Laboratory Exercise on File Sharing

This laboratory exercise explores some capabilities of files and permissions on a Linux-based system.

Some Background

In the Linux world, accounts are organized at three levels:

Personal account: Each user has her or his own account
Group: Each user is part of a group of users
Others: Users not in one's group are considered "others"

Within this structure, files and directories have three types of permissions, and each type of permission has an associated number:

Number	Description
4	ability to view the file (read permission)
2	ability to change the file (write permission)
1	ability to execute the file as a program (execute permission)

Permissions can be combined by adding the corresponding numbers. For example, permission 6 allows a user to read and write (but not execute) a file.

Getting Started

Work in this lab is based on a file my-account-script in the instructor's course account for CSC 105. When this file is run, it prints some basic information about a user's account and the workstation being used.

Run the program my-account-script to determine what it does.
1. Log in to your account, and open a terminal window.
2. Within the terminal window, run the program using the command
```
/home/walker/105/my-account-script
```
3. If you are interested in reviewing how this script works, you can look at it with the statement
```
cat /home/walker/105/my-account-script
```
  You may want to ask your instructor to interpret various details.
Copy file /home/walker/105/my-account-script to your public_html directory, as follows.
1. Within the terminal window, type
```
cd public_html
```
  to move to this directory that you created in previous labs.
2. Type
```
cp /home/walker/105/my-account-script .
```
  (where the final period indicates the file is to be copied to your current directory).

The `ls` Command in Linux

The command


ls -l

provides a long form of listing for files in a directory. The command


ls -l -a

includes information on the current directory (listed as .) and its parent directory (listed as ..).

In deciphering a line of the listings, the first part of each line gives permission information: For example, in the sequence:


-rwx------

the initial dash indicates this is a regular file; a line starting with d indicates a directory.

The next 9 characters indicate permissions:

read, write, execute information for the user
read, write, execute information for the user's group
read, write, execute information for the rest of the world

In the example, rwx------ means the user had full capabilities for the file, while others have no capabilities to work with the file.

Experiments

Use the ls command to determine the protection code of the newly-copied file my-account-script and describe what you encounter.
Try to run this file by typing
```
my-account-script
```
in your terminal window.
Change the permission code for this file so you have execute permission:
```
chmod 700 my-account-script
```
and check the permissions with the ls -l command.
Now try running the file script again.
Use your experience in the previous three steps to explain what is meant by "execute" permission for a regular file
Change the permission code so you can read, but not write or execute, the file. Then try to edit the file with the command
```
emacs my-account-script &
```
For example, you might try to add a line
```
echo "I added this line"
```
1. Can you view the file?
2. What happens if you try to change it?
Set the permission code, so that others can read and execute (but not write) your home directory and your public_html directory. Allow only those in your group to read and execute your file my-account-script. This can be done with the following commands:
```
cd
chmod 755 .
cd public_html
chmod 755 .
chmod 750 my-account-script
```
Your lab partner now should be able to read your file using the name /home/your-account/public_html/my-account-script
With the file permissions set this way, ask your lab partner to try to access and run your script when logged into her or his account. Describe what happens.
Can your lab partner edit your file?
Change the protection code of your file my-account-script to allow group editing (but still no access by others outside your group). What happens now if your lab partner tries to edit the file?
Try accessing the file in your browser, using the URL
```
http://www.cs.grinnell.edu/~your-account/my-account-script
```
Describe what happens.
Change the permission code, so all others can read the file (but not execute or change it).
Now try accessing the file again in your browser, and describe what you see. (Is the file listed; is the file run?)
Change the permission code, so all others can execute the file (but not read or change it). Again, describe what happens when you try accessing this file in your browser.
Summarize your conclusions regarding the nature of permissions for accessing a file with your browser.
Reset the permissions of your file, so that it can be read over the World Wide Web.
Now set the permissions of your public_html directory, so that it can be read, but not executed.
1. What happens when you try to access my-account-script in your browser?
2. What happens in your browser when you use the URL
```
http://www.cs.grinnell.edu/~your-account
```
Now set the permissions of your public_html directory, so that it can be executed, but not read. Again, determine what happens in each case listed in the previous step.
Summarize your conclusions regarding the nature of permissions for accessing a directory with your browser.

Work To Be Turned In

Explanations for steps 3, 7-8, 10-19.

This document is available on the World Wide Web as

http://www.cs.grinnell.edu/~walker/courses/105.sp04/file-sharing.shtml