IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 10 Exercise

Log in to pe15.cs.umb.edu

Run the ssh client

Connect to pe15.cs.umb.edu.

Directory for this Exercise

Go to your ex directory
```
cd  ~/it244/ex
```
Create an ex10 directory
```
mkdir  ex10
```
Go to your ex10 directory
```
cd  ex10
```

Setuid

Find the absolute path of the passwd program
```
which passwd
```
Look at the permissions on this file
```
ls -l /usr/bin/passwd
```
Notice that the execute column for user (owner) has an s in the execute column and that the owner is root.
Look at the permissions of the file that contains account data
```
ls -l /etc/passwd
```
Notice that while everyone can read this file, only the owner, root, can modify it.

But since the passwd command has setuid enabled, anyone who runs this command has the access permissions that root has.

Directory Permissions

Go to the dir_permissions directory in my course_files/it244_files directory
```
cd ~ghoffman/course_files/it244_files/dir_permissions
```
Make sure you are in the right directory
```
pwd
```
Look at the permissions of the contents of this directory
```
ls -l
```
Look at the contents of dir1 and their permissions
```
ls -l dir1
```
You have read permission for this directory, so you can run ls on it.
Enter dir1
```
cd dir1
```
You have execute permission so you can enter the directory.
Make sure you are in the right directory
```
pwd
```
Create a file whose name is your user name in this directory
```
touch YOUR_UNIX_USERNAME
```
Where you replace YOUR_UNIX_USERNAME with your Unix username
Perform a long listing of the contents of dir1
```
ls -l
```
Since you have write permissions to this directory, you can create a file in it.

Notice that you are the owner of this newly created file.
Return to the dir_permissions directory
```
cd ..
```
Make sure you are in the right directory
```
pwd
```
Look at the permissions on dir2
```
ls -ld dir2
```
Running ls -l on dir2 gives you a long listing of the contents of dir2.

If you want to see the permissions on the directory itself, you must use the -d option.
Look at the contents of the dir2 directory and their permissions
```
ls -l dir2
```
You have read permission, so you can do this.
Enter dir2
```
cd dir2
```
You can enter this directory because you have execute permission on it.
Make sure you are in the right directory
```
pwd
```
Try to create a file in this directory.
```
touch YOUR_UNIX_USERNAME
```
This command fails because you do not have write permission for this directory.
Return to the dir_permissions directory
```
cd ..
```
Make sure you are in the right directory
```
pwd
```
Look at the permissions on dir3
```
ls -ld dir3
```
Try to list the contents of dir3
```
ls dir3
```
You can do this because you have read permission on dir3.
Try to cd into this directory
```
cd dir3
```
This command fails because you do not have execute permission on dir3.
Try to create a file in dir3
```
touch dir3/YOUR_UNIX_USERNAME
```
This command fails because you do not have write permission on dir3.
Look at the permissions on dir4
```
ls -ld dir4
```
Try to list the contents of dir4
```
ls dir4
```
This command fails because you do not have read permission on dir4.
Enter dir4
```
cd dir4
```
This command succeeds because you have execute permission on dir4.
Confirm that you are in dir4
```
pwd
```
Try to list the contents of dir4 now that you are inside the directory.
```
ls
```
Again the command fails because you do not have read permission.
Try to create a file in this directory.
```
touch YOUR_UNIX_USERNAME
```
This command fails because you do not have write permission for this directory.

Links

Return to your home directory
```
cd 
```
Confirm that you are in your home directory
```
pwd
```
Create a link to the ex10
```
ln  -s  it244/ex/ex10  ex10
```
List the contents of your home directory along with permissions
```
ls -l
```
Notice that you have at least two links in your home directory, the it244 link that was created when you applied successfully for this course, and the one you just created.

Notice that the it244 link uses an absolute pathname while the one you just created uses a relative pathname.

Notice that the link you just created references another link, it244.
Enter your ex10 directory using this new link
```
cd ex10
```
Display your current location
```
pwd
```
Now show your true location
```
pwd -P
```
What you think of as your it244 directory is really a link to a directory, named after you Unix ID, in the /courses/it244/s19/ghoffman/ directory.
Return to your parent directory
```
cd ..
```
Show your location
```
pwd
```
Notice that you jumped back to your it244 directory, ignoring the fact that your ex10 directory was inside your ex driectory.
Go back to your home directory
```
cd 
```
Remove the link
```
rm ex10
```
This will allow me to test your script.

Bash Script for this Exercise

Go to your ex10 directory
```
cd ~/it244/ex/ex10
```
Make sure you are in you ex10 directory
```
pwd
```
Create the script ex10.sh
```
nano  ex10.sh
```
Write into this file the commands you used above

Include Unix commands from the section Setuid up to and including the section Links.
Save this file

Save and quit.
Test your script
```
~ghoffman/it244_test/ex10.sh
```
Let me check your script