--- title: "Who can read what" subtitle: "The command line · Using a shell · Lesson 5" --- # Who can read what **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Everyone at school logs into the same server. Why can't you read your teacher's marks file? :::teacher-note **In one sentence:** Students read permission strings with ls -l, are refused a file they may not read, turn permissions into digits, and set them with chmod on their own files but not someone else's (AC9TDI10K01 node 3, access control on a real kind of system). **Students should be able to say:** I can read a permission string such as rw-r--r--. I can turn permissions into digits and back. I can protect a file with chmod, and explain why hiding it doesn't. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **A file's permissions for everyone else are r--. Can another student change the file?** ☐ Only if they rename it ☐ Yes: they can read it, so they can change it ☐ Only if it's hidden ☐ No: r means read, and there's no w (write) My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** No. r-- means read yes, write no, run no. They can open it but not change or delete it. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: reading rw-r--r-- :::keep-together ### -rw-r--r--, letter by letter 1. $\text{-}$ — The first character: - for a file, d for a folder. 2. $\text{rw-}$ — Owner: read yes, write yes, run no. 4 + 2 + 0 = 6. 3. $\text{r--}$ — Group: read only. 4. 4. $\text{r--}$ — Everyone else: read only. 4. So this file is 644. 5. $\text{chmod o-r file}$ — Take read (r) away from others (o): rw-r----- (640). Now turn a permission string into digits. **rw-r----- as three digits (read 4, write 2, run 1, for owner, group, everyone else)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 640 rw- = 4 + 2 = 6; r-- = 4; --- = 0. So 640. ::: :::keep-together ### Find the mistake **Sam hides a file. Click the line where it first goes wrong.** ``` 1 Sam's plans are in a file called .plans.txt. 2 The name starts with a dot, so ls does not show it. 3 So nobody else on the computer can read it. 4 Sam's plans are safe. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 3 Line 3: hidden only means plain ls skips it. Anyone can ls -a and cat it if the permissions allow. chmod 600 is what protects it. ::: :::keep-together ### Who can chmod? **Who can change a file's permissions with chmod?** ☐ Anyone who can read it ☐ Anyone who can see it with ls ☐ Nobody, once set ☐ Its owner (and an administrator) ::: :::teacher-note **Answer:** Its owner (and an administrator) That's why you couldn't chmod Lee's file. ::: :::typical ## Vary: change who can do what **Hide it properly.** **Let it run.** **Not yours.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Permissions :::keep-together ### 755 **A file is rwxr-xr-x (755). Can everyone else run it?** ☐ No: only the owner can run anything ☐ Yes: their group is r-x ☐ Only on Mondays ☐ Only if it's hidden ::: :::teacher-note **Answer:** Yes: their group is r-x r-x: read and run, but not change. ::: :::keep-together ### Private **Which digits mean only the owner can read and write, and nobody else can do anything?** ☐ 000 ☐ 600 ☐ 644 ☐ 777 ::: :::teacher-note **Answer:** 600 rw------- = 600: the setting for private files and keys. ::: :::keep-together ### Review: pipes **Review: what does grep error log.txt | wc -l print?** ☐ An error ☐ How many lines log.txt has ☐ The lines with error ☐ How many lines of log.txt contain error ::: :::teacher-note **Answer:** How many lines of log.txt contain error grep picks the lines; wc -l counts them. ::: ## Make: lock your plans Then: on a real Mac or Linux computer, run ls -l in your home folder. Which files are most private? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **What does rw-r----- mean, and how would you make it so only you can read it?** {{lines-3}} ::: :::teacher-note **Answer:** The owner can read and write, the group can read, everyone else nothing (640). chmod 600 file (or chmod g-r file). :::