Skip to main content

I Made My First Grep Command

(Updated: )

Note: This post was migrated from a previous blog.

For my senior project, I’m working on a voice-powered lab assistant for my college’s CS department. Among the assistant’s tasks is getting menu information from the college cafeteria (the Mensa). So basically, a user should be able to ask something like

What’s for dinner in the Mensa tomorrow?


When does the Mensa next serve Chicken Cordon Bleu?

While doing the first is easy, the second one needed more time. The time-consuming aspect was trying to scrape all the possible food items from the Mensa’s menu page because

  • I had to collect records for 200ish days
  • Some food items repeat
  • Sometimes the menu has notes to look out for
  • Sometimes menu items express the same thing and state them differently (“white and wheat rolls” vs “wheat and white rolls”)

After spending a considerable amount of time crafting a Node.js script to collect the menu pages, I needed to pick out the food items.

For the most part, the food items were represented in lists, like this

        <li>Hard Boiled Eggs</li>
        <li>Design Your Own Omlette</li>
        <li>American Fries</li>
        <li>Fruit Salad</li>

Not the best HTML, but at least I could extract the menu items by taking the contents of every list element.

Up to this point, I was using cheerio.js a jQuery implementation for Node.js, allowing you to parse HTML and select and manipulate the DOM.

I was going to make a loop for each webpage and save all the scraped foods into a separate file, but then I thought about grep.

While I’ve never used grep, I know it’s used for searching for text in files. After a little googling (While writing this, I realized that I could have used man grep 🤦🏾), I found created a bash command that will pull out all the list items in all the files, remove some items, sort the file, remove duplicates and save it in a file. Here it is.

$ grep -hoP '(?<=<li>).*?(?=</li>)' * | sed "s/(RFH.*)$//g" |sort --unique > ../foods.txt

Let’s go through it piece by piece.

  • grep -hoP '(?<=<li>).*?(?=</li>)' *
    • grep - The utility responsible for searching files
    • -hoP- configuration for grep. h tells grep to omit the file name from the result so I just have the match. o tells grep to only print the matching text instead of the context around it as it usually would. P means use Perl regular expressions. Note that these flags are case sensitive.
    • '(?<=<li>).*?(?=</li>)' - The regular expression that matches the contents within <li> tags. (?<=<li>) and ?(?=</li>) are the positive lookarounds which essentially means that the expression should look for these elements, but not match them (that is, look for stuff between these). .* means that it will look for any number of (*) any characters (.). Basically, this expression means “match the contents of the <li> HTML elements”.
    • * is the thing we want to grep, in this case, the current directory containing the menu forms.
  • | - this is a pipe command, which tells bash to pass the output of the previous command (grep), to the next command.
  • sed "s/(RFH.*)$//g"
    • sed - sed is a stream editor. Simply put, it parses and edits text in files.
    • "s/(RFH.*)$//g" - This means substitute (s/) anything matching (RFH.*)for nothing (//) across the entire file (globally, g)
  • Another | which pipes sed’s result to sort
  • sort --unique
    • sort does what it says on the tin, sorts the lines in a file alphabetically
    • --unique collapses duplicate lines into one, ensuring that every line has unique values.
  • > ../foods.txt means instead of writing the output to the file, write the results to the file called ../food.txt. Note that this overwrites the prior contents of the file.

And there it is, 14 lines of code put into one. These 14 lines to be exact.

const cheerio = require("cheerio"); // Extract HTML
const fs = require("fs"); // File System
menu_items = new Set(); // Ensures unique elements are added

let files = fs.readdirSync("."); //read current directory (synchronously)
files.forEach((file, i) => {
    // for each element
    data = fs.readFileSync(file); //get the file's contents
    let $ = cheerio.load(data); //build a DOM using cheerio
    $("li").each((i, elem) => {
        // add each list item to the set
menu_items = Array.from(menu_items); //turn the set to an array
fs.writeFileSync("food.txt", menu_items.join("\n")); //write the list items to a file

I think it’s really cool that bash provides all of these utilities. I remember reading the book The Linux Command Line by William Shotts which looks into how the Linux Command Line works. I don’t think I finished it but I had so much fun reading it. I’ll read it again and let you guys know what I feel about it.