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# Reading and finding
The command line · Using a shell
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## Today
**We are learning to** cat prints a whole file and grep prints only the lines that contain a word. Names that start with a dot are hidden: ls skips them, ls -a shows them. With these you can find anything on a computer you can't see.
- I can print a file with cat.
- I can find the lines I want with grep.
- I can find hidden files with ls -a.
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## Why it matters
> ls shows four things in Sam's home folder. There are five. Where's the other one?
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💬 Discussion
## Predict
**In /home/sam, ls shows notes.txt, pets.txt, photos/ and projects/. Is that everything in the folder?**
- Yes, unless the computer is broken
- Yes: ls always shows everything
- Maybe not: names starting with a dot are hidden from ls
- No: ls only shows files, not folders
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fsystems%2Fshell%2Freading%2Fstep-2&code=widget%3A+shell%0Atree%3A+on%0Afind%3A+%2Fhome%2Fsam%2F.plans.txt%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/systems/shell/reading/?full=step-2#step-2)
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✅ Check
## What happened?
* Not everything: there's a hidden file, .plans.txt. ls skips names that start with a dot; ls -a shows them.
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💬 Discussion
## grep cat pets.txt, line by line
Now count the lines.
$$\text{cat} \Rightarrow \text{printed}$$
1) $\text{dog} \Rightarrow \text{skipped}$ — No c-a-t.
2) $\text{catfish} \Rightarrow \text{printed}$ — c-a-t is inside it: grep matches letters anywhere.
3) $\text{parrot, hamster} \Rightarrow \text{skipped}$ — No c-a-t.
4) $\text{cat} \Rightarrow \text{printed}$ — Three lines in all.
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✅ Check
## grep cat pets.txt, line by line
**pets.txt has six lines: cat, dog, catfish, parrot, cat, hamster. How many lines does grep cat pets.txt print?**
* Answer: 3
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✅ Check
## Find the mistake
**Sam looks for a clue in the gym folder. Click the line where it first goes wrong.**
```
1 Sam types ls in the gym folder.
2 ls shows only notice.txt.
3 So notice.txt is the only thing in the gym folder.
4 So there is nowhere else to look for the clue.
```
* Answer: line 3
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✅ Check
## Capital letters
**grep Friday notes.txt prints "Robot club on Friday". What does grep friday notes.txt print?**
- An error
- Every line
- Nothing: grep cares about capital letters (grep -i ignores them)
- The same line
* Answer: Nothing: grep cares about capital letters (grep -i ignores them)
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✍️ Activity
## Vary: find it fast
- **One line:**
- **Count:**
- **Read from here:**
[Open this step](https://studio.learningwaypoints.com/tutorials/systems/shell/reading/?full=step-4#step-4)
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✅ Check
## A huge log
**A log file has 10,000 lines. You want only the lines with ERROR in them. Which command?**
- ls -a log.txt
- grep ERROR log.txt
- cat log.txt
- cd log.txt
* Answer: grep ERROR log.txt
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✅ Check
## Hidden
**Which of these is a hidden file?**
- Config
- .config
- config.
- _config
* Answer: .config
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✅ Check
## Review: paths
**Review: you are in /home/sam/photos. What does cd .. do?**
- Goes home, wherever you are
- Goes to the top, /
- Nothing
- Goes up to /home/sam
* Answer: Goes up to /home/sam
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✍️ Activity
## Make: the treasure hunt, part 1
Then: on a real computer, what kinds of files do you think are hidden, and why?
[Open this step](https://studio.learningwaypoints.com/tutorials/systems/shell/reading/?full=step-6#step-6)
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✅ Check
## Exit ticket
**How would you print only the lines of notes.txt with the word maths in them, and how would you check for hidden files?**
* grep maths notes.txt; ls -a.