The goal of this homework assignment is to allow you to practice manipulating files, redirecting I/O, constructing pipelines, and signaling processes in a Unix environment.
For this assignment, record your responses to the following activities in the
README.md file in the homework01 folder of your assignments GitLab
repository and push your work by 11:59 AM Saturday, January 27.
As discussed in class, each reading and homework assignment must be completed in its own git branch; this will allow you to separate the work of each assignment and for you to use the merge request workflow describe in Reading 01.
To create a homework01 branch in your local repository, follow the
instructions below:
$ cd path/to/cse-20289-sp18-assignments # Go to assignments repository $ git checkout master # Make sure we are in master branch $ git pull --rebase # Make sure we are up-to-date with GitLab $ git checkout -b homework01 # Create homework01 branch and check it out $ cd homework01 # Go into homework01 folder
Once these commands have been successfully performed, you are now ready to add, commit, and push any work required for this assignment.
The /usr/share/pixmaps folder is found on many Linux machines. It
usually contains images such as icons and wallpapers used by desktop
environments and graphical applications.
On the student machines, you are to perform the following operations while using the time command to measure how long each operation took:
Copy all the contents of the /usr/share/pixmaps folder to a new images
folder in your AFS home directory.
Rename this new images folder in your home directory to pixmaps.
Move this new pixmaps folder to /tmp/$USER-pixmaps (where $USER is
your netid)
Remove the /tmp/$USER-pixmaps folder
Record your answers by filling out the following Markdown table:
| Command | Elapsed Time | |-------------------------------------|---------------| | cp ... | 0 seconds | | mv ... | 0 seconds | | mv ... | 0 seconds | | rm ... | 0 seconds |
The time command is typically a Unix shell built-in which executes the specified program and outputs something like the following:
$ time sleep 1 real 0m1.001s user 0m0.000s sys 0m0.002s
In the example above, the time command is used to execute the sleep
command and measure how long it took. Normally, we are interested in the
real or wall-clock time, which in this case is 1.001 seconds.
After performing and timing the operations above, answer the following questions:
Both renaming and moving the pixmaps folder use the mv command but the
former operation takes significantly less time than the latter. Explain why
this is.
Similarly, explain why removing the /tmp/$USER-pixmaps operation is
faster than the move operation.
The bc command, which was introduced in Reading 01, is another example of an interactive interpreter. In this case, bc processes and executes basic arithmetic operations (ie. it is a calculator).
Suppose you had a file named math.txt, with the following contents:
1 + 1 2 * 4 (3 - 7) * 8 2^3 4/0
How would you use I/O redirection to process the math.txt file with
bc?
How would you use I/O redirection to process the math.txt file with
bc and save the output to results.txt?
How would you use I/O redirection to process the math.txt file with
bc, save the output to results.txt, and suppress any errors?
How would you use cat and a pipeline instead of I/O redirection to
process the math.txt file with bc?
Why is this considered less efficient than using I/O redirection?
Recall that the Unix Philosophy can be summarized as:
Write programs that do one thing and do it well.
Write programs to work together.
Write programs to handle text streams, because that is a universal interface.
The ultimate example of these tenets in action is the Unix pipeline.
For each of the following tasks, construct a pipeline that combines multiple commands to accomplish each task.
Determine how many accounts in the /etc/passwd file have /sbin/nologin
as their shell.
Determine how many instances of bash are running on the current machine
(including all users).
Determine the top 5 largest files or directories in the /etc directory
while suppressing any errors.
Determine how many unique users are logged into the current system.
You can use either awk or cut to extract fields from a line of text. For instance, the following returns the first field separated by spaces of each line in the input file:
cat file | awk '{print $1}'
You can accomplish something similar with cut:
cat file | cut -d ' ' -f 1
This returns the first field separated by a single space. We will learn about constructing pipelines with text filters in a few weeks.
For the last activity, you are to run the following program on the student machines:
/afs/nd.edu/user15/pbui/pub/bin/TROLL
This is a program that will taunt you and insult you... until you figure out how to terminate the process.
Using a single terminal window, run the TROLL program and figure out
a way to terminate it:
a. Describe what commands or operations failed to terminate the process.
b. Describe what commands or operations finally succeeded in terminating the process.
Using two terminal windows, run the TROLL program in the first window
and figure out a way to terminate it from the second terminal window:
a. Describe a pipeline that can be used to terminate the process from the second terminal window.
b. Describe a single command that can be used to terminate the process from the second terminal window.
Explore passing different signals to the TROLL program. Which ones
lead to interesting messages that are not taunts or insults?
To verify that the TROLL is actually terminated and not merely
stopped, you can use the following command:
$ pgrep -u $(whoami) TROLL
If any PIDs are displayed, then you still have TROLLs running and thus
have not terminated it properly. Ensure that the sequence of commands you
figured out will reliably terminate all instances of the TROLL.
For extra credit, read the following chapters from The Linux Command Line:
Once you have read these chapters, you are to customize your bash shell
prompt by adding useful information and colors and setting $EDITOR to your
preferred text editor.
Here are some additional resources to help you customize your bash shell:
Normally, to change your default shell, you would use the chsh command. Unfortunately, the student machines do not allow for this operation and so we cannot use this mechanism. Instead, you have to use the following unsupported workaround to make bash the default shell:
if ($?prompt) then setenv SHELL /bin/bash exec /bin/bash endif
Append the code above to the bottom of your ~/.cshrc file. This code
snippet checks if the session is interactive; if so, it sets the SHELL
environment variable to /bin/bash and then executes the bash shell.
With this in place, you will automatically be given the bash shell whenever you login rather than tcsh.
To get credit for this Guru Point, show your bash shell customizations to the instructor or a TA to verify.
If you have any questions, comments, or concerns regarding the course, please
provide your feedback at the end of your README.md.
To submit your assignment, please commit your work to the homework01 folder
of your homework01 branch in your assignments GitLab repository:
$ cd path/to/cse-20289-sp18-assignments # Go to assignments repository $ git checkout master # Make sure we are in master branch $ git pull --rebase # Make sure we are up-to-date with GitLab $ git checkout -b homework01 # Create homework01 branch and check it out $ cd homework01 # Go to homework01 directory ... $ $EDITOR README.md # Edit appropriate README.md $ git add README.md # Mark changes for commit $ git commit -m "homework01: complete" # Record changes ... $ git push -u origin homework01 # Push branch to GitLab
Remember to create a merge request and assign the appropriate TA from the Reading 01 TA List.