ce03 Multiuser System Fun Times

This class exercise is designed to familiarize students with file and directory permissions in a multiuser Unix environment.

Prerequisite Knowledge

Understanding of Unix basics, including commands, text editors, etc.
CSCI 1302 Package Tutorial
Unix Tutorial Five
CSCI 1302 Octal Mode Reading

Course-Specific Learning Outcomes

LO1.a: Navigate and modify files, directories, and permissions in a multi-user Unix-like environment.
LO1.b: (Partial) Execute, redirect, pipe, and manage programs/processes in a multi-user Unix-like environment.
LO1.c: Create and modify text files and source code using a powerful terminal-based text editor such as Emacs or Vi.
LO1.d: (Partial) Use shell commands to compile new and existing software solutions that are organized into multi-level packages and have external dependencies.

Questions

In your notes, clearly answer the following questions. These instructions assume that you are logged into the Odin server.

NOTE 1: For this exercise, at least two group members will need access to Odin at the same time. When relevant, the instructions will refer to [user1], [user2], and [group] to indicate who should perform the actions for that step. Make sure you execute them in order.

NOTE 2: If a step requires you to enter in a command, please provide in your notes the full command that you typed to make the related action happen. If context is necessary (e.g., the command depends on your present working directory), then please note that context as well.

Exercise Steps

Checkpoint 1 Steps - Multiuser Intro

[user1]: Launch a terminal emulator, login to Odin, then create the subdirectory structure seen below. What single command can be used to create this hierarchy of directories? Ignore Loop.java for now.
```
exercise3
|--- bin
|--- src
      |--- cs1302
            |--- loop
                 |--- Loop.java
4 directories, 1 file
```
[user1]: Change into the exercise3 directory. For the rest of this exercise, do not change out of this directory unless the instructions explicitly ask you to do so.
[user1]: Use Emacs to open Loop.java in src/cs1302/loop without changing your present working directory. Remember, the file will not actually be created until you save it.
[user1]: In Loop.java, create the cs1302.loop.Loop class (fully qualified name) and write an infinite loop in the main method that doesn't cause any output. For example, you might write the following:
```
while (true) {
    int a = 2;
} // while
```
Be sure to save Loop.java, then close Emacs.
[user1]: What is the size and mode of Loop.java? What exact command did you use to find this?
[user1]: Compile Loop.java specifying the default package for compiled code as bin. As always, be sure to write the full command you used in your notes.
[user1]: Run cs1302.loop.Loop using java. Be sure to indicate the default package for compiled code by appropriately setting the class path option. The program will hang due to the infinite loop. To terminate the program, you can use C-c.
[user1]: Now run cs1302.loop.Loop as a background process. What exact command did you use? How do you know it worked (i.e., Is there a command you can use to verify this)?
[user2]: Launch a terminal emulator on your laptop, connect to odin on your account (not the same account as user1).
[user2]: Find the Process ID (PID) of user1's executing cs1302.loop.Loop process using the following command:
```
$ ps aux
```
The second column in the output denotes the PID for each process. However, this command shows every process that it running on the system, which is a little overwhelming. Remember, you can pipe the output of this program into grep to filter the output to only include certain entries (e.g., ones containing user1's username).

In your notes, include the piped ps command that you used to produce the desired output as well as the PID that you found. Note that the PID is execution specific, i.e., a different one is used every time a new process is created.
[user2]: Let's see if you can interfere with user1's process on the system. Ordinarily, this would be against the UGA Policies on the Use of Computers, however, we will make a brief exception for you to do the following: try to kill user1's process. Were you successful? What was the output from the kill command you used?
[user1]: Most Unix systems are setup to only allow the user who launches a program to terminate it. The usual exception to this are the superusers (i.e., the system administrators). It looks like user2 is not a superuser. Phew! Please go ahead and terminate the process yourself. What command or sequence of commands did you use?

Checkpoint 2 Steps - File Permissions

[group]: Each person in the group should execute the following command:
```
$ umask
```
If the output is 0022, then proceed to the next step. Otherwise, please see an instructor so that they can inspect your Bash configuration to ensure that you can successfully complete this exercise.
[user1]: What is the mode of the exercise3 directory in both octal and symbolic notation?
[user1]: Carefully change the permission of your home directory to add execute permission for other. Be very careful! If you mess up the user permissions on your home directory, then that can lead to unintended, bad consequences. Since this is a high stakes step, the exact recommended command is provided below:
```
$ chmod o+x ~
```
This step is needed because student home directories on Odin are in a group that is different from ugrads. Subdirectories of your home directory should belong to the ugrads group as expected. You should remove execute permission for other at the end of this activity using:
```
Don't execute the command below right now. Wait until the end of the class period.

$ chmod o-x ~
```
[user2]: Can you successfuly perform an ls on user1's exercise3 directory? Don't be afraid to ask them for the absolute path of their exercise3 directory. Write the full command you used in your notes.
[user1]: Change the mode for Loop.java so that group members only have write permission and others have no permissions. What command(s) did you use? What is the new mode for Loop.java?
[user2]: Try to see the contents of user1's Loop.java using cat. In your notes, explain why this doesn't work (saying permission denied is not a sufficient answer).
[user2]: We're about to ask you to use output redirection to modify another student's file. Remember that there is a BIG difference between > and >> when performing output redirection. Take a moment to refresh your memory regarding this before continuing with this step.

Now that you remember the difference between > and >>, use output redirection to append a single line Java comment to the end of user1's Loop.java.
[user1]: Inspect your Loop.java. Did user2 successfully append a comment?
- If not, then recreate the previous contents of the file, save it, then have user2 try the previous step again.
- If yes, then delete user2's message from Loop.java, save, then enable group read permission on Loop.java. In your notes, include the command you used.
[user2]: Now that you have read permission, open user1's Loop.java file and modify it so that it is no longer has an infinite loop. What program did you use to edit the file?
[user1]: Now that user2 has finished helping you modify your Loop.java, remove group write permission from Loop.java to prevent user2 or other in the same group from making further changes. As always, write the full command you used in your notes.

Checkpoint 3 Steps - Directory Permissions

[user1]: Create a subdirectory within exercise3 called thoughts. Inside of the thoughts directory, add a file called notes.txt. Explicitly set the mode for each of the following to the indicated octal:
- exercise3 --> 770
- exercise3/thoughts --> 700
- exercise3/thoughts/notes.txt --> 660
group: In your notes, create the table below. For a directory, read and write permission are generally only useful in conjunction with execute permission. For this reason, some modes are omitted.

Mode a b c d e

700

710

730

750

770

Now, for each row in the table:
1. [user1]: Change the mode of the thoughts directory to the value specified in the Mode column.
2. [user2]: Try each of the actions listed below record a yes/no in your table. If anything surprises you, then be sure to write that down in your notes.
  - (a) Read notes.txt using cat.
  - (b) Delete notes.txt.
  - (c) List the contents of thoughts using ls.
  - (d) Add a new file to thoughts called notes2.txt.
  - (e) Use cd to change to thoughts.
3. [user1]: If user2 successfully deleted notes.txt, then recreate it and change its mode to 660.
4. [user1]: If user2 successfully created notes2.txt, then delete it.
[user1]: Explicitly set the mode for each of the following to the indicated octal (these are different from step 1):
- exercise3 --> 700
- exercise3/thoughts --> 770
- exercise3/thoughts/notes.txt --> 660
group: In your notes, create the following table:

Mode a b c d e

700

710

720

Now, for each row in the table:
1. [user1]: Change the mode of the exercise3 directory to the value specified in the Mode column.
2. [user2]: Try each of the actions listed below record a yes/no in your table. If anything surprises you, then be sure to write that down in your notes.
  - (a) Read notes.txt using cat.
  - (b) Delete notes.txt.
  - (c) List the contents of thoughts using ls.
  - (d) Add a new file to thoughts called notes2.txt.
  - (e) Use cd to change to thoughts.
3. [user1]: If user2 successfully deleted notes.txt, then recreate it and change its mode to 660.
4. [user1]: If user2 successfully created notes2.txt, then delete it.
[group]: In the previous question, you probably noticed something interesting about the row for mode 710. Is the same set of actions possible with other group permissions? If so, list the three digit octal mode for each of them.
[group]: What permissions do you feel are appropriate for a user's home directory? Explain your choice.
[group]: Change the mode of your home directory back to 750.

Copyright © Michael E. Cotterell, Bradley J. Barnes, and the University of Georgia. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License to students and the public and licensed under a Creative Commons Attribution-NonCommercial 4.0 International License to instructors at institutions of higher education. The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.

Mode	a	b	c	d	e
700
710
730
750
770

Mode	a	b	c	d	e
700
710
720

cs1302uga/cs1302-ce03