% CST8207 Assignment 08 - hard links, shell PATH, disk usage, du % Ian! D. Allen - - [www.idallen.com] % Fall 2013 - September to December 2013 - Updated Tue Nov 19 10:47:30 EST 2013 Due Date and Deliverables ========================= - **Due Date**: `08h00 (8am) Monday November 11, 2013 (start of Week 11)` - Material covered in this assignent will appear on [Midterm Test #2] - You have almost three weeks to do this assignment, but your next assignment will be available in two weeks and will overlap this assignment by a week. Don’t delay! - Late assignments or wrong file names may not be marked. Be accurate. - **Available online**: - Version 1 – 14:00pm Tuesday October 22, 2013 (part C not ready yet) - Version 2 – 16:30pm Tuesday October 22, 2013 - **Prerequisites**: - All [Class Notes] since the beginning of term. - All your previous [Assignments]. - an ability to **READ ALL THE WORDS** to work effectively - **Deliverables**: 1. One text file uploaded to Blackboard according to the steps in the [Checking Program] section below. 2. Directory structure created and left for marking on the [Course Linux Server] (**CLS**).\ **Do not delete any assignment work from the CLS until after the term is over!** **WARNING:** Some inattentive students upload Assignment #7 into the Assignment #6 upload area. Don’t make that mistake! Be exact. Purpose of this Assignment ========================== > Do not print this assignment on paper. On paper, you cannot follow any of > the hyperlink URLs that lead you to hints and course notes relevant to > answering a question. This assignment is based on your weekly [Class Notes]. 1. To understand and work with hard links `ln`, and disk use `du`. 2. To modify a shell `PATH` so the shell can find a command name in a different directory. Remember to **READ ALL THE WORDS** to work effectively and not waste time. Introduction and Overview ========================= This is an overview of how you are expected to complete this assignment. Read all the words before you start working. 1. Complete the **Tasks** listed below. 2. Verify your own work before running the [Checking Program]. 3. Run the [Checking Program] to help you find errors. 4. Submit the output of the [Checking Program] to Blackboard before the due date. 5. **READ ALL THE WORDS** to work effectively and not waste time. You must understand hard links and know how to use the `ln`, `find` and `du` commands to do this assignment. See the [Class Notes], especially all the notes dealing with the [File System], [Inodes and Links], [Disk Usage], and [Symbolic Links]. Non-empty files occupy disk space. Hard links to files only occupy a little extra space in a directory for the extra file name; they don’t create new disk file space. To completely remove a file, you must remove all the names a file has. (The link count must go to zero.) Your job in Part C of this assignment is to remove all the files in a directory and reduce the disk space used. You will be given a series of directories containing linked files. You have to remove *all* the names for some files so that the system actually frees up the disk space. Recall that the `rm` command does not remove files; it only removes names. Your job is to make the system remove the disk blocks occupied by the *files*, to make more disk space, which means you need to find and remove *all* the names for the files so that the link counts go to zero. There are three levels of difficulty. Do the easy one first. Assignments may be re-marked at any time on the CLS; you must have your term work available on the CLS right until term end. > Since I also do manual marking of student assignments, your final mark may > not be the same as the mark submitted using the current version of the > [Checking Program]. I do not guarantee that any version of the [Checking > Program] will find all the errors in your work. Complete your assignments > according to the specifications, not according to the incomplete set of the > mistakes detected by the [Checking Program]. The Source Directory -------------------- All references to the “Source Directory” below are to the CLS directory `~idallen/cst8207/13f/assignment08/` 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). Tasks ===== - Do the following tasks in order, from top to bottom. - These tasks must be done in your account on the [Course Linux Server]. - **READ ALL THE WORDS!** and do not skip steps. - Your instructor will mark on the due date the work you do in your account on the CLS. Leave all your work on the CLS and do not modify it. - **Do not delete any assignment work from the CLS until after the course is over.** Set Up ------ 1. 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: CST8207-13F `-- Assignments `-- assignment08 Run the [Checking Program] to verify your work so far. Part A - Linking exercise ------------------------- 1. In your `assignment08` directory, create a sub-directory and an empty file `ln/`*abcd0001*`.txt` (no spaces), where the text *abcd0001* is replaced by your *own* eight-character userid in the file name. NOTE: The sub-directory name is `ln` (two letters), not `1n` (digit letter). 2. For every unique character in your own eight-character userid, create a sub-sub-directory under `ln` with that single-character name. For example, the userid `abca0151` would result in six unique sub-sub-directory names under the `ln` directory – one sub-sub-directory for each of the unique characters `a`, `b`, `c`, `0`, `1`, `5`. Do this for your *own* userid, which means you may have more or fewer sub-sub-directories, depending on the letters and digits in your own userid. 3. Inside each of those new sub-sub-directories, create a single [Hard Link][File System] to the empty file from the first step. Keep the same file name as the original for each hard link. Use hard links, not symbolic links. Continuing the above example, the `abca0151` user would hard link the original empty file name `abca0151.txt` into each of those six new sub-sub-directories, creating six additional names for the same file. Keep the same file name as the original for each hard link. Check the link counts on everything to make sure that you have created links to the same file and not made copies of the file. Use hard links, not symbolic links. 4. For every lower-case letter directory name you created, create a short, relative [Symbolic Link][Symbolic Links] that is its upper-case equivalent. If you created directory `a`, then create symlink `A` that points to `a` so that both `ls ln/a` and `ls ln/A` give identical results. (You must use symbolic links, because you cannot make hard links to directories.) 5. In every file you have just created in this Part, enter the following information, one name per line: Enter the names of the three common file system commands that are “directory only” commands that require permission only on the directory inode to work properly, and that do *not* require permissions on the file inode to work. The answer is three lines, one command name per line. (See your in-class notes for the three names I wrote on the board in Week 8, or read the course notes about links and inodes.) The right answer has this format (three lines; three words; nine characters): $ wc abcd0001.txt 3 3 9 abcd0001.txt Again, the text `abcd0001` must be *your own* userid, in all cases. Hints: All the file names you created in this Part should be hard links to the same file; you have very little editing to do. The three command names are all commands that are [directory operations][Inodes and Links] that manipulate file names; they don’t touch the file data and don’t need any permissions on the file data. Run the [Checking Program] to verify your work so far. Part B – Search `PATH` ---------------------- Create a file named `check.sh` under your `assignment08` directory. Edit this file to contain exactly three lines of text: 1. The first line must set the [shell search `PATH`] variable to include the [Source Directory] for this assignment appended at the (right) end of the path. 2. The second line must safely `echo` the new value of `PATH`. 3. The third line must have the shell find and execute a command named `assignment08check` (which it will do using the modified `PATH`). Save your three-line shell script and run `sh check.sh` to execute your script. The script should display the modified `PATH` on your screen and then run the checking program for you. (This will only work if the shell search `PATH` is set correctly in the file, otherwise it will say `command not found`.) Hints: You need to know how to [append to `PATH`][shell search `PATH`]. Remember to [quote all variable expansions]. The shell uses `PATH` to [find and run commands][shell search `PATH`]. Part C – Disk Usage Exercise ---------------------------- Using the copy option that means `archive` that preserves hard links (as well as all the other attributes), recursively copy the directory named `blocks` from the [Source Directory] to your `assignment08` directory. You will know you got it right if your copy of the `blocks` directory has the same disk use summary (`du -s`) as the one in the [Source Directory]. The `blocks` directory contains three sub-directories and many other files and sub-directories. Some of the files are hard links to each other. (If there are no hard links anywhere, you didn’t use the `archive` option to the copy command. Delete everything and re-copy.) If you make errors in this assignment, you can remove or rename the `blocks` directory and re-execute the above recursive copy command (with the `archive` option) to re-create the `blocks` directory and start over. This Part has three levels of difficulty. If you remove the entire `blocks` directory to start over, you will need to redo the all three levels. If you are smart and rename the directory, you can salvage from the saved directory the parts of the assignment you have already done successfully. There are three levels of difficulty. Do the easy one first. ### The first level of difficulty is “easy” There is a directory named `easy` under `blocks`. This easy subdirectory contains `236` blocks (recursive total for everything) and a `foo` subdirectory that contains `84` blocks of those `236` blocks: $ du -s . 236 . $ du -s foo 84 foo Read this section all the way through before you delete anything, or else you will have to start over again. **YOUR JOB:** Remove all the files in the `foo` subdirectory (but keep the directory), so that the total disk use in the `easy` directory drops to `236-80=156` blocks. Don’t try to do this until you’ve read this whole section through, including the *Hints* below. *Hints:* Some of the files in the `foo` subdirectory have more than one name. Those other names are located somewhere else under `easy`. (You don’t have to search the whole disk partition to find them.) The disk blocks for these files in `foo` will not be freed until you find and remove *all* their names. Do not remove any names from `foo` until you also know how to find and remove all the other names for these files. You will need to look at inode numbers to know which files in directory `foo` are also named in the other directories. You read about how to do this in [Disk Usage]. If you succeed in the `easy` directory you will see this: $ du -s . ; du -s foo ; find . | wc 156 . 4 foo 39 39 431 If you don’t get the right answer, you can start over by re-copying all or part of the `blocks` directory from the [Source Directory]. Run the [Checking Program] to verify your work so far. ### The next level of difficulty is “medium” There are two hidden directory under `blocks`. One name contains the string `medium` as part of its name. This medium subdirectory contains `368` disk blocks (recursive total for everything) and a `foo` subdirectory that contains `132` of those `368` blocks. **YOUR JOB:** Remove all the files in the `foo` subdirectory (but keep the directory), so that the total disk use in the medium directory drops to `368-128=240` blocks. *Hints:* See the previous question for hints on finding all the file names. If you succeed in the medium directory you will see this: $ du -s . ; du -s foo ; find . | wc 240 . 4 foo 60 60 744 If you don’t get the right answer, you can start over by re-copying all or part of the `blocks` directory from the [Source Directory]. Run the [Checking Program] to verify your work so far. ### The next level of difficulty is “hard” There are two hidden directory under `blocks`. One name contains the string `hard` as part of its name. This hard subdirectory contains `304` disk blocks (recursive total for everything) and a `foo` subdirectory that contains `132` of those `304` disk blocks. **YOUR JOB:** Remove all the files in the `foo` subdirectory (but keep the directory), so that the total disk use in the hard directory drops to `304-128=176` blocks. *Hints:* An option to `ls` to display nongraphic (unprintable) characters will be needed. Many file names will need to be quoted to protect shell metacharacters. See the previous question for hints on finding all the file names. If you succeed in the hard directory you will see this: $ du -s . ; du -s foo ; find . | wc 176 . 4 foo 60 165 864 If you don’t get the right answer, you can start over by re-copying all or part of the `blocks` directory from the [Source Directory]. Run the [Checking Program] to verify your work so far. 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. 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 `assignment08check` in the [Source Directory] on the CLS. You can execute this program by typing its (long) pathname into the shell: `$ ~idallen/cst8207/13f/assignment08/assignment08check` 2. Execute the above “check” program. 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. **Some task sections require you to finish the whole section before running the checking program at the end; you may not always be able to run the checking program successfully after every single task step.** 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 `assignment08.txt` under your `assignment08` directory on the CLS. Use the *exact* name `assignment08.txt` in your `assignment08` directory. Case (upper/lower case letters) matters. Be absolutely accurate, as if your marks depended on it. Do not edit the file. Make sure the file actually contains the output of the checking program! 4. Transfer the above `assignment08.txt` file from the CLS to your local computer and verify that the file still contains all the output from the checking program. Do not edit this file! No empty files, please! Edited or damaged files will not be marked. You may want to refer to your [File Transfer] notes. 5. Submit the `assignment08.txt` file under the correct Assignment area on Blackboard (with the exact name) before the due date. Upload the file via the **assignment08** “Upload Assignment” facility in Blackboard: click on the underlined **assignment08** 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 `assignment08` 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!** -- | 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/ | Defend digital freedom: http://eff.org/ and have fun: http://fools.ca/ [Plain Text] - plain text version of this page in [Pandoc Markdown] format [www.idallen.com]: http://www.idallen.com/ [Midterm Test #2]: 000_test_instructions.html [Class Notes]: indexcgi.cgi#XImportant_Notes__alphabetical_order_ [Assignments]: indexcgi.cgi#XAssignments [Checking Program]: #checking-marking-and-submitting-your-work [Course Linux Server]: 070_course_linux_server.html [File System]: 450_file_system.html [Inodes and Links]: 455_links_and_inodes.html [Disk Usage]: 457_disk_usage.html [Symbolic Links]: 460_symbolic_links.html [shell search `PATH`]: 400_search_path.html [Source Directory]: #the-source-directory [quote all variable expansions]: 320_shell_variables.html [File Transfer]: 015_file_transfer.html [Plain Text]: assignment08.txt [Pandoc Markdown]: http://johnmacfarlane.net/pandoc/