3.1 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.

3.2 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.

4.1 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.

4.2 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.

4.3 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

4.4 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.