% CST8177 Assignment 05 - Shell Script Parameters and Flow Control % Todd Kelley Richard Donnelly Ian! D. Allen - idallen@idallen.ca - www.idallen.com % Winter 2013 - January to April 2013 - Updated Mon Mar 4 07:58:27 EST 2013 Due Date and Deliverables ========================= - **Due Date**: `23h59 (11:59pm) Saturday March 2, 2013 (end of Week 7)` - Late assignments or wrong file names may not be marked. Be punctual. - **Available online**: - Version 1: 12h00 Wed Feb 13, 2013 - Version 2: 13h45 Wed Feb 13, 2013 (example for BONUS added) - Version 3: 16h30 Wed Feb 13, 2013 (BONUS sections removed) - Version 4: 12h40 Mon Feb 25, 2013 (added more EOF examples) - Version 5: 02h50 Tue Feb 26, 2013 (added more pathname examples) - Version 6: 02h30 Sat Mar 2, 2013 (changed example ../../.. to ../..) - Version 7: 08h00 Mon Mar 4, 2013 (added missing Source Directory link) - **Prerequisites**: - [CST8207 GNU/Linux Operating Systems I] - 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 and files 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!** Purpose of this Assignment ========================== 1. Create shell scripts that deal with parameters and flow control. 2. Practise with a text editor. 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. Complete the [Tasks] listed below on the [Course Linux Server] (**CLS**). Test your script using the given examples. Run a [Checking Program] to verify your work after you have run your own tests. Submit the output of the checking script to Blackboard before the due date. Each of the tasks below (except the first) asks you to write a small executable shell script, based on the lecture notes and slides. *None* of the scripts need command pipelines (“`|`”) or Boolean expressions (“`||`” or “`&&`” or `-a` or `-o`); they are all simple scripts with simple conditional logic. Each script should begin with the International Script Header you used for your previous assignment. When you have completed each script, ensure that it is executable, so that it can be run as `./scriptname.sh`. Run the given tests on your scripts to make sure they work. Sample output for each of the scripts is given, so that you may check your work as you proceed. Make sure your script handles *all* of the sample inputs given, especially the inputs containing shell metacharacters. (System crackers often attack your system using special characters as input.) 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. The previous 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. The Source Directory -------------------- All references to the “Source Directory” below are to the directory `~idallen/cst8177/13w/assignment05/` and that name starts with a *tilde* character followed by a userid with no intervening slash. 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. Set Up ------ 1. Make the directory `~/Assignments/assignment05`, in which you will create the scripts resulting from the following tasks. Basic Scripts ------------- 1. **Arguments on the command line and positional parameters:**\ Create a two-line (plus header) script named `arguments.sh` that prints to the screen (standard output) these two things: 1. the number of arguments given to the script 2. all the arguments themselves, preserving blanks Examples: $ ./arguments.sh The number of arguments passed is: 0 The arguments are: $ ./arguments.sh one two 'three four' '*' The number of arguments passed is: 4 The arguments are: one two three four * $ ./arguments.sh foo bar >out $ cat out The number of arguments passed is: 2 The arguments are: foo bar $ ./arguments.sh /bin/* >out $ head -n 1 out The number of arguments passed is: 151 (number may differ) $ ./arguments.sh /usr/bin/* >out $ head -n 1 out The number of arguments passed is: 3491 (number may differ) Note that GLOB characters do *not* expand when output by the script. 2. **Reading input from the user/stdin:**\ Create a three- or four-line (plus header) script named `user_input.sh` that prompts (on standard error) for input and reads one line of input from the user and then displays that input back to the user on standard output. The prompt may or may not end in a newline. Examples: $ ./user_input.sh Enter input: (may not end in newline) foo bar You entered: foo bar $ ./user_input.sh >out Enter input: (may not end in newline) * $ cat out You entered: * $ echo a b c | ./user_input.sh Enter input: (this line may not appear) You entered: a b c Note that GLOB characters do *not* expand when output by the script. Hint: The shell `read` command can be made to prompt for input (`man bash`), and it correctly displays the prompt on standard error. You can also use the shell syntax described in [Good Error Message] below to echo a message to standard error. Prompts must never disappear into the output file when standard output is redirected! 3. **Conditional statements `if then else`, and `test`:**\ Create a script named `path_test.sh` that outputs a line saying whether a pathname (any kind of pathname) exists or not. The pathname will be passed to the script as the only argument to the script. The script must ensure that exactly one argument is supplied. If the argument count is wrong, the script will issue both a [Good Error Message] and a Usage Message (how to use the script) on stderr and exit with a bad status of `2`. The script will exit with a good status of `0` if the pathname exists and a bad status of `1` if the pathname does not exist. The script must have the following structure and use full `if then else` statements and not conditional operators such as `&&`: if the number of arguments is not 1, print a Good Error Message (see notes) print a Usage Message (how to use this script) on stderr exit the script with a status 2 if the argument is a pathname that exists, then print a statement that the pathname 'xxx' exists exit the script with status 0 else print a statement that the pathname 'xxx' doesn't exist exit the script with status 1 where `xxx` is whatever argument the user supplied on the command line. Examples: $ ./path_test.sh ./path_test.sh: Expecting one pathname argument; found 0 () Usage: ./path_test.sh pathname $ echo $? 2 $ ./path_test.sh a '*' c >out ./path_test.sh: Expecting one pathname argument; found 3 (a * c) Usage: ./path_test.sh pathname $ echo $? 2 $ ./path_test.sh path_test.sh Pathname exists: path_test.sh $ echo $? 0 $ ./path_test.sh .. Pathname exists: .. $ echo $? 0 $ ./path_test.sh /dev/null Pathname exists: /dev/null $ echo $? 0 $ ./path_test.sh /dev/sda Pathname exists: /dev/sda $ echo $? 0 $ ./path_test.sh /dev/log Pathname exists: /dev/log $ echo $? 0 $ ./path_test.sh nosuchfile Pathname nonexistent: nosuchfile $ echo $? 1 $ ./path_test.sh '*' >out $ echo $? 1 $ cat out Pathname nonexistent: * Note that GLOB characters do *not* expand when output by the script. Make sure you know how to write a [Good Error Message] to stderr. 4. **Loop statements `while` and `test`:**\ Combine elements from the previous two scripts to create a script named `path_test_loop.sh` that is a looping version of the previous `path_test.sh` script. The new script will read pathname input from the user instead of using command line arguments. Copy the previous `path_test.sh` script to `path_test_loop.sh`. Modify the new script to generate a [Good Error Message] and Usage Message on stderr if any arguments are found on the command line. If there are any arguments, the script should state that it must be run without any arguments and exit with status `2`. Create a `while` loop that prompts and reads input from the user (until EOF) and then uses the input as a pathname to test that *write permission is granted* (`man test`). (Re-use parts of the previous scripts.) You will need an option to the shell `read` statement that issues a prompt to the user (`man bash`). The script must have the following structure and use full `if then else` statements and not conditional operators such as `&&`: while the script prompts and reads input from the user successfully if the input from the user is a pathname with write permission print a statement that the pathname 'xxx' is writable else print a statement that the pathname 'xxx' does not exist or is not writable Examples: $ ./path_test_loop.sh foo bar ./path_test_loop.sh: Expecting no arguments; found 2 (foo bar) Usage: ./path_test_loop.sh $ echo $? 2 $ ./path_test_loop.sh x y z >out ./path_test_loop.sh: Expecting no arguments; found 3 (x y z) Usage: ./path_test_loop.sh $ echo $? 2 $ ./path_test_loop.sh Enter pathname: path_test_loop.sh Pathname is writable: path_test_loop.sh Enter pathname: . Pathname is writable: . Enter pathname: ../.. Pathname is writable: ../.. Enter pathname: nosuchfile Pathname is nonexistent or not writable: nosuchfile Enter pathname: / Pathname is nonexistent or not writable: / Enter pathname: foo bar Pathname is nonexistent or not writable: foo bar Enter pathname: * Pathname is nonexistent or not writable: * Enter pathname: ^D (signal CTRL-D EOF to end the script) $ $ ./path_test_loop.sh >out Enter pathname: out Enter pathname: *** Enter pathname: ^D (signal CTRL-D EOF to end the script) $ cat out Pathname is writable: out Pathname is nonexistent or not writable: *** $ $ echo "." | ./path_test_loop.sh Pathname is writable: . $ $ ./path_test_loop.sh out ./acol.sh: Expecting one column number argument; found 6 (1 2 3 a b c) Usage: ./acol.sh colnum $ echo $? 2 $ echo a b c | ./acol.sh '' ./acol.sh: column number argument is empty Usage: ./acol.sh colnum $ echo $? 2 $ echo a b c | ./acol.sh ' ' ./acol.sh: column number argument is a blank (space) Usage: ./acol.sh colnum $ echo $? 2 $ echo one,two three,four | ./acol.sh NF three,four $ echo $? 0 6. Link `acol.sh` into your personal `bin/` directory using the name `acol` and use it whenever you need to see just one column of data. Take your `acol` script with you to your next job! $ last | acol 1 | sort | uniq -c | sort -nr | head -5 93 kelleyt 90 user1234 87 idallen 82 user2345 77 donn0067 $ last | acol 2 | sort | uniq -c | sort -nr | head -5 450 pts/2 403 pts/3 328 pts/5 235 pts/11 226 pts/8 When you are done ----------------- When you are finished and all of the shell scripts have been created, run the [Checking Program] program to create an overall mark. Good Error Messages =================== Good shell script error messages must obey these four rules: 1. Error messages must appear on standard error, not standard output. You can use the shell syntax `1>&2` to send to stderr any output normally destined for stdout. See the examples below. - Usually `1>&2` is used on `echo` statements, to send the text to standard error instead of standard output. 2. Must contain the name of the program that is issuing the message (from `$0`). - Do not put the name of the script into the script; always use `$0`. 3. Must state what kind of input was expected (e.g. `expecting one file name`). - Do not say only “expecting one argument”, since that doesn’t say what *kind* of argument is needed. Be explicit. 4. Must display what the user actually entered (e.g. `found 3 (a b c)` - Display both the number of arguments and their values. *Never* say just `missing argument` or `illegal input` or `invalid input` or `too many`. Always specify what is needed and how many is “too many” or “too few”: echo 1>&2 "$0: Expecting 3 file names; found $# ($*)" echo 1>&2 "$0: Student age $student_age is not between $min_age and $max_age" echo 1>&2 "$0: Modify days $moddays is less than zero" After detecting an error, the usual thing to do is to exit the script with a non-zero return code. Don’t keep processing bad data! Checking, Marking, and Submitting your Work =========================================== Check your work a final time using the `assignment05check` program symlink and save the output as described below. Submit your final mark following the directions below. **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 `assignment05check` in the [Source Directory] on the CLS. Create a symbolic link to this program named `check` under your new `assignment05` 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 `assignment05.txt` under your `assignment05` directory. Use the *exact* name `assignment05.txt` in your `assignment05` 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 `assignment05.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 `assignment05.txt` file under the correct Assignment area on Blackboard (with the exact name) before the due date. Upload the file via the **assignment05** “Upload Assignment” facility in Blackboard: click on the underlined **assignment05** 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 `assignment05` 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!** -- | Todd Kelley / 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/ | Defend digital freedom: http://eff.org/ and have fun: http://fools.ca/ [Plain Text] - plain text version of this page in [Pandoc Markdown] format [CST8207 GNU/Linux Operating Systems I]: ../../../cst8207/12f [Checking Program]: #checking-marking-and-submitting-your-work [Course Linux Server]: 000_course_linux_server.html [Tasks]: #tasks [Good Error Message]: #good-error-messages [Source Directory]: #the-source-directory [CST8207 Search Path]: ../../../cst8207/12f/notes/400_search_path.html [File Transfer]: 220_file_transfer.html [Plain Text]: assignment05.txt [Pandoc Markdown]: http://johnmacfarlane.net/pandoc/