Week 4

Week 4 - File Exploration and Text Analysis from the Command Line

Lecture Materials

Video Shorts

In-class notes

Wednesday Notes (1)

Wednesday Notes(2)

Note: Links will require you to log in as a UCSD student

Lab Tasks

Discuss with your group:


Write down your answers (and why you chose them!) in your group’s shared doc.

In this lab you’ll work with scripts to do several tasks, and explore programs that do a recursive traversal of directories.

To get started, fork and clone this repository:


The technical/ directory is a sample of writing in English from https://anc.org/data/oanc/download/, a free and open corpus of English text samples. We’ll use it as sample data to explore how to search through files. We’ll do two main tasks:

  1. Answer several questions about the dataset by using command-line tools and bash scripts
  2. Write a web server that can respond to queries for files within this directory

Answering Questions about Text Files

In this section we’ll use a few different command-line tools to build scripts that can answer interesting questions about these text files – they’d work on any directories containing plain text files! We’ll also generally get practice with using tools purely from the command-line.

Counting Text Files

First question: How many text files (files ending in .txt) are there? We’ll walk through this together.

First, let’s try the find command. find will take a directory path as an argument and list files and directories inside that directory. Try using

find technical/

What do you see? (If your local computer is Windows, make sure you have a bash terminal open!)

That’s a lot of files, and all that output kind of takes over the terminal!

One really useful thing we can do with any command is use output redirection to put whatever would be printed into a file. Then we can process that file with other commands. The > character does output redirection in bash. Try:

find technical/ > find-results.txt

What do you see? Nothing, right? Do ls and you’ll see that find-results.txt has been created in the current directory. You can use cat on it and see the long listing of all the files and directories.

Sometimes we want to explore a file at the command line (because we’re on the remote), and we don’t want the long output from cat. Another command, called less, is really good for this. Try:

less find-results.txt

This will “take over” your terminal with just the first screenful of lines. You can press q to exit out of less and get back to the normal terminal (try it, then restart less). You can scroll up and down using the up and down arrows, and go down by a screen at a time by using the space bar. less is a great way to quickly check the contents of a file when you don’t have a convenient visual editor (like VScode) to use to explore it.

OK, so we can confirm that this file that we’ve made find-results.txt, has a bunch of lines and each line is a path. Let’s get back to our question:

How many text files are there?

There are a few ways we could do this. Since we’d (eventually) like an answer that works in a script, it would be useful to find a command that does this, rather than, say, counting them by hand or using the line number in a text editor. That leads us to introduce one more command, wc, which stands for “word count”. wc takes a path and prints out some information about that file.

Try this:

wc find-results.txt

You’ll see output that looks something like this:

    1402     1402   54468 find-results.txt

The first is the number of lines in the file. The second is the number of words (wc uses a pretty simple definition of words – strings separated by whitespace; since the paths don’t have spaces, each counts as one word). The third is the number of characters in the file.

Since there’s one line per path, it seems like 1402 is our answer. We used a few commands and concepts to get here:

  • find «directory-path», which searches (recursively) in a directory for files and lists them all
  • less «file-path», which helps explore files from the command line
  • wc «file-path», which counts words in a file
  • «any-command» > «a-file», which isn’t a command, but we can put after a command to redirect its output to a file

Write down in notes: Show screenshots of using the above commands to get to this answer. Are you sure it’s the right answer? How do you know? Can you see anything that might be inconsistent about that answer when you use less?











Turns out this answer (1402) is wrong. You might say it’s only a little bit wrong, but it’s still not right! It’s wrong because find includes all of the directory names as well as the file names. (It would also be wrong if there were non-.txt files in the directory structure – are there any?)

There are a lot of ways we can do this—I encourage you to do a web search for the -name and -type options for find—we will use it as an excuse to introduce one more really cool command: grep.

At its simplest, grep takes a string and a file, and prints out all the lines in that file that match the string. Try:

grep ".txt" find-results.txt

Then, let’s store the results in a file so we can work with them:

grep ".txt" find-results.txt > grep-results.txt

The, use wc to check the line count in this new file (you try that yourself!)

Write down in notes: What’s the actual count of .txt files?

Putting it Into a Script

That’s a lot of exploration at the terminal! It’s useful to also consider how to turn this into a script that prints the answers. Let’s see what that might look like. We can put the commands in a row in a file called count-txts.sh:

find technical > find-results.txt
grep ".txt" find-results.txt > grep-results.txt
wc grep-results.txt

Then we can run it with count-txts.sh.

$ bash count-txts.sh
    1391     1391   54178 grep-results.txt

Write down in notes: Show putting this into a script and running it to get this answer.

Sometimes it’s useful to parameterize a script with command line arguments. Make it so this script takes the name of the directory to traverse as the first command-line argument, so you use it like this instead:

bash count-txts.sh technical

Then, use it to count the number of files in some of the subdirectories like biomed and plos.

Write down in notes: How many files are in those directories?

Write down in notes: What happens to the find-results.txt and grep-results.txt files when you run the script? What are some consequences of that for where you should be careful when using output redirection?

Counting Sizes of Text Files

Here’s another question that would be nice to answer: How many total words are in the files in technical/biomed?

For this, it would be nice to be able to use wc on all the files in that directory. wc can take multiple filenames. For example, we could give two paths, and wc will tell us the number of lines, words, and characters in each:

$ wc technical/biomed/1468-6708-3-1.txt technical/biomed/1468-6708-3-3.txt 
     432    3380   24112 technical/biomed/1468-6708-3-1.txt
     296    2166   16882 technical/biomed/1468-6708-3-3.txt
     728    5546   40994 total

We can use a * pattern to make wc work on all the files in that directory:

$ wc technical/biomed/*.txt
     432    3380   24112 technical/biomed/1468-6708-3-1.txt
     296    2166   16882 technical/biomed/1468-6708-3-3.txt
     547    4301   31378 technical/biomed/1468-6708-3-4.txt
     317    2312   18114 technical/biomed/1468-6708-3-7.txt
     533    3630   29585 technical/biomed/1468-6708-3-10.txt
     ... lots of lines! ...
  490673 3437323 26328271 total

Here we have our answer – 3437323. That’s a lot of words!

Write down in notes How many total words are in technical/plos? How many total characters?

Another related question we might want to answer is which file in technical/biomend has the most lines? If wc reported the files’ counts in order, we could simply read off the first or last one. But we can see in the output above that there is no particular ordering relative to line, word, or character counts in the output.

There’s another command that’s great for many situations like this: sort. That’s right – there’s a sorting command built-in! sort takes a file and prints out the lines in that file in sorted string order. The way wc is designed, this ends up exactly matching a sort based on line number!

Let’s try it:

$ wc technical/biomed/*.txt > biomed-sizes.txt
$ sort biomed-sizes.txt
... a bunch of lines ...
    1656   12212   89104 technical/biomed/1472-6904-2-5.txt
    1773   10309   83990 technical/biomed/gb-2002-3-12-research0086.txt
    1803    8968   73428 technical/biomed/gb-2002-3-7-research0036.txt
    2236    9393   78562 technical/biomed/1471-2105-3-18.txt
    2359   17408  136424 technical/biomed/1471-2105-3-2.txt
  490673 3437323 26328271 total

The last file output has 2359 lines, and it’s technical/biomed/1471-2105-3-2.txt.

Write down in notes: What is the article in that file about?

Write down in notes: Answer the following questions using grep, find, ** patterns, > redirection, wc, and sort:

  • What is the file with the fewest words in technical/plos? What are the first few lines of that file? (Hint: the line count comes first. You can make wc report just the word count with the -w option)
  • What is the file with the most characters in either technical/plos or technical/biomed? What are the first few lines of that file? (Hint: try the -c option to wc)
  • How many lines in technical/plos contain the string "base pair"? What about in technical/biomed? (Hint: look up the -r option to grep)
  • How many files in technical/plos contain the string "base pair"? What about in technical/biomed? (Hint: look up the -l option to grep)

Copy the commands you used to get these answers along with the answers themselves! You can make scripts out of them (especially if they needed multiple commands).

Discuss: What other interesting questions can you answer with what you know?

A Search Server

The repository also has a file DocSearchServer.java, which has a (fixed) version of getFiles from last week’s lab, and a server that uses it.

  • Add start.sh and test.sh scripts as we did in lecture, and make sure they start the server and run the tests, respectively.
  • Start the server and check that the following URL paths have the described behavior:
    • / prints "There are NNNN files to search" where NNNN is the total number of files returned by getFiles
    • /search?q=search-term prints "There were NNNN files found:" and then a list of all the paths of files that contain that search term. For example, if the search term is base pair it should print the same paths you found in your search above.
  • Add a few tests that give meaningful search results (you can use some of the ideas from using grep above), and take some screenshots of the working server loaded from a browser.

Write down in notes: How long did it take you to make the scripts? Now that you’ve made them how long does it take you to run the tests and start the server? Was that an overall savings on your time? What if we run the tests and server 100 more times this quarter, will it be worth it?

Push to Github: The scripts you added to your fork

Experiment: Add a new text file somewhere in technical with the contents of your choice. Then, get the code and data onto ieng6 if you haven’t already (you could push and then git clone on the server). Start the server and have our partner do a search that finds the file you added. Then do the same with their server (they add a new file that you find). Where are those files stored? What does that say about how the filesystem and paths work for searching for these files?

Then, make an extension to the behavior of the server:

  • Accept queries of the form ?title='<some string>'. This should return all the file paths where the given string is part of the path of the file (including its file name)
  • Write two tests in the test file that use this query
  • Include a few screenshots demonstrating this query
  • Start your enhanced server on ieng6 and get someone else to try it out from another computer

If you want a programming challenge, try making it so you can support queries of the form title=str&q=str that check for both the title and the file contents containing the respective strings.

Getting AI to Do It

What’s a question you want to answer, but aren’t sure how to answer about these files with the commands you have? Maybe someone in your group or your lab tutor would have good guesses! Or maybe…. ChatGPT would.

Come up with at least one idea that you don’t know how to answer with the commands you’ve seen so far. Ask ChatGPT to help! You (or one of the members of your group) can make a free account by logging in with Google.

We’re not giving any examples here because we are all new to this technology. We want you to experiment and teach each other (and us) what works and what doesn’t for you in using it to explore different command-line options.

The crucial thing here is that you should both try out and attempt to explain the results from ChatGPT. As we saw in class, it’s completely capable of lying or giving inconsistent results. So we have to actually run the commands to check that they’re producing something reasonable (and maybe check by hand that some of the answers are correct!)

You’ll probably see new commands (ChatGPT doesn’t know which commands we learned this week), see new options and symbols, and so on. Try asking your tutor, your group, Google, and ChatGPT for help understanding them. Write down in your notes the prompts that worked especially well, and what you learned.

Write down in notes: At least 4 prompts you gave to ChatGPT where it suggested command lines to try, with screenshots showing what happened when you tried out those commands, and explanations of how they work. Don’t just copy-paste the explanation from ChatGPT if it gives one (we’ve seen those be wrong in class, too!) – try to verify the explanation.