This is Lab Worksheet 8 - not an Assignment

This Lab Worksheet contains some practical examples that will prepare you to complete your Assignments.
You do not have to hand in this Lab Worksheet. Make sure you complete the separate Assignments on time. Quizzes and tests may refer to work done in this Lab Worksheet; save your answers.

Before you get started - REMEMBER TO READ ALL THE WORDS

You must have an account on the Course Linux Server to do this lab. Log in to the server and use the shell. Review the Class Notes related to this worksheet as you work through it. Leave your work on the Linux server. Do not delete any work from the Linux server until the term is over and your marks are complete!

Linux File System Permissions (modes)

Commands, topics, and features covered

Use the on-line help (man command) for the commands listed below for more information.

• chmod – (change mode) Change the permissions (mode) on an existing inode (file, directory, etc.)

• ls -lid – (list structure, long version, inode, directory) See the permissions of an inode

• umask [value] – (user mask - shell built in) Display or change the octal umask value for this shell
  
  

See the course notes and man pages regarding the new commands umask, and chmod.

Log in to the Course Linux Server to do all commands in this lab. Set your bash PS1 shell prompt to show your login name, computer name, and the basename of your current directory, just as you did in the previous Lab. Leave your finished work on the server; do not delete it when you are finished the worksheet.

1Exercise: Conversion between symbolic mode and octal mode

For each nine-character symbolic permission, give the equivalent three-digit octal permission and the three-character symbolic permissions that apply to each of User/Owner, Group, and Other:

2Exercise: Conversion between octal mode and symbolic mode

For each three-digit octal permission in the following table, give the equivalent nine-character symbolic permission and the three symbolic permissions that apply to each of User/Owner, Group, and Other:

Did you read all the words before completing the above exercise? Symbolic or numeric? Read all the words.

3Exercise: Understanding directory permissions

This exercise chooses eight different permissions for a directory and then for each of the eight permissions attempts some basic commands such as cd, touch, mkdir, and ls. Different permissions will allow or deny each of these actions. You are to set the permissions, try each of the actions, and record whether or not it worked. Are the results as you would expect?

1. Execute the following three commands to create a testing directory:

[user@host ]$ cd ; rm -rf lab08 ; mkdir -p lab08/top ; cd lab08

For each row of the table below, repeat these next two steps (B and C) that will set some permissions on the top directory and then try four commands to see if those commands work with those directory permissions:

2. From in the lab08 directory change the permission of the top directory using the chmod command given in the second column (Command line) of the table. Execute this chmod command in the lab08 directory to set the permissions on the top subdirectory.

3. Next, for the row and permission value you just set in Step A, try to execute the four commands listed across the top of the table. For each of the four commands, record in the table whether or not the command line executes successfully. Enter in the table PD for Permission Denied; OK for success. The four commands to try are these (also listed across the top of the table):

1. cd top followed immediately by "cd .." only if it worked (gave no error messages)
   You need to "cd .." to get back up to the lab08 directory only if the cd command worked.

2. touch top/file followed immediately by "rm top/file" if it worked

3. mkdir top/dir followed immediately by "rmdir top/dir" if it worked

4. ls -l top

• You will carry out Steps B and C above (one chmod and four other commands) for each of the rows of the table below. Do the chmod, then try each of the four commands and record PD or OK.

• Before you run each chmod command in the table below, ensure your current directory is lab08.

• If the cd into top works (no error), follow it immediately with "cd .." to return up to the lab08 directory again, otherwise you will be in the wrong directory for the next command in the table.

• If the touch works (no error), immediately remove the file you just created.

• If the mkdir works (no error), immediately remove the directory you just created.

• If you get any error other than Permission Denied, you either made a typing error or your current directory is not lab08 – you probably forgot to do "cd .." to return up to the lab08 directory again after a successful cd top command. You must always work in the lab08 directory.

Did you read all the words before completing the above exercise? Your table must contain only OK or PD. If you see any other error such as "no such file or directory", you are either typing incorrectly or you are not in the correct lab08 directory when you are running that command.

4Exercise: Minimum Permissions for common file operations

In the table below, give in symbolic rwx form the minimum permissions an ordinary (non-root) user requires to successfully complete the commands listed in the left column below. "Minimum" means the least amount of rwx permissions needed. How many of rwx can you take away and still perform the given command successfully? What are the minimum permissions needed? Test your guess to make sure you are correct.

Recall that directories store only names and inode numbers, not data. Recall that some commands require permissions only on the directory inode; some require permissions on the directory and the data inodes.

1. [user@host]$ cp srcdir/srcfile targetdir/ (new file name and content)

2. [user@host]$ mv srcdir/srcfile targetdir/ (rename to new file name)

3. [user@host]$ ln srcdir/srcfile targetdir/ (add new file name)

4. [user@host]$ rm srcdir/srcfile (remove an existing file name)

5. [user@host]$ cat srcdir/srcfile (display an existing file)

6. [user@host]$ date >> dir/oldfile (append to an existing file)

7. [user@host]$ date > dir/newfile (create a new file in an existing directory)

For commands 6 and 7 above, the directory is not a "source" directory since nothing is being read from it, and the file is not a "source" file since it is being written to (it is an output target). Use the first two columns in the table below to record permissions for the directory and the target file for 6 and 7 above.

Now go back and re-read all the words of this question, including the word "symbolic" in the first line.

Make sure all the answers are the minimum permissions needed to do the given operation. (You could not remove any more of the given permissions and still have the operation work.)

5Shell umask and permissions for new files and directories

The shell umask value restricts the permissions given to new files and directories when they are first created. Without the umask value, or with a zero value, a new file would be created with full permissions 666 and a new directory with full permissions 777. The umask masks out permissions - each bit of the umask turns off the corresponding permission in the newly created file or directory. A umask value of of 777 turns off all permissions on new files and directories; a umask of 000 doesn't turn off any permissions (not recommended!).

Note that masking is not the same as subtracting, e.g. 666 masked with 001 is still 666 and 666 masked with 003 is 664. The mask turns off permission bits. If they are already off, the umask makes no change:

• rw- (6) masked with --x (1) is rw- (6) because the x bit in the mask does not change any permissions

• rw- (6) masked with -wx (3) is r-- (4) because only the w bit is changed (turned off) by the mask

Each process, and thus each shell, has its own umask value. Different Linux distributions set different default (at login) umask values. The values in your particular distribution of Linux may not be the same as other distributions. The values set by the system administrator may differ from the distribution defaults. Do not rely on the umask having any standard value.

1. What built-in shell command displays or sets the umask value: ______________

2. Give the default (at full login) umask for your account: ____________

For each row of the following table set the given umask and then fill in the four new permission columns:

3. Set and use the given umask in column two before filling in the permissions in the rest of the row.

4. Based on the umask in column two, write down the permissions that would be given to a new file and a new directory. Give both the symbolic and numeric forms. You can create a new file and a new directory to verify your answer, but you should calculate and know the answer without needing to create anything. Using the given umask, what would be the permissions set on a new file and a new directory?
   
   

After you are finished the above exercise, exit your shell or reset your umask back to the normal umask. Do not continue to use a shell that has a umask of 0777.

1. True or False - the umask can change the permissions of existing files and directories? __________

2. What command name changes the permissions of existing files and directories: __________

3. What value umask gives the owner and group full permissions on new files and directories: _____

4. What value umask gives only the owner full permissions on new files and directories: _______