CSE 303, Winter 2007, Assignment 2

Due: Monday, January 22 2007, 6:00pm


Assignment goal

You will get experience writing bash scripts. You will experiment with the grep, sed, and awk utilities. You will learn about a program called gnuplot for plotting files.

Note: The instructions for this assignment may seem a bit long. This is because we try to give you plenty of sample outputs and hints for each question. We hope this will help you complete the assignment faster.


In addition to the lecture notes, you may find "The Linux Pocket Guide" a useful reference for completing this assignment (in particular, pages 166 and following).

Online manuals:

In general, whenever you need to use a new tool, you should get into the habit of looking for documentation online. There are usually good tutorials and examples that you can learn from. As you work on this assignment, if you find that you would like more information about a tool (awk, sed, grep, or gnuplot), try searching for the name of the tool or the name of the tool followed by keywords such as "tutorial" or "documentation".


Getting ready

Download the file: hw2.tar.gz.

Extract all the files for this assignment using the following command:

> tar zxvf hw2.tar.gz

You should now see a directory called hw2.

If you see it, you are ready to start the assignment. If this procedure did not work for you, please contact a TA or talk to another student in the class.



Because you did very well in CSE303, you were offered a summer position in a research group. As part of your summer job, you would like to run the following experiment. Given a list of 100 popular websites, you would like to measure the sizes of their index pages (the first index.html file that the browser downloads when you visit the site). You suspect that popular sites must have very small index pages because they need to handle a heavy user load.

We provide you the list of popular websites in the file popular.html (this list comes from 100bestwebsites.org)

One approach would be to manually download each index page in the list, and use wc to compute its size in bytes. You could then manually put all the results in a table and plot some graphs to display the results. This approach would be long, tedious, and error prone. It would also be painful if you wanted to repeat the experiment on the 100 least popular websites. Instead, you decide to automate the experiment by writing a set of scripts.

1. Download a page and compute its size

In a file called perform-measurement.sh, write a bash script that takes a URL as argument and outputs the size of the corresponding page in bytes.

For example, executing your script with the URL of the class website as argument:

> ./perform-measurement.sh http://www.cs.washington.edu/education/courses/303/07wi/syllabus.html

should output only the number 7819:

> 7819

If the user does not provide any arguments, the script should print an error message and exit.

If the user provides an erroneous argument or downloading the requested page fails for any other reason, the script should simply print the number "0" (zero).



2. Parsing the html list of websites

The list of popular websites is in html format. To run an experiment automatically on each URL in this list, we need to extract the URLs and write them into a text file. There are several ways in which this can be done, and different utilities (awk, sed, grep) can help.

We would like you to use grep and sed.

In a file called parse.sh, write a script that extracts the URLs and writes them into a text file. The script should take two arguments: the name of the input html file and the name of the output file for the results.

For example, executing:

> parse.sh popular.html popular.txt

Should write the following content into popular.txt:


If the user provides fewer than 2 arguments, the script should print an error message and exit.

If the html file provided as argument does not exist, the script should print an error message and exit.

If the txt file provided as argument (for the output) exists, the script should simply overwrite it without any warning.


Q: How come popular.txt contains only 88 urls? Is it a bug in your script or a bug in the data? You don't need to answer this question in writing, just think about it for yourself.


Hints: step-by-step instructions

Note: the first line that will appear in the result file (http://www.100bestwebsites.org/oledata.mo) should technically not be there, but don't worry about it. Just leave it there or remove it manually.


3. Running the experiment

To perform the experiment, your need to execute the script perform-measurement.sh on each URL inside the file popular.txt. Once again, you would like to do this automatically with a script.

In a file called run-experiment.sh, write a shell script that:

To debug your script, instead of trying it directly on popular.txt, we provide you with a smaller file: popular-small.txt. You should execute your script on popular-small.txt until it works. Only then try it on popular.txt.

Executing your script as follows:

> run-experiment.sh popular-small.txt results-small.txt

Should produce the following output:

Performing measurement on http://www.cs.washington.edu/education/courses/303/07wi/
Performing measurement on http://www.cs.washington.edu/education/courses/303/07wi/syllabus.html
Performing measurement on http://i.will.return.an.error
Performing measurement on http://www.cs.washington.edu/education/courses/303/07wi/schedule.html
Performing measurement on http://www.cs.washington.edu/education/courses/303/07wi/homework.html
Performing measurement on http://www.cs.washington.edu/education/courses/303/07wi/resources.html

And the content of results-small.txt should be similar to the ones below. Note that the exact value will change as we edit the class website! In particular, schedule.html will grow with every lecture!

1 http://www.cs.washington.edu/education/courses/303/07wi/ 554
2 http://www.cs.washington.edu/education/courses/303/07wi/syllabus.html 7819
4 http://www.cs.washington.edu/education/courses/303/07wi/schedule.html 13083
5 http://www.cs.washington.edu/education/courses/303/07wi/homework.html 765
6 http://www.cs.washington.edu/education/courses/303/07wi/resources.html 1231

As another example, after executing your script as follows:

> run-experiment.sh popular.txt results.txt

The file result.txt, should contain results similar to the ones shown below (when you run your experiment, the exact values may differ)

2 http://www.yahoo.com/ 10047
3 http://www.google.com/ 3951
4 http://www.amazon.com/ 60700


4. Plotting the results

It is hard to understand the results just by looking at a list of numbers, so you would like to produce a graph. More specifically, you would like to produce a scatterplot, where the x-axis will show the rank of a website and the y-axis will show the size of the index page.

Luckily, you talk about your problem to your friend Alice. She suggests that you use a program called gnuplot to produce the graph. Because she used it many times before, Alice helps you write the necessary gnuplot script called produce-scatterplot.gnuplot. Note that the gnuplot file expects your experimental results to be stored in a file called results.txt.

Produce the graph with the following command:

> gnuplot produce-scatterplot.gnuplot

The script should produce a file called scatterplot.eps. You can view it with gv:

> gv scatterplot.eps


You find that it is still difficult to draw conclusions about the sizes of the index pages by looking only at the scatterplot. You decide to produce a CDF instead.

The CDF, or cumulative distribution function, shows for each file-size x, the fraction of index.html files that have a size no greater than x. Writing the script to produce the CDF is an extra-credit question (see problem 5 below), but we would like everyone to see the result. We thus provide you read and execute permissions on the binary executable version of our solution script. Plot the CDF by executing the following two commands. Please note that transform.sh.x will only work on attu1. You can thus produce the CDF by executing the following two commands:

> /cse/courses/cse303/07wi/bin/transform.sh.x results.txt transformed-results.txt

> gnuplot produce-cdf.gnuplot

The script should produce a file called cdf.eps.

Note: because you need both execute and read permissions to execute a bash script, we replaced transform.sh with a binary executable version transform.sh.x. We created this version with a publicly available shell script compiler: http://www.datsi.fi.upm.es/~frosal/frosal.html

Write your answers to the following questions in a file called problem4.txt:

Q1: Examine the gnuplot file produce-scatterplot.gnuplot. Ignoring the first line, explain what the rest of the script does.

Q2: Looking at the scatterplot, what can you conclude about the relationship between the popularity of a site and the size of its index.html file? Are these result what you expected?

Q3: Looking at the CDF, what is the median size of the index.html files?

Q4: Examine the file transformed-results.txt. Which website has the smallest index.html? Which website has the largest index.html? Which website has the median-size index.html? Note that in file transformed-results.txt, the first column shows the file size in Kilobytes (KB). The second column shows the cumulative fraction (expressed as a percentage value between 0 and 100) of files with an index.html size no higher than the value in the first column. The third column shows the URL of the website whose index.html has exactly the size shown in the first column (the URL appears without the leading "http://" and without the trailing "/").

Q4: Looking at the CDF, what can you conclude about the distribution of the file sizes? Is it what you expected?


5. Extra credit: transform.sh

Write the script transform.sh.

To write transform.sh, one approach is to proceed in three steps:

Note: Please attempt this question only if you found the rest of the assignment easy.

Turn-in instructions

You should work on this assignment in a group of two. Each group should turn-in only one copy of the solution. Please make sure to write both names inside each file that you hand in.

Here is the list of files that you need to turn in:

Please find the detailed turn-in instructions here.