Assignment 1: Shell We?

Table of Contents

1. Overview
   1. Learning Goals
2. Part I: Write Simple Regular Expressions (1.5 points)
3. Part II: Write Data Wrangling Pipelines (1.5 points)
   1. Subpart 1: Three O’s
   2. Subpart 2: Separated O’s
   3. Subpart 3: Common Suffixes
4. Feedback Survey (0.5 points)
5. Submitting Your Assignment

Overview

This assignment consists of two different components:

1. You’ll familiarize yourself with RegEx by completing a few exercises
2. You’ll combine your RegEx and shell expertise to wrangle some words

We expect this assignment to take 1-3 hours depending on your proficiency level with the tools. If you find yourself unproductively stuck or unproductively struggling (spinning in circles), ask on Ed and/or go to office hours!

As a reminder, you can use man pages to learn more about commands you don’t yet know (especially what flags they support). The website cheat.sh also has many examples of how to use different commands.

Learning Goals

We think it’s important that our assignments exercise the tools and concepts we’re learning in class. In general, lectures will provide a conceptual framework for understanding these systems, while assignments will give examples of the tools used in practice. Here’s what you can expect to take away from this assignment:

• You’ll get practice with designing regular expressions to match interesting patterns in text.
• You’ll get practice using shell tools and controlling their behaviors with flags and arguments.
• You’ll gain a significant amount of experience in using the shell, including:
  • calling different programs to perform specific tasks
  • chaining programs together with pipelines
• You’ll grow your ability to discover information about tools and utilities. This is really important, because in the “real world” (a job or another class), this will be one of the major ways you self-teach yourself whatever tools are relevant to the task at hand.

Part I: Write Simple Regular Expressions (1.5 points)

For this part of the assignment, you will get some practice writing simple regular expressions. You should complete up through Lesson 10 in the regular expressions tutorial found here.

Place all of your answers in a simple text file called regex.txt. Remember that you can make a new text file using the touch command. You can open a file by using the open command (on macOS), the explorer.exe command (on Windows) or the xdg-open command (on Linux). The answer for each lesson (including lesson 1½) should go on a new line inside of regex.txt. Note that there are many possible correct answers to each exercise!

Important: Make sure you name your file regex.txt and not something else. Make sure that your file is exactly 11 lines long, one line per lesson.

Part II: Write Data Wrangling Pipelines (1.5 points)

For this part of the assignment, you will get some practice with sed and grep by writing commands to parse data inside a dictionary file we provide, which is a newline-delimited list of English words. Each exercise in this part can be solved multiple different ways (as is often the case with data wrangling!). In particular, many can be solved with either sed or grep, since there’s a lot of overlap in what they do; for this assignment, feel free to use either tool.

To begin, use the command below to download a dictionary file. (Different machines have different dictionaries installed, so we want to make sure everyone is using the same one):

curl -Lo dictionary.txt https://cs45.stanford.edu/res/assign1/dictionary.txt

You can run cat dictionary.txt to see what the dictionary of words looks like.

As a reminder, the regex debugger at regex101 is really helpful! We’d recommend making use of it while you develop your regexes, to make sure it’s doing what you think it is.

Subpart 1: Three O’s

First, find the number of words in dictionary.txt that have at least three o‘s. Take a look at the man page of grep or sed for possible ways to do this.

Subpart 2: Separated O’s

Next, find the number of words in dictionary.txt that have exactly three o’s and where all o’s are separated by at least one non-o character. In other words, you should match a word such as microbiology (as all three o’s have at least one non-o character separating them) but you should not match a word such as zootrophy (as there are two adjacent o’s that are not separated by a non-o character). This exercise is a little tricky as you need to consider that some words begin with an uppercase O and regexes are case sensitive by default.

Subpart 3: Common Suffixes

Finally, find the three most common last three letters for words in dictionary.txt that have two adjacent o‘s. We recommend doing this in two steps:

1. Find a list of all words in dictionary.txt that have two adjacent o‘s. (The words can have more than two o’s as long as at least two of them are adjacent).
2. Then find the three most common final three letters of these words. (Hint: take a look at how you might use grep to strip off the final three letters and how you might use sort and uniq to find the most frequent occurrences for a given set of data).

To submit your answers, create a file named words_commands.txt and place the commands you used to generate your answers for each subpart on a new line. This file should be exactly three lines long. Each line should contain a single command pipeline that, if typed in at the shell, would output the requested data.

Feedback Survey (0.5 points)

Once you have completed the assignment, you can earn an additional 0.5 points by completing our anonymous feedback survey. Given this is the first offering of the course, we want to collect as much feedback as possible to improve the course in the future. You can complete the survey here.

Once you complete the survey, you will receive a completion code which you should place in a text file named survey.txt. The survey is anonymous so submitting the completion code is the only way to verify that you completed the survey. Please do not share this code with anyone, as that would constitute a breach of the honor code.

Submitting Your Assignment

Once you have finished this assignment, you will need to upload your files to Gradescope. Make sure to upload both files to the Assignment 1 submission page. You should also upload survey.txt if you completed the survey.

Here are the files we’re expecting:

• regex.txt: eleven lines, each of which contains a regular expression
• words_commands.txt: three lines, each of which contains a valid command pipeline
• (optional) survey.txt: one line, which is the survey code

Gradescope will autograde your submission and let you know if your solutions worked. You may resubmit as many times as you’d like before the deadline.

All files must have the same names as specified above.