% CST8177 Assignment 06 - Shell Script Parameters and Control Flow
% Todd Kelley, Ian! D. Allen – <idallen@idallen.ca> – [www.idallen.com]
% Winter 2014 - January to April 2014 - Updated Thu Mar 20 20:17:44 EDT 2014

Due Date and Deliverables
=========================

> **Do not print this assignment on paper!**
>
> -   On paper, you will miss updates, corrections, and hints added to the
>     online version.
> -   On paper, you cannot follow any of the [hyperlink URLs] that lead you
>     to hints and course notes relevant to answering a question.
> -   On paper, scrolling text boxes will be cut off and not print properly.

-   **Due Date**: `12h00 (12 noon) Monday March 10, 2014 (start of Week 9)`
    -   **NEW DUE DATE**
    -   Late assignments or wrong file names may not be marked. Please be
        punctual.

-   **Available online**
    -   Version 1 – 18:00 February 23, 2014
    -   Version 2 – 22:00 February 23, 2014 – fixed links and wording
    -   Version 3 – 14:10 March 9, 2014 – new due date (Blackboard problems)

-   **Prerequisites**
    -   [CST8207 GNU/Linux Operating Systems I]
    -   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 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
==========================

> **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.

1.  Create shell scripts that deal with parameters and flow control.
2.  Practise with a text editor.

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.

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 Standard Script Header you used for your
previous script assignments. 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.

Searching the course notes
--------------------------

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].

The Source Directory
--------------------

All references to the “Source Directory” below are to the directory
`~idallen/cst8177/14w/assignment06/` 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.
-   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 – The Base Directory
---------------------------

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.  Make the directory `~/CST8177-14W/Assignments/assignment06`, in which you
    will create the scripts resulting from the following tasks.

**This `assignment06` directory is the *base* directory for most pathnames in
this assignment. Store your files and answers below in this *base*
`assignment06` directory.**

Run the [Checking Program] to verify your work so far.

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 </dev/null
        $

    You need an option to the shell `read` statement that issues a prompt to
    the user (`man bash`). Note that GLOB characters do *not* expand when
    output by the script. Make sure the script exits on EOF.

`awk` wrapper script: extracting a column of input
--------------------------------------------------

Recall in lecture that we used used the `awk` program to extract the first
space-delimited column of an input stream. We will develop a script named
`acol` (Awk COLumn) that extracts *any* column of input.

1.  Create a one-line (plus header) script named `acol1.sh` that uses `awk`
    to read its standard input and extract the first column, exactly as used
    in the lecture notes. Example usage:

        $ echo a b c | ./acol1.sh
        a

        $ date | ./acol1.sh
        Wed

        $ last | ./acol1.sh
        idallen
        idallen
        kelleyt
        donnelr
        [...etc...]

2.  Create a one-line (plus header) script named `acol2.sh` that uses `awk`
    to read its standard input and extract the *second* column of input.
    Example usage:

        $ echo a b c | ./acol2.sh
        b

        $ date | ./acol1.sh
        Feb

        $ last | ./acol2.sh
        pts/9
        pts/14
        pts/50
        pts/50
        [...etc...]

3.  Create a one-line (plus header) script named `acolNF.sh` that uses `awk`
    to read its standard input and extract the *last* (`NF`) column of input.
    Example usage:

        $ echo a b c | ./acolNF.sh
        c

        $ echo a b d e f g h i j | ./acolNF.sh
        j

        $ date | ./acolNF.sh
        2013

        $ last | ./acolNF.sh
        (00:31)
        (02:11)
        (02:02)
        (00:01)
        [...etc...]

It should be clear that having a separate script for every possible number of
columns is not a good thing. Let’s write one script that takes as its only
argument the column number we want `awk` to print.

4.  Create a one-line (plus header) script named `acolnew.sh` that uses `awk`
    to read its standard input and extract the column of input given as an
    argument on the command line. This script is a simple modification of
    your previous script; you only need to change about a half-dozen
    characters to make it work. Example usage:

        $ echo a b c d e f | ./acolnew.sh 1
        a

        $ echo a b c d e f | ./acolnew.sh 2
        b

        $ echo a b c d e f | ./acolnew.sh NF
        f

    The script takes a single argument that is either a number (e.g. `2`) or
    the string `NF` and passes that argument directly to `awk` as the column
    to print. The script is a simple modification of your previous script; it
    is one line (plus header); it does not do any input checking or argument
    processing. If you don’t supply an argument, or you pass something that
    `awk` doesn’t understand, you will get an error from `awk` – this is
    okay:

        $ echo a b c d e f | ./acolnew.sh
        ./acolnew.sh: 8: ./acolnew.sh: 1: parameter not set

        $ echo a b c d e f | ./acolnew.sh ''
        awk: { print $ }
        awk:           ^ syntax error

        $ echo a b c d e f | ./acolnew.sh ' '
        awk: { print $  }
        awk:            ^ syntax error

        $ echo a b c d e f | ./acolnew.sh @
        awk: { print $@ }
        awk:          ^ invalid char '@' in expression

    If you supply more than one argument, the script won’t detect the error
    and will simply ignore the extra arguments – this is okay:

        $ echo a b c | ./acolnew.sh NF these arguments are ignored
        c

    This one-line script (plus header) does not validate its input, so it
    gives cryptic errors if the argument is incorrect and it does not warn
    you if you give too many arguments. This is not a production-quality
    script. We will fix it in the next step.

5.  Copy `acolnew.sh` to `acol.sh` and add input validation to the new
    script. The new script must check to make sure it has exactly one input
    argument and do minimal validation of that one argument before using it
    with `awk` to print the given column number. You do not need to change
    the `awk` line at all. You only need to add input validation to the
    script, before calling `awk`.

    Your script should enforce the “only one argument” requirement. Print
    both a [Good Error Message] and Usage Message on stderr and exit with a
    bad error status if the number of arguments is not exactly one, or if the
    one argument is equal to the empty string (`''`), or equal to a single
    blank character (`' '`).

    You are *not* required to validate that the argument is a number or the
    string `NF`, since that kind of pattern-matching needs more advanced
    scripting knowledge.

    Examples:

        $ echo a b c | ./acol.sh
        ./acol.sh: Expecting one column number argument; found 0 ()
        Usage: ./acol.sh colnum
        $ echo $?
        2

        $ echo a b c | ./acol.sh 1 2 3 a b c >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
-----------------

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.

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
===========================================

**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.

> 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].

1.  There is a [Checking Program] named `assignment06check` in the [Source
    Directory] on the CLS. Create a symbolic link to this program named
    `check` under your new `assignment06` 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.

2.  Execute the above “check” program using its new symbolic link. (Review
    the [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. **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 `assignment06.txt` under your `assignment06` directory. Use
    the *exact* name `assignment06.txt` in your `assignment06` 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 `assignment06.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 `assignment06.txt` file under the correct Assignment area on
    Blackboard (with the exact name) before the due date. Upload the file via
    the **assignment06** “Upload Assignment” facility in Blackboard: click on
    the underlined **assignment06** 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 `assignment06` 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 Kelly 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 

  [www.idallen.com]: http://www.idallen.com/
  [hyperlink URLs]: indexcgi.cgi#XImportant_Notes__alphabetical_order_
  [CST8207 GNU/Linux Operating Systems I]: ../../../cst8207/13f
  [Assignments]: indexcgi.cgi#XAssignments
  [Checking Program]: #checking-marking-and-submitting-your-work
  [Course Linux Server]: ../../../cst8207/14w/notes/070_course_linux_server.html
  [Remote Login]: ../../../cst8207/14w/notes/110_remote_login.html
  [Good Error Message]: #good-error-messages
  [Source Directory]: #the-source-directory
  [Search Path]: ../../../cst8207/14w/notes/400_search_path.html
  [File Transfer]: ../../../cst8207/14w/notes/015_file_transfer.html
  [Plain Text]: assignment06.txt
  [Pandoc Markdown]: http://johnmacfarlane.net/pandoc/