I wanted to know what was in the program. 
It turns out this program is just a simple python 2 script, that is only 21 lines long (17 if you don't include blank lines!)

Here is the excerpt from the documentary in which the script is briefly discussed. 

https://www.youtube.com/watch?v=j0DLmgmh2N8

I managed to find the script online. But I didn't immediately understand it. I got the general gist, but there were a few things I had not seen before like the one line class which Aaron defines which is just an exception that appears to just do nothing. There's also a URL which has been reacted by the courts, which further makes this program difficult to understand. 

So let's look through the program and try and understand it line-by-line.

Here is the file keepgrabbing.py:

```import subprocess, urllib, random
class NoBlocks(Exception): pass
def getblocks():
r = urllib.urlopen("http://{?REDACTED?}/grab").read()
if '<html' in r.lower(): raise NoBlocks
return r.split()

import sys
if len(sys.argv) > 1:
prefix = ['--socks5', sys.argv[1]]
else:
prefix = []#'-interface','eth0:1']
line = lambda x: ['curl'] + prefix + ['-H', "Cookie: TENACIOUS=" + str(random.random())[3:], '-o', 'pdfs/' + str(x) + '.pdf', "http://www.jstor.org/stable/pdfplus/" + str(x) + ".pdf?acceptTC=true"]

while 1:
blocks = getblocks()
for block in blocks:
print block
subprocess.Popen(line(block)).wait()```

This is written in python2.
I because we don't have the URL which is reacted, some of the notes here are my best interpretation.

The imports:

• subprocess - allows you to create new processes from the python script.   subprocesses are child processes created from another process.
• urllib - allows you to retrieve resources from URL.
• random - generates pseudo-random numbers (i.e. not truly random numbers)
• sys - imported later. Enables access to parameters and functions specific  to the machine that is running the code.

import subprocess, urllib, random

After a little bit of investigation, I realised the one-line class NoBlocks that Aaron had included in this script was a way of terminating the script (more on this later).

class NoBlocks(Exception): pass

The next bit of code is the method getblocks().
This function reads from a URL which has been reacted by the courts.

 It looks like this function called to custom url which was likely set up by Swartz himself, which had a list of URLs linking to PDFs to download. It is likely the case that Swartz had this grab page setup and would control and modify it from out side of the MIT campus in order to direct the script to download the PDFs he wanted. The first line in this function calls urllib.urlopen().read and saves the response of this call to a variable called 'r'. The urlopen function reads from a URL and returns a file-like object from the contents of the external resource that the URL points to. The read function that is called on this simply reads the byte contents and returns them.

The second line of this function checks to see if there is HTML in the retrieved page. If there is then it raises a NoBlocks exception and exits the script. It is likely that the URL that is reacted simply was a text file with the PDFs Swartz wanted to download. When he wanted to stop the script he could simply swap this text file for a HTML file and the script would exit.

The split function simply takes a string and splits it into a list and by default, it will split the string at every space, which is what Aaron is doing here.

def getblocks():
r = urllib.urlopen("http://{?REDACTED?}/grab").read()
if '<html' in r.lower(): raise NoBlocks
return r.split()

The next 5 lines of code are concerned with taking the arguments to the script from the sys.argv and if there is one present adding it to a variable as a list with the string - socks5 as the first string in the list.

Note that it takes the second element in the sys.argv list as the first element in the sys.argv list is the name of the script.

This prefix variable will be used in a lamda expression below.
Basically, this prefix is used to make the script connect to JSTOR via a proxy or just though the computer's internet connection (which Aaron left a command about, suggesting this was an ethernet connection, which makes sense as the computer that Aaron used to run this script was in a store cupboard connected to the MIT network)

This line may mean that Aaron could run this script from outside the MIT network, but that is just speculation.

import sys
if len(sys.argv) > 1:
prefix = ['--socks5', sys.argv[1]]
else:
prefix = []#'-interface','eth0:1']

The next line declares a lambda function which is saved to the variable called line.

This lambda expression takes a single argument, which is the name of the PDF that the script is going to download.

This lambda expression will later be used as part of a subprocess call later in the script. It defines a curl request. The curl command is a command which allows you
to transfer data to or from a URL, i.e upload or download from a url. The curl request is with a proxy to connect via (depending on the conditional above as mentioned). Next, it defines a cookie, which is simply the string TENACIOUS= followed by a random 3-digit number. This cookie, will make the server responding to this curl request think that it is coming from a real user as opposed to a script. The next thing this function does is define the output of this curl request: the pdf file name to a directory called pdfs. The rest of this lambda creates the url of the PDF from which to download the PDF with using curl.

line = lambda x: ['curl'] + prefix + ['-H', "Cookie: TENACIOUS=" + str(random.random())[3:], '-o', 'pdfs/' + str(x) + '.pdf', "http://www.jstor.org/stable/pdfplus/" + str(x) + ".pdf?acceptTC=true"]

This next section of code. Defines an infinite loop, which is the part of the code that composes everything else.

First it calls the getblocks from earlier and saves the resulting list of PDFs to a variable called blocks.
It then iterates over these, printing them to the console and then calling the line lambda from earlier in a subprocess.Popen call. Subprocess Popen will create a new process, in this case the curl request that will download the current PDF. Then the script will block until this subprocess finishes, i.e. it will wait until the PDF is finished downloading and then it will move on to the next PDF.

while 1:
blocks = getblocks()
for block in blocks:
print block
subprocess.Popen(line(block)).wait()

And that's it!

I have also made a video where I go through this code, if that sort of thing floats your goat:

https://www.youtube.com/watch?v=0sJ7CVos5bQ