CSE 374 15au - Homework 3
Due: Thursday, Oct. 22, at 11 pm.
Assignment goal
The main goal of this assignment is to learn more about shell
scripting and using regular expressions and string processing
programs, particularly grep
and sed
. You
will also learn about accessing files from the web and using a program
called gnuplot
for plotting files.
Searching the Web
Note: The instructions for this assignment may seem a bit long. This is because we try to give you plenty of samples and hints for each part. We hope this will help you complete the assignment faster.
Documentation
In addition to the lecture slides, you may find "The Linux Pocket Guide" a useful reference for completing this assignment (in particular, pages 195 and following [pp. 166 and later in the first edition]).
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 (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". Also be sure to use the regular Unix documentation (man pages and info command), and experiment with commands to try out various options and see what they do.
Getting ready
Download the file: hw3.tar. Extract all the files for this assignment using the following command:
> tar -xvf hw3.tar
You should now see a directory called hw3
.
If you see it, you are ready to start the assignment. If this did not work for you, please post a message on the discussion list describing the problem to see if someone has any ideas, or contact a TA or the instructor (if you send mail, please use cse374-staff[at]cs), or talk to another student in the class.
Background
Because you did very well in CSE 374, 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 particular list was taken
from 100best websites.org a
while back, but even though it is a bit old, it will server our purposes for this assignment).
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 an argument and outputs the size of the
corresponding page in bytes.
For example, executing your script with the URL of homework 1 on the class website as argument:
> ./perform-measurement.sh http://courses.cs.washington.edu/courses/cse374/15au/hws/hw1.html
should output only 12172:
>12172
(This number was correct at the time this assignment was prepared, but might be somewhat different if the page is modified some time in the future.)
If the user does not provide any arguments, the script should print an appropriate 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).
Hints:
- Remember to change permissions
on
perform-measurement.sh
to make it executable. - The
wget
program downloads files from the web. Useman wget
to see its options. - Your script may create temporary files if you
want. The
mktemp
program produces unique file names for temporary files. If you create a temporary file, you should remove it before your script exits. Generally it is best to create temporary files like this in/tmp
. - Experiment with the following commands:
wc a-test-file
andwc < a-test-file
. - To suppress the output of a command, try to redirect its output
to
/dev/null
. For example tryls > /dev/null
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
(sed
, grep
) can help.
You must use grep
and/or sed
even if you know other programs or languages
(awk
, perl
, python
, ...)
that could do similar things in different ways. But it's fine to
use egrep
and extended regular expressions
in sed
and grep
if you wish.
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 content similar to the following into popular.txt
:
http://www.yahoo.com/
http://www.google.com/
http://www.amazon.com/
...
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 appropriate 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
might not contain exactly 100
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. You do need to explain in a readme
file submitted with the rest of the assignment how you chose to
handle any extra urls.
Hints: step-by-step instructions
- First, use
grep
to find all the lines that contain the stringhttp
. Test if it works before proceeding to step 2. - Second, use
sed
to replace everything that precedes the URL with the empty string. Test if it works before proceeding to step 3. Your sed command(s) must match thehttp://...
URL strings themselves, not surrounding text in the table. (i.e., your sed command must use a pattern that matches the URLs although, of course, it may contain more than that if needed to isolate the URL strings. But it can't just be .* surrounded by patterns that match whatever appears before and after the URLs in this particular data file.) - Third, use
sed
to replace everything after the URL with the empty string as well. Test if everything works.
Note: there are some URLs at the beginning and at the end of the
file (such as http://www.100bestwebsites.org/criteria
)
that are not actually part of the list of 100 best web sites. It's
up to you to figure out a reasonable way to handle this so they
don't get included in the output - either by removing them somehow
(by hand? with some sort of script?), or leaving them in and
figuring out how best to deal with them. You should explain what
you did in a readme
file that you turn in with your
code. This shouldn't be extensive or long-winded, just a sentence
or two about what the problem was and how you dealt with it.
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:
- Takes a file with a list of URLs as argument and
executes
perform-measurement.sh
on each URL in the file. - For each URL,
run-experiment.sh
should produce the following output, separated by spaces:rank URL page-size
Therank
of a page is the line number of the corresponding URL inpopular.txt
(or whatever the input file containing the URLs is named). The URL on the first line of the table has rank 1, the URL on the second line has rank 2, and so on until the last URL/line in the file. TheURL
is the same string as the argument you gave toperform-measurement.sh
. Thepage-size
is the result ofperform-measurement.sh
. run-experiment.sh
should write its output to a file. The name of that file should be given by the user as second argument.- If
perform-measurement.sh
returns zero for a URL,run-experiment.sh
should not write any output to the file for that URL. - Because it can take a long time for the experiment to finish,
your script should provide feedback to the user. The feedback should
indicate the progress of the experiment.
- Before executing
perform-measurement.sh
on a URL, your script should print the following message: "Performing measurement on <URL>...
". - Once
perform-measurement.sh
produces a value, if the value is greater than zero, the script should output the following message: "...success
". If the value is zero, this means some error has occurred, and the script should output the following message: "...failed
".
- Before executing
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 output similar to the following:
Performing measurement on http://courses.cs.washington.edu/courses/cse374/15au/
...success
Performing measurement on http://i.will.return.an.error
...failed
Performing measurement on http://courses.cs.washington.edu/courses/cse374/15au/syllabus.html
...success
And the content of results-small.txt
should be similar
to the ones below. Note that the exact values will change as we edit the
class website!
1 http://courses.cs.washington.edu/courses/cse374/15au/ 10042
3 http://courses.cs.washington.edu/courses/cse374/15au/syllabus.html 13987
As another example, after executing your script as follows:
> run-experiment.sh popular.txt results.txt
The file result.txt
, should contain results somewhat
like the ones shown below (when you run your experiment, the exact values
will likely differ)
...
3 http://www.amazon.com/ 157147
4 http://www.about.com/ 96537
5 http://www.bartleby.com/ 41402
...
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 evince
or any other program that knows how to
display an eps file.
> evince scatterplot.eps
If you are working on klaatu or some other remote machine, you can either transfer the .eps file to your local machine and view it there, or you can see it by running the viewer program remotely. In the later case you may need to use the -Y option on ssh (ssh -Y klaatu.cs....) or the equivalent on your remote login application (Putty, for example). This sets up the connection so the remote viewer program can open a window on your local machine to display the results.
If you are using the CSE Fedora VM and evince is not installed, use Fedora's software installation program to add it.
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?
Turn-in instructions
Here is the list of files that you need to turn in:
- Problem 1: perform-measurement.sh
- Problem 2: parse.sh
- Problem 3: run-experiment.sh
- Problem 4: problem4.txt and scatterplot.eps
- The
readme
file containing your (brief) notes about how you dealt with extraneous urls or other problems in the input data.
Every file you turn in should have your name and information identifying the problem in a comment (except for the .eps file where this won't be feasible). If you do combine your files into an archive, use some straightforward format like tar or zip and don't use exotic compression formats. (We want to be able to unscramble what you turn in without too much guessing.)
Please turn in your files using the regular turnin dropbox.
Computer Science & Engineering University of Washington Box 352350 Seattle, WA 98195-2350 (206) 543-1695 voice, (206) 543-2969 FAX
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