--- marp: true theme: default html: true --- # Pipes The command line ยท Using a shell --- ## Today **We are learning to** a pipe (|) sends one command's output into the next command instead of the screen, so small commands combine: grep finds lines, wc -l counts them, sort puts them in order, uniq drops repeats, head keeps the first few, and > saves the result. - I can join commands with a pipe. - I can count, sort and remove repeats. - I can save a command's output into a file. --- ## Why it matters > No command counts how many different pets are in a list. Three tiny commands joined together do. --- ๐Ÿ’ฌ Discussion ## Predict **grep cat pets.txt prints cat, catfish and cat. What does grep cat pets.txt | wc -l print?** - An error - 6 - 3 - cat catfish cat --- โœ๏ธ Activity ## Try it {{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fsystems%2Fshell%2Fpipes%2Fstep-2&code=widget%3A+shell%0A&fresh=1}} [Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/systems/shell/pipes/?full=step-2#step-2) --- โœ… Check ## What happened? * 3. The three lines go into wc -l instead of the screen, and wc -l counts them. --- ๐Ÿ’ฌ Discussion ## sort pets.txt | uniq | wc -l Now count the lines after uniq. $$\text{sort pets.txt}$$ 1) $\text{| uniq}$ โ€” Neighbouring repeats go: cat, catfish, dog, hamster, parrot. 2) $\text{| wc -l}$ โ€” Counts the lines it receives: 5. 3) $\text{one line, three jobs}$ โ€” Each command is tiny; the pipe joins them into something new. --- โœ… Check ## sort pets.txt | uniq | wc -l **pets.txt is cat, dog, catfish, parrot, cat, hamster. How many lines does sort pets.txt | uniq print?** * Answer: 5 --- โœ… Check ## Find the mistake **Sam explains a pipe. Click the line where it first goes wrong.** ``` 1 Sam types grep cat pets.txt | wc -l. 2 grep finds the lines with cat in them: cat, catfish, cat. 3 wc -l counts the lines in pets.txt, which has 6 lines. 4 So the answer is 6. ``` * Answer: line 3 --- โœ… Check ## Why sort first **Why does uniq need sort before it?** - uniq only works on sorted words - uniq only drops repeats that are next to each other - It doesn't - sort makes uniq faster * Answer: uniq only drops repeats that are next to each other --- โœ๏ธ Activity ## Vary: different pipes - **Different pets:** - **First two:** - **Save it:** [Open this step](https://studio.learningwaypoints.com/tutorials/systems/shell/pipes/?full=step-4#step-4) --- โœ… Check ## Count visits **A server log has a line for every visit. Which pipe counts the visits from Australia (lines with AU)?** - cat log.txt | sort - grep log.txt | AU - grep AU log.txt | wc -l - wc -l log.txt | grep AU * Answer: grep AU log.txt | wc -l --- โœ… Check ## First name **What does sort names.txt | head -n 1 print?** - The name that comes first alphabetically - The number of names - Every name - The first name in the file * Answer: The name that comes first alphabetically --- โœ… Check ## Review: rm **Review: what does rm -r projects do?** - Deletes the projects folder and everything in it, for good - Moves it to the recycle bin - Renames it - Lists it * Answer: Deletes the projects folder and everything in it, for good --- โœ๏ธ Activity ## Make: the treasure hunt, part 2 - **The GOLD line:** - **The code:** - **Count with a pipe:** Then: think of a question about a long list (your playlist, a class list) that a pipe could answer in one line. [Open this step](https://studio.learningwaypoints.com/tutorials/systems/shell/pipes/?full=step-6#step-6) --- โœ… Check ## Exit ticket **Write one line that counts how many lines of pets.txt contain the letter o.** * grep o pets.txt | wc -l (or grep -c o pets.txt).