Updated: 2013-02-02 17:58 EST

1 Due Date and Deliverables

2 Purpose of this Assignment

  1. More review of basic Linux command line operations and utilities, e.g. from CST8207 GNU/Linux Operating Systems I.
  2. Explore the PATH mechanism of the shell
  3. Verify why putting the current directory in your PATH is undesirable
  4. Create simple shell scripts

Remember to READ ALL THE WORDS to work effectively and not waste time.

3 Introduction and Overview

This is an overview of what you will do in this assignment. Read all the words before you start working.

Complete the Tasks listed below on the Course Linux Server (CLS). Run a Checking Program to verify your work as you go. Submit your marks.

Task 1 guides you through the process of exploring the PATH mechanism of the shell. In the process, you will gain some practice working with shell variables.

The remainining tasks involve creating simple shell scripts. You can use the Checking Program to check your work after you create each script. You can check your work with the checking program as often as you like before you submit your final mark.

When you are finished the tasks, leave the files and directories in place as part of your deliverables. Do not delete any assignment work until after the term is over! Assignments may be re-marked at any time; you must have your term work available right until term end.

This lab involves the simplest of scripting techniques, to be built upon in future labs. You will draw on your knowledge of the material from the prerequisite course CST8207 GNU/Linux Operating Systems I; however, you will probably need to refresh your memory of various topics by referring to the CST8207 course notes and the Linux man pages. Your lab instructor is there to help you, but he will want you to have tried consulting the notes and man pages first.

The prevous term’s course notes are available on the Internet here: CST8207 GNU/Linux Operating Systems I. All the notes files are also on the CLS. You can learn about how to read and search these files using the command line on the CLS under the heading Copies of the CST8207 course notes near the bottom of the page Course Linux Server.

Remember to READ ALL THE WORDS to work effectively and not waste time.

4 Tasks

4.1 The Source Directory

All references to the “Source Directory” below are to the directory ~idallen/cst8177/13w/assignment02/ and that name starts with a tilde character followed by a userid with no intervening slash.

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” in http://en.wikipedia.org/wiki/Password_strength#Guidelines_for_strong_passwords

  2. Now read this XKCD comic: http://xkcd.com/936/

  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

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 perhaps then start a new shell, or re-login if you exited your login shell).

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

  1. Create the new directory assignment02 under the existing Assignments directory in your CLS account. (You should have created the Assignments directory in a previous assignment.)

  2. Start a second copy of bash (a nested shell). (Watch the video on “subshells” from the Course Notes). You can use the ps command to see that you have two copies of bash running. Re-read the NOTE, above.

  3. 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. (Hint: 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.) (See your instructor if /bin is not in your PATH.)

  4. 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.

  5. Store the current value of your PATH variable in a new shell variable named oldpath by entering 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 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 section of the assignment, you will lose the value of oldpath and will have to re-save it.

  6. 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.)

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

  8. Set your PATH variable to have no value. The method shown in class was to type nothing after the equals sign - just push Enter after the equals: PATH= 
    Display the new (empty) value of PATH on your screen.

  9. 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 if no commands can be found using the current shell?)

  10. Run the ls program using its absolute pathname (the one you discovered earlier). Note that you do not need any directories in your PATH to run a command using a pathname (containing slashes) instead of a command name (without slashes). The PATH is not used to look for command names containing slashes.

  11. 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

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

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

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

4.4 Creating your first shell script

  1. Create the directory bin in your HOME directory as a place to keep your own personal commands.

  2. Create the directory Assignments/assignment02 under your HOME directory if you have not already done so.

  3. In the assignment02 directory, 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 quotes and dollar sign correct.
  4. 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.

  5. Execute the script. You will note that, just as with the check script from an earlier assignment, the shell cannot find myfirst.sh as an executable command name (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 worldout in the assignment02 directory.

  6. 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 that, 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.

  7. Make assignment02 your current directory. Now, without changing directories again, check the inode numbers of both names to verify that they are hard links to the same inode. Use one command with two relative pathnames to check. When the output is correct, save the output into file myinodes in the assignment02 (current) directory.

  8. Change to your HOME directory. Without changing directories again, execute your new myfirst command that is inside your bin directory using a relative pathname (from your HOME).

  9. In a manner similar to adding “.” to the start of your PATH (see the last of the “03” lecture slides), add 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.

  10. 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 shell and then start another one.

  11. 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).

  12. 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.

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

  14. 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.

  15. Log out and log in. 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 still works.

4.5 Putting the current directory in PATH is a bad idea

  1. Create a fake ls command by copying the script you created above to a file in your home directory and name the new file ls; make sure you use an option to the copy command that preserves the mode (permissions) and timestamps of the file. (RTFM: You should have an alias that adds this option already!)

  2. Edit that new file, changing the text message to read: You are running fake ls with all your privileges - this script could now remove your files!

  3. Make the new fake ls command 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.

  4. Run the ls command and note that you ran 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 directory.

  5. As you did earlier, save the current value of your PATH variable in an oldpath variable.

  6. Make yourself a victim: temporarily put the current directory . at the front of your PATH variable – see the last of the 03 lecture slides for how to do this.

  7. Display the new value of your PATH variable and verify that . is the first directory in the sequence.

  8. Change into your home directory (where the fake ls command resides) and run the ls command as you normally would. Notice that you’ve run a fake ls command as yourself, and it has privileges to do anything your user can do.

  9. 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 command runs with root privileges – and it can destroy your system. NEVER PUT THE CURRENT DIRECTORY IN YOUR PATH!

  10. Using the saved oldpath variable, restore your PATH variable to its former proper value, removing the current directory.

4.6 Creating a script

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 and some without, 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 option letters. The output will be several hundred lines long, with every line starting with the userid that owns that process. The lines will look something like this:

    root     29556   555  0 Jan17     ? 00:00:00 /sbin/udevd --daemon
    cst8207b 26103 26102  0 02:15 pts/2 00:00:00 -bash
  2. Create a new two-line script file assignment02/processes.sh by copying your existing myfirst.sh file and changing the second line of the new file to use the process listing command you used from a 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. Recall from class that the command awk '{print $1}' reads from standard input and prints just the first space-delimited column from that input, and notice the output of the process listing command 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, preserving the mode and timestamps as before. (System administrators always preserve the mode and timestamps of copied files! That’s why we have an alias for the copy command!)

  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-mentioned 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, preserving the mode and timestamps as before. (System administrators always preserve the mode and timestamps of copied files! That’s why we have an alias for the copy command!)

  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. 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 page Redirection.

    Pipe the output of your new processes_sorted.sh script into this uniqueness command with its counting option. The output will be counts of the number of processes being run by each unique userid.

    Instead of hundreds of lines of output, you will see a few dozen lines, depending on how many users are logged in. The lines will all look like this:

     3 postfix
    78 root
  13. Copy processes_sorted.sh to processes_counted.sh. (From here on, we assume you will always preserve the mode and timestamps when you copy a file, and we won’t keep reminding you. Make sure you have your copy alias defined!)

  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 output of the process listing
  1. Copy processes_counted.sh to processes_counted2.sh.

  2. 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.
  1. 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!
  2. Copy processes_counted2.sh to processes_counted_header.sh.

  3. Enhance the new processes_counted_header.sh script so that it prints out its own header 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 ...
  4. Run the new script processes_counted_header.sh to make sure it works. It should generate a 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!
  5. 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 into your bin directory and name the link pch.

  6. 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 a final time using the assignment02check program symlink. Submit your final mark following the directions below.

5 Checking, Marking, and Submitting your Work

Summary: Do some tasks, then run the checking program to verify your work as you go. You can run the checking program as often as you want. When you have the best mark, upload the marks file to Blackboard.

  1. There is a Checking Program named assignment02check in the Source Directory on the CLS. Create a symbolic link to this program named check under your new assignment02 directory so that you can easily run the program to check your work and assign your work a mark. Note: You can create a symbolic link to this executable program but you do not have permission to read or copy the program file. To verify the symbolic link, try executing it.

  2. Execute the above “check” program using its symbolic link. (Review the CST8207 Search Path notes if you forget how to run a program by pathname from the command line.) This program will check your work, assign you a mark, and display the output on your screen. (You may want to paginate the long output so you can read all of it.)

    You may run the “check” program as many times as you wish, to correct mistakes and get the best mark.

  3. When you are done with checking this assignment, and you like what you see on your screen, redirect the output of the Checking Program into the text file assignment02.txt under your assignment02 directory. Use the exact name assignment02.txt in your assignment02 directory. You only get one chance to get the name correct. Case (upper/lower case letters) matters. Be absolutely accurate, as if your marks depended on it. Do not edit the file.

  4. Transfer the above assignment02.txt file from the CLS to your local computer and verify its contents. Do not edit this file! No empty files, please! Edited or damaged files will not be marked. You may want to refer to this term’s updated File Transfer notes.

  5. Submit the assignment02.txt file under the correct Assignment area on Blackboard (with the exact name) before the due date. Upload the file via the assignment02 “Upload Assignment” facility in Blackboard: click on the underlined assignment02 link in Blackboard. Use “Attach File” and “Submit” to upload your plain text file.

    No word-processor documents. Do not send email. Use only “Attach File”. Do not enter any text into the Submission or Comments boxes on Blackboard; I do not read them. Use only the “Attach File” section followed by the Submit button. (If you want to send me comments about your assignment, use email.)

  6. Your instructor may also mark the assignment02 directory in your CLS account after the due date. Leave everything there on the CLS. Do not delete any assignment work from the CLS until after the term is over!

Use the exact file name given above. Upload only one single file of plain text, not HTML, not MSWord. No fonts, no word-processing. Plain text only.

Did I mention that the format is plain text (suitable for VIM/Nano/Pico/Gedit or Notepad)?

NO EMAIL, WORD PROCESSOR, PDF, RTF, or HTML DOCUMENTS ACCEPTED.

No marks are awarded for submitting under the wrong assignment number or for using the wrong file name. Use the exact name given above.

WARNING: Some inattentive students don’t read all these words. Don’t make that mistake! Be exact.

READ ALL THE WORDS. OH PLEASE, PLEASE, PLEASE READ ALL THE WORDS!

Author: 
| Todd Kelly / Richard Donnelly and
| Ian! D. Allen  -  idallen@idallen.ca  -  Ottawa, Ontario, Canada
| Home Page: http://idallen.com/   Contact Improv: http://contactimprov.ca/
| College professor (Free/Libre GNU+Linux) at: http://teaching.idallen.com/
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