Updated: 2015-09-06 00:38 EDT
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) Friday September 26, 2014 (end of Week 4)
WARNING: Some inattentive students upload Assignment #02 into the Assignment #01 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.
PATH
mechanism of the shellPATH
is undesirableThis 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.
The first tasks guide you through the process of reviewing the Search Path mechanism of the shell. In the process, you will gain some practice working with Shell Variables.
The remaining tasks involve creating simple shell scripts. This lab uses basic scripting techniques, to be built upon in future labs. You can use the Checking Program to check your work after you create each script.
You will create file system structure in your HOME directory, with various directories, files, and links. You can use the 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.)
When you are finished the tasks, leave these files, directories, and links 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.
This is partially a review lab, and some of the tasks can be completed with knowledge of the material from the prerequisite course CST8207 GNU/Linux Operating Systems I; however, you will probably need to refresh your memory of various topics by referring to the CST8207 course notes and the Linux man pages. Your lab instructor is there to help you, but s/he will want you to have tried consulting the notes and man pages first.
The prevous 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.
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.
All references to the “Source Directory” below are to the CLS directory ~idallen/cst8177/14f/assignment02/
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).
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.
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:
CST8177-14F
`-- Assignments
`-- assignment02
This
assignment02
directory is the Base Directory for most pathnames in this assignment. Store your files and answers in this Base Directory.
assignment02check
in the Source Directory on the CLS. Follow the instructions in the first two steps at the start of Checking Program to create a working symbolic link to this program.Check your work so far using the checking program symlink.
PATH
mechanismThis task explores the shell Search Path.
NOTE: In this task, you will change the PATH
variable temporarily for the current shell session only. If at any time you want to undo these temporary changes, you can simply exit
the shell that has the changes and then start a new shell, or re-login if you exited your login shell. (Below, we suggest you create a subshell so that you don’t log out when you exit!)
Since this task involves temporary changes to shell variables, you cannot resume this task in the middle if you log out or exit the shell. You must always start this task over from the beginning.
Do NOT edit your .bashrc
to make permanent changes that affect future login sessions, unless explicitly told to do so.
Start a second copy of bash
(a nested shell or subshell). (Watch the MP4 video on Subshells. Some sysadmin like to put the nesting level of the shell in their prompt using the $SHLVL
variable.) You can use the ps
command to verify that you have two copies of bash
running. Re-read the NOTE, above.
Display the value of the shell environment variable PATH
on your screen and verify that the standard system directory /bin
is in the list of directories in your shell’s search PATH
. (If the /bin
directory is not listed in your PATH
, something is very wrong; contact your instructor.)
Hints: There are several ways to see the value of an environment variable (or local variable) on your screen. You could use a command that shows single lines of text on your screen. Review Shell Variables.
Use a command to discover which of your PATH
directories contains the ls
program. Use redirection to put the output of this command (the absolute path to the ls
command) in file lspath.txt
in your Base Directory. You will need to know this pathname later.
Store the current value of your PATH
variable in a new shell variable named oldpath
by entering a shell assignment statement oldpath=$PATH
(no blanks). Make sure that both variables PATH
and oldpath
have the same values.
Hints: Some commands that can display the value of environment variable PATH
cannot be used to display the value of the new local non-environment variable oldpath
. Choose correctly. Since setting the value of a shell variable doesn’t save it on disk, the saved value in oldpath
is temporary, only valid for this shell. The value will be lost if you exit this shell and start a new shell. If you exit this shell before completing this task, you will lose the value of oldpath
and must start the task over from the beginning.
Put the value of the shell variable oldpath
into a new file named savepath.txt
in your Base Directory. Hint: Display the value of the variable and use output redirection to put it into the file. It’s easy and accurate.
Remove all write permissions from the savepath.txt
file, so that you don’t accidentally overwrite it.
Set your PATH
variable to have no value. One method is to type nothing after the equals sign – just push Enter after the equals: PATH=
Use echo
to display the new (empty) value of PATH
on your screen. (Most other commands will not be found; echo
is built-in to the shell and doesn’t need a valid search PATH
.)
Try to run any non-builtin command (for example, ls
, who
, date
, etc.) and notice that the attempt fails because the shell cannot find a program with that name in your current (empty) search PATH
. Type the ls
command name and use the correct syntax to redirect just the shell error message into a file called path_error.txt
in your Base Directory.
Hints: Recall how to redirect errors using shell Shell I/O Redirection. Display the contents of the file to make sure the error message is in the file: If you use the right absolute command pathname, you can run a file display command even if no commands can be found because of the empty PATH
variable of the current shell. See the next question for an example of using an absolute pathname as a command name.
Run the ls
program using its absolute pathname (the one you discovered and saved earlier). You do not need any directories in your PATH
to run a command using a pathname containing slashes. The PATH
is not used to look for command names containing slashes.
Issue a shell built-in command (for example, echo
, pwd
, shopt
) and verify that built-in commands are part of the shell process itself and also do not need to be found in PATH
directories.
Redirect the output of shell built-in shopt
into a file called shopt.txt
in your Base Directory. Because this is a shell built-in command, the search PATH
is not used.
Restore your PATH
variable to its original value by restoring its value from the oldpath
variable.
Hints: This is the reverse shell assignment statement compared to what you did earlier to save the value: simply exchange the names. If you make a mistake, re-read the NOTE, above.
Display your PATH
variable to verify it is restored (has the same value as in the variable oldpath
).
Try a few non-builtin commands to verify they are found with the restored PATH
in place.
Exit the nested sub-shell and return to your original login shell.
Check your work so far using the checking program symlink.
firstscript.sh
, containing the following two lines:
#!/bin/sh -u
(plus one more character for the newline at the end of the line).*** It's not a "Micro$oft" World ***
on the screen. Make sure you count the asterisks and get the capitalization, quotes, and dollar sign correct. You will test that you got this right in the next few steps.Make the firstscript.sh
file executable for only the owner (you). Group and others can read the file but not execute it. Hint: Review Permissions from last term.
Execute the script and edit it to fix any errors. When the output of the script is correct on your screen, redirect the output into file firstscriptout
in the Base Directory.
Hints: Execute the script by typing its name. You will note that the shell cannot find firstscript.sh
as an executable command name with no slashes, because the command is not on your search PATH
. You need to avoid the search PATH
by using a relative pathname containing a slash to execute this file, just as you do when executing the check
symlink. Review Executing a program in the current directory.)
You have just finished creating your own very simple Linux command – a script that prints some text to its standard output. Linux sysadmin create dozens of command scripts that do various system administration tasks. The scripts eliminate typing errors and make repeating tasks much easier.
Create the directory bin
in your own HOME directory as a place to keep your own personal commands. Hint: Make sure you create this directory in your own HOME directory.
We will give our new command a more command-like name, without the file extension. We will at the same time link the new name into our private bin
directory that we just created. To do this, create a hard link from your firstscript.sh
script to the name firstscript
(no extension) inside your bin
directory. You now have one file inode with two names. Hint: Review Links and Inodes.
Make the Base Directory your current directory. Now, without changing directories again, use a command to display the inode numbers and names of both script files to verify that they are hard links to the same inode. Use one command with two relative pathnames. When the two output lines are correct (showing the relative pathnames and the identical inode numbers), save the output into file sameinodes.txt
in the current (base) directory.
Change to your HOME directory. Without changing directories again, execute your new firstscript
command that is inside your bin
directory using the shortest relative pathname (from your HOME). Put the relative pathname you used into file binrel.txt
in your Base Directory.
Hint: A fast way to create binrel.txt
is to echo
the pathname that you just used and use redirection to create the file in the base directory.
Start a subshell. Append the absolute pathname of your new bin
directory to the right end (not the start) of the PATH
variable of your current shell. Remember how you did this – you will need to do it again later in your .bashrc
file. Hint: Review Appending to PATH.
Display your PATH
to confirm that the absolute path of your bin
directory is on the right end (not at the start). do not add the directory more than once – it should only appear once in your PATH
variable. If you need to start over, exit this subshell and then start another one to get a fresh, good copy of PATH
.
if you have done the previous steps correctly, you can now type the command name firstscript
(no slashes, no file extension) and the shell will find your firstscript
script in your bin
directory and execute it (because your bin
directory is now in the PATH
for this shell).
Hint: Now that your bin
directory is in your search PATH
, you can use the TAB key to do command name completion on the firstscript
command name, so you don’t have to type the whole name. Let the shell do your typing for you!
When you have your own firstscript
command working using your modified PATH
variable, save a copy of your modified PATH
variable in file binpath.txt
in your Base Directory. Make sure there are no duplicate directories in PATH
before you save it. Your HOME bin
directory must be at the end.
The change we just made to this shell’s
PATH
variable will be lost when the shell exits. We need to save this change so that it happens every time we log in. We do this using the.bashrc
file.
Create a backup copy of your .bashrc
file, in case you need to start over.
Make the necessary changes to your .bashrc
file to add the absolute path of your bin
directory to the right end of your PATH
variable. The syntax to do this is exactly as you did earlier with your interactive shell. Save the file.
Log out and log in to have your .bashrc
execute. Check your PATH
variable to make sure the absolute path of your bin
directory is at the right end of your PATH
. Verify that your firstscript
command from your own bin
directory still works (no slashes). The shell will do command completion on this name if you use the TAB key.
You will note that every time you start a new nested subshell, your
.bashrc
appends another copy of your HOMEbin
to your shell’s searchPATH
variable. We will learn in later assignments how to avoid this duplication.
Check your work so far using the checking program symlink.
PATH
is a bad ideaMake your HOME directory your current directory.
ls
command using these steps:
firstscript
script you created above to a new file named ls
in the current (your HOME) directory.You are running fake ls with all your privileges - this script could remove all your files!
ls
script in the current directory and make sure it prints your text message on the screen. Hint: To execute the ls
script, you need a relative command pathname that includes a slash.You have just finished setting a trap for anyone who has the current directory
.
near the beginning theirPATH
and runsls
while in your HOME directory.
Still in your HOME directory, type the ordinary ls
command name and note that you get the real ls
command, because your PATH
does not include the current directory. You do not have .
at the start of your PATH
so you didn’t run the fake ls
in the current (HOME) directory.
Start a subshell (a second copy of the shell).
Make yourself a victim: temporarily put the current directory .
at the front (start) of your current shell’s PATH
variable. Hint: Review Appending to PATH, but make sure you add to the front (left) end of the PATH
this time.
Display the new value of your PATH
variable and verify that the current directory name .
is the first directory in the list, followed by a colon character (:
) and the rest of the search PATH
. Redirect and save this new PATH
into file badpath.txt
in the Base Directory.
Still in your HOME directory (where the fake ls
command resides), type the ls
command name as you normally would. Notice that your PATH
setting means you’ve just run the fake ls
command, not the real ls
command!
This fake ls
script executes with your permissions, and it has privileges to do anything to your account that your own userid can do.
Imagine you are running as the super-user
root
account (not a good idea) and you have made the mistake of putting the current directory in yourPATH
as we have done here. (DON’T DO THIS!) Imagine further that you notice a new directory on the system that shouldn’t be there. Asroot
you change into this directory and typels
to see what it contains. A user has placed a fakels
command in this directory. Because you’reroot
, that fakels
command in the current directory runs withroot
privileges – and it can destroy your system.NEVER PUT THE CURRENT DIRECTORY IN YOUR PATH!
Exit the subshell that has the bad PATH
.
Make sure your current shell’s PATH
does not contain the current directory before you continue! Log out and back in, if needed.
Check your work so far using the checking program symlink.
This task explores Processes and Jobs.
In this task you will create a script that prints out the number of processes each user is currently running on the system. You will create a series of scripts, each being an enhancement of the previous one, until you have the final product.
NOTE: The command that gives you a “snapshot of the current processes” has several kinds of options, some with dashes (UNIX style) and some without (BSD style), because it is really two or three commands merged into one program. You sometimes find that mixing options from the different types (with and without dashes) gives error messages. It’s best not to mix option types. See the first few paragraphs of the man page.
Run a command that prints out a “full-format listing” and “select all processes” running on the system. Search in the man page for the terms “Select all processes” and “full-format” to discover the correct two UNIX-style options. (UNIX-style means the option letters must be preceded by a dash, unlike BSD-style options.) When you run the command with these two UNIX-style options, the output will be several hundred lines long, with every line starting with the userid that owns that process. The first few lines will look similar to this:
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 Sep12 ? 00:00:11 /sbin/init
root 2 0 0 Sep12 ? 00:00:01 [kthreadd]
(Hint: Pipe the huge output into a command that displays only the first few lines, so you can confirm the above format.)
Create a new two-line script file procs.sh
in your Base Directory by copying your existing two-line firstscript.sh
file and replacing the second line of the new file to use the process listing command you used from the previous step. Do NOT change the first line.
Run the new script procs.sh
to make sure it works. It should generate the full-format, all-processes listing for you, just as if you had typed it yourself on the command line.
As described in the section on
awk
in Data Mining, the commandawk '{print $1}'
reads lines from standard input and prints just the first space-delimited field of each line from that input. The output of the process listing command from your new script is conveniently in space-delimited columns.
Run a command pipeline that pipes the output from your procs.sh
script into an awk
command line that selects just the first leading (userid) column of each line. You should end up with a list of several hundred userids on your screen. The first three lines will be the same as the first column of the sample output given above.
Copy procs.sh
to procs_users.sh
in the same directory.
Change the second line in the new procs_users.sh
file to be a command pipeline prints just the first column of the process listing. (Add the previously-used awk
command to the end of the line to make it a command pipeline, as you did in a previous step.) Do not change the first line.
Run the new script procs_users.sh
to make sure it works. It should generate a list of several hundred userids, just as if you had typed the two commands yourself on the command line.
Pipe the output of your new procs_users.sh
script into the sort
command. The output will be the same list of several hundred userids, now in sorted order.
Copy procs_users.sh
to procs_sorted.sh
in the same directory.
Enhance the command pipeline in the new procs_sorted.sh
file, adding a further pipeline into the sort
command so that the one column of userids comes out sorted.
Run the new script procs_sorted.sh
to make sure it works. It should generate a list of several hundred userids in sorted order, just as if you had typed the three commands yourself on the command line.
Pipe the output of your new procs_sorted.sh
script into the command that counts adjacent identical lines. Instead of hundreds of lines of output, the output will be counts of the number of processes being run by each unique userid. You will see a few dozen lines, depending on how many users are logged in. The lines will all have this output format of number followed by userid, though the actual numbers and accounts may differ:
1 102
1 UID
2 avahi
1 colord
2 cst8207a
8 cst8207c
...etc...
Hint: There is a command that can count the number of occurrences of adjacent input lines, displaying each unique line preceded by the count of the number of times that line appeared. To recall this unique command name and its option to count adjacent lines, review the Command List from last term and the many counting Examples of pipes in the pages Shell I/O Redirection and Data Mining.
Copy procs_sorted.sh
to procs_counted.sh
.
Enhance the command pipeline in the new procs_counted.sh
script file to show the counts of processes run by each unique userid.
Run the new script procs_counted.sh
to make sure it works. It should generate a list of more than a dozen unique userids, each preceded by a count, just as if you had typed the four commands yourself on the command line.
We can now see that there are two odd things in the output that are not account userids:
102
– a number, not a userid. This appears to be a bug in the process listing program, since thepasswd
file does have an account entry for uid 102:messagebus
UID
– the column heading from the first line of output of the process listing. We don’t want to see this in our output.
Copy procs_counted.sh
to procs_counted2.sh
.
procs_counted2.sh
file to eliminate the bogus UID
column heading in the output. There are at least two ways to do this; pick either way and make the change:
UID
. Hint: the command that searches inside files for lines matching a pattern has an option to invert the match and only show non-matching lines. Search the man
page.man
page for no header
will find it. You will need to read about the problematic use of the single-letter option and how they solved it using long options.procs_counted2.sh
to make sure it works. It should generate a list of more than a dozen userids in sorted order, with no bogus UID
in the output.
UID
line!Copy procs_counted2.sh
to procs_counted_header.sh
.
Enhance the new procs_counted_header.sh
script so that it prints out its own header (title) line before it generates the output. The header line should be the exact text “NumProc Account
” (15 characters, including the single space, with upper-case for the N
, P
, and A
) and you can optionally output some dashed underlining under it as well, to make the heading stand out. Line up the blank in the title with the blank in the two columns of output. The output will look similar to this, though the actual numbers and accounts may differ:
NumProc Account
------- -------
1 102
2 avahi
1 colord
2 cst8207a
10 cst8207c
... etc ...
procs_counted_header.sh
to make sure it works. It should generate its own header line, some optional underlining, followed by a list of more than a dozen userids in sorted order, with no bogus UID
in the output.
UID
line!The long script name procs_counted_header.sh
is helpful for knowing what the script does, but it’s much too long for a command name. Make a hard link from procs_counted_header.sh
to the name pch
in your HOME bin
directory.
Because your bin
directory is in your PATH
, you should now be able to run pch
as a command name with no slashes. You have written your second Linux command script.
Check your work so far using the checking program symlink.
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.
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 assignment02check
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.
assignment02.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
assignment02.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
assignment02.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 assignment02 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!