3.1 The Source Directory

All references to the “Source Directory” below are to the CLS directory ~idallen/cst8177/14w/assignment02/ and that name starts with a tilde character ~ followed by a userid with no intervening slash. The leading tilde indicates to the shell that the pathname starts with the HOME directory of the account idallen (seven letters).

4.1 Set Up

1. Do a Remote Login to the Course Linux Server (CLS) from any existing computer, using the name appropriate for whether you are on-campus or off-campus. All work in this assignment must be done on the CLS.

2. Create the following directory structure in your CLS HOME directory and record (for study purposes) the series of Unix commands you used to create it. Spelling and capitalization must be exactly as shown:

   CST8177-14W
   `-- Assignments
       `-- assignment02

   This directory is the base directory for most pathnames in this assignment. Store your files and answers here.

3. There is a Checking Program named assignment02check in the Source Directory on the CLS. Follow the instructions in the first two steps at the start of Checking Program to create a working symbolic link to this program.

Check your work so far using the checking program symlink.

4.2 Change your password from the default

System technicians must be highly security-conscious. The CLS is directly on the Internet and is being attacked every minute of the day. Passwords for accounts are being tested for weaknesses.

If you have not already changed your password from the default given to you by your instructor, do so now using the following guidelines. Accounts with unchanged passwords will be disabled next week.

1. Read Guidelines for strong passwords

2. Read this XKCD comic about easy-to-remember passwords.

3. Change your password to one that is more secure than the one you were given. Find a way to remember your new password. If you forget your password, contact your Linux instructor to have it reset.

The CLS is on the public Internet; security is important. Choose your password carefully.

Accounts that do not have their passwords changed before the due date of this assignment will be disabled. See your instructor.

4.3 Exploring the search PATH mechanism

This task explores the shell Search Path.

NOTE: In this task, you will change the PATH variable temporarily for the current shell session only. If at any time you want to undo these temporary changes, you can simply exit the shell that has the changes and then start a new shell, or re-login if you exited your login shell. (Below, we suggest you create a subshell so that you don’t log out when you exit!)

Since this task involves temporary changes to shell variables, you cannot resume this task in the middle if you log out or exit the shell. You must always start this task over from the beginning.

Do NOT edit your .bashrc to make permanent changes that affect future login sessions, unless explicitly told to do so.

1. Start a second copy of bash (a nested shell or subshell). (Watch the MP4 video on Subshells. Some sysadmin like to put the nesting level of the shell in their prompt using the $SHLVL variable.) You can use the ps command to verify that you have two copies of bash running. Re-read the NOTE, above.

2. Display the value of the shell environment variable PATH on your screen and verify that the directory /bin is in the list of directories in PATH. (See your instructor if /bin is not in your PATH.)

   (Hints: Review Shell Variables. There are several ways to see the value of an environment variable. You could use a command that shows single lines of text on your screen.)

3. Use a command to discover which of your PATH directories contains the ls program. Use redirection to put the output of this command (the absolute path to the ls command) in file lspath under your new assignment02 directory. You will need to know this pathname later.

4. Store the current value of your PATH variable in a new shell variable named oldpath by entering a shell assignment statement oldpath=$PATH (no blanks). Make sure that both PATH and oldpath have the same values. (Some commands that can display the value of environment variable PATH cannot be used to display the value of local non-environment variable oldpath.)

   Since setting the value of a shell variable doesn’t save it on disk, the saved value in oldpath is temporary. The value will be lost if you exit this shell and start a new shell. If you exit this shell before completing this task, you will lose the value of oldpath and must start the task over from the beginning.

5. Put the value of the shell variable oldpath into a new file named oldpath under your new assignment02 directory. (Hint: Use output redirection. It’s easy and accurate.)

6. Remove all write permissions from the oldpath file, so that you don’t accidentally overwrite it.

7. Set your PATH variable to have no value. One method is to type nothing after the equals sign – just push Enter after the equals: PATH= 
   Use echo to display the new (empty) value of PATH on your screen. (Other commands may not be found; echo is built-in to the shell.)

8. Try to run any non-builtin command (for example, ls, who, date, etc.) and notice that the attempt fails because the shell cannot find a program with that name in your current (empty) PATH. Type the ls command name and use the correct syntax to redirect just the shell error message into a file called path_error in your assignment02 directory. Verify that the error message is in the file. (How can you verify this using a command if no commands can be found using the PATH variable of the current shell?)

9. Run the ls program using its absolute pathname (the one you discovered earlier). You do not need any directories in your PATH to run a command using a pathname containing slashes. The PATH is not used to look for command names containing slashes.

10. Issue a shell built-in command (for example, echo, pwd, shopt) and verify that built-in commands are part of the shell process itself and do not need to be found in PATH directories. Redirect the output of shell built-in shopt into a file called assignment02/builtin

11. Restore your PATH variable to its original value by restoring its value from the oldpath variable. This is the opposite shell assignment statment of what you did earlier to save the value. If you make a mistake, re-read the NOTE, above.

12. Display your PATH variable to verify it is restored (has the same value as oldpath).

13. Try a few non-builtin commands to verify they are found with the restored PATH in place.

Check your work so far using the checking program symlink.

4.4 Creating your first shell script

1. Create the directory bin in your own HOME directory as a place to keep your own personal commands. (Make sure you create this directory in your own HOME directory.)

2. In the assignment02 directory you created in Set Up, create the file myfirst.sh, containing the following two lines:
   1. The first line of the file should be exactly these 12 characters: #!/bin/sh -u (plus one more character for the newline at the end of the line).
   2. The second line of the file should be a command that displays the exact text: *** It's not a "Micro$oft" World *** on the screen. Make sure you count the asterisks and get the capitalization, quotes, and dollar sign correct.

3. Make the myfirst.sh file executable for only the owner (you). Group and others can read the file but not execute it. You have just finished creating a very simple Linux command – a script that prints some text to its standard output.

4. Execute the script. You will note that, just as with the check symlink, the shell cannot find myfirst.sh as an executable command name with no slashes. You need to use a relative pathname (with slashes) to execute this file. When the output is correct, redirect the output into file myfirstout in the assignment02 directory. (Hint: Review Executing a program in the current directory.)

5. We will give our new command a more command-like name, without the file extension. We will at the same time put the new name into our private bin directory that we just created. To do this, create a hard link from your myfirst.sh script to the name myfirst (no extension) inside your bin directory. You now have one file inode with two names. (Hint: Review Links and Inodes.)

6. Make assignment02 your current directory. Now, without changing directories again, use a command to display the inode numbers of both names to verify that they are hard links to the same inode. Use one command with two relative pathnames. When the two output lines are correct (showing the relative pathnames and the identical inode numbers), save the output into file sameinodes in the assignment02 (current) directory.

7. Change to your HOME directory. Without changing directories again, execute your new myfirst command that is inside your bin directory using the shortest relative pathname (from your HOME). Put the relative pathname you used into file binrel in your assignment02 directory.

8. Start a subshell. Append the absolute pathname of your new bin directory to the right end (not the start) of the PATH variable of your current shell. Remember how you did this – you will need to do it again later in your .bashrc file. (Hint: Review Appending to PATH.)

9. Display your PATH to confirm that the absolute path of your bin directory is on the right end (not at the start). do not add the directory more than once – it should only appear once in your PATH variable. If you need to start over, exit this subshell and then start another one to get a fresh, good copy of PATH.

10. if you have done the previous steps correctly, you can now type the command name myfirst (no slashes, no file extension) and the shell will find your myfirst script in your bin directory and execute it (because your bin directory is now in the PATH for this shell).

11. When you have your own myfirst command working using your modified PATH varaible, save a copy of your modified PATH variable in file binpath in your assignment02 directory. Make sure there are no duplicate directories in PATH before you save it. Your HOME bin directory must be at the end.

12. Create a backup copy of your .bashrc file, in case you need to start over.

13. Make the necessary changes to your .bashrc file to add the absolute path of your bin directory to the right end of your PATH variable, exactly as you did earlier with your interactive shell. Save the file.

14. Log out and log in to have your .bashrc execute. Check your PATH variable to make sure the absolute path of your bin directory is at the right end of your PATH. Verify that your myfirst command from your own bin directory still works (no slashes).

Check your work so far using the checking program symlink.

4.5 Putting the current directory in PATH is a bad idea

1. Make your HOME directory your current directory.

2. Create a fake ls command using these steps:
   1. Copy the myfirst script you created above to a new file named ls in the current (your HOME) directory.
   2. Edit the new file, changing the text message to read exactly this (one long line): You are running fake ls with all your privileges - this script could remove all your files!
   3. Make the new file executable if it is not already.

You have just finished setting a trap for anyone who has the current directory . near the beginning their PATH and runs ls while in your HOME directory.

3. Still in your HOME directory, type the ls command name and note that you get the real ls command, because your PATH does not include the current directory. You do not have . at the start of your PATH so you didn’t run the fake ls in the current (HOME) directory.

4. Start a subshell, or, as you did earlier, save the current value of your PATH variable in an oldpath variable.

5. Make yourself a victim: temporarily put the current directory . at the front (start) of your current shell’s PATH variable. (Hint: Review Appending to PATH.)

6. Display the new value of your PATH variable and verify that the current directory name . is the first directory in the list.

7. Still in your HOME directory (where the fake ls command resides), type the ls command name as you normally would. Notice that you’ve just run the fake ls command in the current directory as yourself, with your permissions, and that fake script has privileges to do anything to your account that your own userid can do.

8. Imagine you are running as root and you have made the mistake of putting the current directory in your PATH as we have done here. Imagine further that you run the ls command after changing into the HOME directory of a user that has placed a fake ls command there. Because you’re root, that fake ls command in the current directory runs with root privileges – and it can destroy your system. NEVER PUT THE CURRENT DIRECTORY IN YOUR PATH!

9. Using the saved oldpath variable, restore your PATH variable to its former proper value without the current directory, or simply exit the subshell that has the bad PATH.

10. Make sure your current shell’s PATH does not contain the current directory before you continue! Log out and back in, if needed.

Check your work so far using the checking program symlink.

4.6 Creating a script

This task explores Processes and Jobs.

In this task you will create a script that prints out the number of processes each user is currently running on the system. You will create a series of scripts, each being an enhancement of the previous one, until you have the final product.

NOTE: The command that gives you a “snapshot of the current processes” has several kinds of options, some with dashes (UNIX style) and some without (BSD style), because it is really two or three commands merged into one program. You sometimes find that mixing options from the different types (with and without dashes) gives error messages. It’s best not to mix option types. See the first few paragraphs of the man page.

1. Run a command that prints out a “full-format listing” and “select all processes” running on the system. Search in the man page for the terms “Select all processes” and “full-format” to discover the correct two dash (UNIX style) option letters. When you run the command with these two dash options, the output will be several hundred lines long, with every line starting with the userid that owns that process. The first few lines will look similar to this:

   UID        PID  PPID  C STIME TTY          TIME CMD
   root         1     0  0 Jan06 ?        00:00:40 /sbin/init
   root         2     0  0 Jan06 ?        00:00:04 [kthreadd]

   (Hint: Pipe the huge output into a command that displays only the first few lines, so you can confirm the above format.)

2. Create a new two-line script file assignment02/processes.sh by copying your existing two-line myfirst.sh file and replacing the second line of the new file to use the process listing command you used from the previous step. Do NOT change the first line.

3. Run the new script processes.sh to make sure it works. It should generate the full-format, all-processes listing for you.

4. As described in the section on awk in Data Mining, the command awk '{print $1}' reads lines from standard input and prints just the first space-delimited field of each line from that input. The output of the process listing command from inside your new script is conveniently in space-delimited columns. Run a command pipeline that feeds the output from your processes.sh script into this awk command line that selects just the first leading (userid) column. You should end up with a list of several hundred userids on your screen.

5. Copy processes.sh to processes_users.sh in the same directory.

6. Change the second line in the new processes_users.sh file to be a command pipeline prints just the first column of the process listing. (Add the previously-used awk command to the end of the line to make it a command pipeline, as you did in a previous step.) Do not change the first line.

7. Run the new script processes_users.sh to make sure it works. It should generate a list of several hundred userids.

8. Pipe the output of your new processes_users.sh script into the sort command. The output will be the list of several hundred userids in sorted order.

9. Copy processes_users.sh to processes_sorted.sh in the same directory.

10. Enhance the command pipeline in the new processes_sorted.sh file, adding a further pipeline into the sort command so that the one column of userids comes out sorted.

11. Run the new script processes_sorted.sh to make sure it works. It should generate a list of several hundred userids in sorted order.

12. Pipe the output of your new processes_sorted.sh script into the command that counts adjacent identical lines. Instead of hundreds of lines of output, the output will be counts of the number of processes being run by each unique userid. You will see a few dozen lines, depending on how many users are logged in. The lines will all have this output format of number followed by userid, though the actual numbers and accounts may differ:

     1 102
     1 UID
     2 avahi
     1 colord
     6 cst8207b
     ...etc...

    (Hint: There is a command that can count the number of occurrences of adjacent input lines, displaying each unique line preceded by the count of the number of times that line appeared. To recall this unique command name and its option to count adjacent lines, review the Command List from last term and the many counting Examples of pipes in the pages Redirection and Data Mining.)

13. Copy processes_sorted.sh to processes_counted.sh.

14. Enhance the command pipeline in the new processes_counted.sh script file to show the counts of processes run by each unique userid.

15. Run the new script processes_counted.sh to make sure it works. It should generate a list of more than a dozen unique userids, each preceded by a count.

We can now see that there are two odd things in the output that are not account userids:

• 102 – a number, not a userid (this appears to be a bug in the process listing program)
• UID – the column heading from the first line of output of the process listing

16. Copy processes_counted.sh to processes_counted2.sh.

17. Enhance the command pipeline in the new processes_counted2.sh file to eliminate the bogus UID column heading in the output. There are at least two ways to do this; pick either way and make the change:
    • One way is to insert a pipeline filter that removes any line containing the string UID. (Hint: the command that searches inside files for lines matching a pattern has an option to invert the match and only show non-matching lines.)
    • Another way is to see if the process listing command has an option to print no header line. (Hint: It does!)
    • DO NOT use any removal method that relies on the number of lines output or the order of the lines output, since the number of lines and the user names in those lines will change from day to day.

18. Run the new script processes_counted2.sh to make sure it works. It should generate a list of more than a dozen userids in sorted order, with no bogus UID in the output.
    • Re-read the note above about how NOT to remove the UID line!

19. Copy processes_counted2.sh to processes_counted_header.sh.

20. Enhance the new processes_counted_header.sh script so that it prints out its own header (title) line before it generates the output. The header line should be the exact text “NumProc Username” (16 characters, including the single space, with upper-case for the N, P, and U) and you can optionally output some dashed underlining under it as well, to make the heading stand out. Line up the blank in the title with the blank in the two columns of output. The output will look similar to this, though the actual numbers and accounts may differ:

    NumProc Username
    ------- --------
          1 102
          2 avahi
          1 colord
          6 cst8207b
    ... etc ...

21. Run the new script processes_counted_header.sh to make sure it works. It should generate its own header line, some optional underlining, followed by a list of more than a dozen userids in sorted order, with no bogus UID in the output.
    • Re-read the note above about how NOT to remove the UID line!

22. The long script name processes_counted_header.sh is helpful for knowing what the script does, but it’s much too long for a command name. Make a hard link from processes_counted_header.sh to the name pch in your HOME bin directory.

23. Because your bin directory is in your PATH, you should now be able to run pch as a command name with no slashes. You have written your second Linux command script.

Check your work so far using the checking program symlink.

4.7 When you are done

That is all the tasks you need to do.

Check your work a final time using the Checking Program and save the output as described below. Submit your mark following the directions below.