Updated: 2015-03-02 15:20 EST
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.
23h59 (11:59pm) Tuesday March 3, 2015 (Week 8)
WARNING: Some inattentive students upload Assignment #05 into the Assignment #04 upload area. Don’t make that mistake! Be exact.
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 is an overview of how you are expected to complete this assignment. Read all the words before you start working.
For full marks, follow these directions exactly.
You can use a Checking Program to check your work as you do the tasks. You can check your work with the checking program as often as you like before you submit your final mark. (Some tasks sections below require you to finish the whole section before running the checking program; you may not always be able to run the checking program successfully after every single task step.)
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.
You will create file system structure in your HOME directory on the CLS, with various directories, files, and links. When you are finished the tasks, leave these files, directories, and links in place as part of your deliverables on the CLS. 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.
All references to the Source Directory below are to the CLS directory ~idallen/cst8177/15w/assignment05/
and that name starts with a tilde character ~
followed by a user name 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).
You do not have permission to list the names of all the files in the Source Directory, but you can access any files whose names you already know.
The previous term’s course notes are available on the Internet here: CST8207 GNU/Linux Operating Systems I. All the notes files are also searchable on the CLS. You can recall 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 Course Notes.
Most of the tasks below ask 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 below must begin with the Standard Script Header you used for your previous script assignments. See the class notes.
Though the header is executable code, in the descriptions below we don’t count those lines, or any comment or blank lines, in the size of the script. We only count the new lines of code that you write.
For example, a “one-line script” is really several lines of header, a blank line, a block of several comment lines that Document Your Script, another blank line, and then your one line of actual script code. The description below calls this a one line script, even though it may contain a dozen lines.
When you have completed each script, ensure that it is executable, so that it can be executed as ./scriptname.sh
from the shell command line.
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.)
The examples below do not fully test your script; you will need to try other examples to make sure your scripts work properly for all possible inputs, especially inputs with blanks and shell meta-characters.
Remember to double quote all variable expansions to prevent syntax errors and other unwanted problems in your script.
For full marks, follow these task directions below exactly as written. READ ALL THE WORDS to work effectively and not waste your time.
Your instructor will also 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.
Do a Remote Login to the Course Linux Server (CLS) from any existing computer, using the host name appropriate for whether you are on-campus or off-campus. All work in this assignment must be done on the CLS.
Base Directory: Make the CLS Base Directory named ~/CST8177-15W/Assignments/assignment05
, in which you will create the files for the following tasks.
This CLS assignment05
directory is the Base Directory for most pathnames in this assignment. Store your files and answers in this Base Directory on the CLS.
Create the check
symbolic link needed to run the Checking Program, as described in the section Checking Program below.
Use the symbolic link to run the Checking Program to verify your work so far.
These scripts deal with command line arguments and reading input from the user. The concepts here will be used in the next section.
Review Properties of all Scripts, above.
arguments.sh
Arguments on the command line and positional parameters:
Create a two-line script named arguments.sh
that prints to the screen (standard output) exactly two lines:
nargsxxx
with your description.argdescyyy
with your description.Examples:
$ ./arguments.sh
nargsxxx: 0 # use your own words for nargsxxx
argdescyyy: # use your own words for argdescyyy
$ ./arguments.sh one two 'three four' '*'
nargsxxx: 4
argdescyyy: one two three four *
$ ./arguments.sh foo bar >out
$ cat out
nargsxxx: 2
argdescyyy: foo bar
$ ./arguments.sh /bin/* >out
$ head -n 1 out
nargsxxx: 151 # number may differ
$ ./arguments.sh /usr/bin/* >out
$ head -n 1 out
nargsxxx: 1782 # number may differ
Notes:
nargsxxx
and argdescyyy
above. Explain in your own words what the output is.Add comments to Document Your Script.
Check your work so far using the checking program symlink.
user_input.sh
Reading input from the user/stdin:
Create another two-line script named user_input.sh
that does two things:
promptxxx
with the words of your prompt for user input.descyyy
with your description.Examples:
$ ./user_input.sh
promptxxx: foo bar # use your own words for promptxxx
descyyy: foo bar # use your own words for descyyy
$ ./user_input.sh >out
promptxxx: *
$ cat out
descyyy: *
$ echo a b c | ./user_input.sh
descyyy: a b c
Notes:
promptxxx
and descyyy
above. Explain in your own words what the output is.Hint: The shell read
command can be made to prompt for input (see help read
or look up read
inside man bash
), and it correctly displays the prompt on standard error only if standard input is coming from a keyboard. Prompts must never disappear into the output file when standard output is redirected!
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
These path checking scripts use concepts from the above Basic Scripts and add error checking and conditional logic. You may find it useful to copy and adapt some of your working code from the above Basic Scripts.
Review Properties of all Scripts, above.
path_test_args.sh
Arguments and conditional statements if then else
, and test
:
Combine the concepts from the previous scripts and add error checking. Create a script named path_test_args.sh
that outputs a line saying whether an argument 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, and that the argument is not the empty string. If anything 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 must have exactly the following structure and use full if then else
statements and not conditional operators such as &&
:
# Follow this exact structure (3 IF statements, 1 ELSE) for your script:
if the number of arguments is not 1, then
print a Good Error Message (see notes) on stderr
print a Usage Message (how to use this script) on stderr
exit the script with a status 2
if the argument is empty (empty string ""), then
print a Good Error Message (see notes) on stderr
print a Usage Message (how to use this script) on stderr
exit the script with status 2
if the argument is a pathname that exists, then
print a statement saying that the pathname 'xxx' exists
exit the script with status 0
else
print a statement saying 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.
The script will exit with a status of:
0
if the pathname exists.1
if the pathname does not exist.2
if the number of arguments is not 1, or, if the one argument pathname is the empty string.Examples:
$ ./path_test_args.sh >out
./path_test_args.sh: Expecting one pathname argument; found 0 ()
Usage: ./path_test_args.sh pathname
$ echo $?
2
$ ./path_test_args.sh a '*' c >out
./path_test_args.sh: Expecting one pathname argument; found 3 (a * c)
Usage: ./path_test_args.sh pathname
$ echo $?
2
$ ./path_test_args.sh "" >out
./path_test_args.sh: Argument is an empty string; expecting one pathname.
Usage: ./path_test_args.sh pathname
$ echo $?
2
$ ./path_test_args.sh path_test_args.sh
Pathname exists: path_test_args.sh
$ echo $?
0
$ ./path_test_args.sh ..
Pathname exists: ..
$ echo $?
0
$ ./path_test_args.sh /dev/null
Pathname exists: /dev/null
$ echo $?
0
$ ./path_test_args.sh /dev/sda
Pathname exists: /dev/sda
$ echo $?
0
$ ./path_test_args.sh /dev/log
Pathname exists: /dev/log
$ echo $?
0
$ ./path_test_args.sh nosuchfile
Pathname nonexistent: nosuchfile
$ echo $?
1
$ ./path_test_args.sh '*' >out
$ echo $?
1
$ cat out
Pathname nonexistent: *
Notes:
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
path_test_user.sh
Reading input from the user/stdin and testing pathnames:
Combine the concepts from the previous scripts and add error checking. Create a script named path_test_user.sh
that has the same function as the previous path_test_args.sh
script, but where the input pathname is read from the user instead of coming from an argument on the command line.
The script must have exactly the following structure and use full if then else
statements and not conditional operators such as &&
:
# Follow this exact structure (3 IF statements, 1 ELSE) for your script:
if the number of arguments is more than zero, then
print a Good Error Message (see notes) on stderr
print a Usage Message (how to use this script) on stderr
exit the script with a status 2
Prompt and read one line of input (a pathname) from the user
if the input line is empty (empty string, no input), then
print a Good Error Message (see notes) on stderr
print a Usage Message (how to use this script) on stderr
exit the script with status 2
if the input is a pathname that exists, then
print a statement saying that the pathname 'xxx' exists
exit the script with status 0
else
print a statement saying that the pathname 'xxx' doesn't exist
exit the script with status 1
where xxx
is whatever pathname the user entered in response to the prompt.
The script will exit with a status of:
0
if the pathname exists.1
if the pathname does not exist.2
if there are any arguments on the command line, or, if the user entered no input.Notes:
path_test_args.sh
script, above.Add comments to Document Your Script.
Check your work so far using the checking program symlink.
path_write_loop.sh
Loop statement while
:
Modify the previous script to create a script named path_write_loop.sh
that is a looping version of the previous path_test_user.sh
script. The new script will loop, reading pathname input from the user over and over and testing whether each one is writable, until EOF or no input is entered.
You might want to copy the previous path_test_user.sh
script to path_write_loop.sh
as as starting point and modify it to test for writable pathnames instead of existing pathnames.
The new script must have the following structure and use full if then else
statements and not conditional operators such as &&
:
# Follow this exact structure (1 WHILE, 3 IF, 1 ELSE):
if the number of arguments is more than zero, then
print a Good Error Message (see notes) on stderr
print a Usage Message (how to use this script) on stderr
exit the script with a status 2
while prompt and read one line of input (a pathname) from the user
if the input line is empty (empty string, no input), then
print a message saying there was no input
print a friendly good-bye message and exit with status 0
if the input is a pathname that is writable, then
print a statement saying that the pathname 'xxx' is writable
else
print a statement saying that the pathname 'xxx' doesn't exist or is not writable
print a message saying EOF was received
print a friendly good-bye message and exit with status 0
Examples:
$ ./path_write_loop.sh x y z >out
./path_write_loop.sh: Expecting no arguments; found 3 (x y z)
Usage: ./path_write_loop.sh
$ echo $?
2
$ ./path_write_loop.sh
Enter pathname: path_write_loop.sh
Pathname is writable: path_write_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 # type CTRL-D for EOF
EOF received
Goodbye!
$
$ ./path_write_loop.sh >out
Enter pathname: out
Enter pathname: ***
Enter pathname: # no input (empty line)
$ cat out
Pathname is writable: out
Pathname is nonexistent or not writable: ***
Empty input line.
Goodbye!
$
$ echo "." | ./path_write_loop.sh
Pathname is writable: .
EOF received
Goodbye!
$
$ ./path_write_loop.sh </dev/null
EOF received
Goodbye!
$
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
awk
wrapper script: extracting a column of inputRecall 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.
Review Properties of all Scripts, above.
acol1.sh
Create a one-line 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...]
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
acol2.sh
Create a one-line 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...]
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
acolNF.sh
Create a one-line 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...]
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
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.
acolnew.sh
Create a one-line 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 new one-line script is a very small modification of your previous one-line script; you only need to change about a half-dozen characters in the line 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. 1
, 2
, etc.) or the string NF
and substitutes that argument directly into the awk
command line inside the script as the column to print, instead of using the previous hard-coded number.
Hint: Instead of hard-coding the column number in the awk
command line, as you did in the above three previous scripts, use the first script argument variable instead of the hard-coded number. You will need to adjust the Quoting around the awk
command arguments to allow the shell argument variable to be expanded by the shell but not the $
used by awk
to select the column number. You cannot use the awk
-v
option here.
The new one-line script is a simple modification of your previous one-line script: it is still a one-line script; it does not yet 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 the shell or from awk
– this is expected:
# examples of errors when script or awk gets bad input:
$ 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 for this beginning script:
$ echo a b c | ./acolnew.sh NF these arguments after NF are ignored
c
This one-line script 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.
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
acol.sh
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 in your script at all. You only need to add input validation to the script, before calling awk
. The input validation will cause the script to exit if the input is bad, so that awk
doesn’t process bad input.
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.
Also print an error and Usage and exit 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 (e.g. a shell case
statement). Don’t worry about errors from awk
due to non-digit arguments for this script (but do make sure your script itself can test and process them without errors).
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 1
one,two
$ echo $?
0
$ echo one,two three,four | ./acol.sh NF
three,four
$ echo $?
0
$ ./acol.sh '*' # test GLOB pattern handling
awk: { print $* } # awk errors are allowed here
awk: ^ syntax error
Add comments to Document Your Script.
Check your work so far using the checking program symlink.
OPTIONAL (advanced): If you want to make the script check for an argument that is not a number, you can use a shell
case
statement to match an argument containing any non-digit character and exit with an error message. Arguments that do not contain non-digits should pass unchanged through thecase
statement.
acol
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 jiangw
90 user1234
87 idallen
82 user2345
77 cst8207a
$ 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
Check your work so far using the checking program symlink.
That is all the tasks you need to do.
Read your CLS Linux EMail and remove any messages that may be waiting. See Reading eMail for help.
Check your work a final time using the Checking Program and save the standard output as described below. Submit your mark following the directions below.
Add at least four comment lines (script lines starting with the comment character #
) to each script containing the following information (in any order):
4.2.1 arguments.sh
Obey these rules for your script comments:
-p
option to the read
command.Good shell script error messages must obey these four rules:
1>&2
to send to stderr any output normally destined for stdout. See the examples below.
1>&2
is used on echo
statements, to send the text to standard error instead of standard output.$0
).
$0
.expecting one file name
).
found 3 (a b c)
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' must be greater than zero"
echo 1>&2 "$0: File '$file' does not exist; expecting accounting file"
Put quotes around anything entered by a user, otherwise your error messages may be confusing. Compare these example messages without and with quotes around the user input file name:
$ ./total.sh still
./total.sh: File still does not exist; expecting accounting file
Usage: ./total.sh account_file
$ ./total.sh still
./total.sh: File 'still' does not exist; expecting accounting file
Usage: ./total.sh account_file
After detecting an error, the usual thing to do is print a Good Error Message explaining the error, followed by a Usage message telling how to use the script, then exit the script with a non-zero return code. Don’t keep processing bad data!
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.
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 Base Directory on the CLS so that you can easily run the program to check your work and assign your work a mark on the CLS. Note: You can create a symbolic link to this executable program but you do not have permission to read or copy the program file.
Execute the above check
program on the CLS using its 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.
assignment05.txt
under your Base Directory on the CLS. Use that exact name. Case (upper/lower case letters) matters. Be absolutely accurate, as if your marks depended on it.
YOUR MARK for
assignment05.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.
YOUR MARK for
assignment05.txt
file from your local computer to the correct Assignment area on Blackboard (with the exact name) before the due date:
Use only Attach File on the Upload Assignment page. Do not enter any text into the Text Submission or Comments boxes on Blackboard; I do not read them. Use only the Attach File section followed by the Submit button. If you need to comment on any assignment submission, send me EMail.
You can revise and upload the file more than once using the Start New button on the Review Submission History page to open a new Upload Assignment page. I only look at the most recent submission.
You must upload the file with the correct name from your local computer; you cannot correct the name as you upload it to Blackboard.
You will also see the Review Submission History page any time you already have an assignment attempt uploaded and you click on the underlined assignment05 link. You can use the Start New button on this page to re-upload your assignment as many times as you like.
You cannot delete an assignment attempt, but you can always upload a new version. I only mark the latest version.
Your instructor may also mark files in your 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!
I do not accept any assignment submissions by EMail. Use only the Blackboard Attach File. No word processor documents. Plain Text only.
Use the exact file name given above. Upload only one single file of Linux-format plain text, not HTML, not RTF, not MSWord. No fonts, no word-processing. Linux plain text only.
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 16-character, lower-case 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!