CS376: Operating Systems - Shell Scripting (100 Points)

Assignment Goals

The goals of this assignment are:
  1. To use standard UNIX shell tools
  2. To write a Bash shell script

Background Reading and References

Please refer to the following readings and examples offering templates to help get you started:

The Assignment

In this tutorial, we’ll create a Bash script to monitor and alert about increases in CPU usage on a Linux system. We’ll break down the process into simple steps, making it easy to understand even for beginners in Bash scripting. You will write a script that calculates the average CPU usage over all cores and checks if there’s an increase beyond a predefined threshold compared to the previous minute.

Prerequisites

  1. Access to a Linux system with standard command-line tools installed (Windows users can install the Windows Subsystem for Linux (WSL) for access to these tools).
  2. Basic familiarity with the Linux command line and Bash scripting, or completion of this Bash Scripting tutorial.

Step 1: Creating a Shell Script

Create a new Bash script file:

nano cpu_monitor.sh

And add the shebang line at the top of the file, which tells the system that the script should be passed to the /bin/bash program for executing (as if the user had typed these commands themselves):

#!/bin/bash

Step 2: Declaring Variables

Declare variables to store previous CPU usage and the threshold for alerting:

previous_cpu_usage=0
threshold=10  # You can modify this threshold as needed

Step 3: Writing the Monitoring Loop

Use a while loop to continuously monitor the CPU usage:

while true; do
    # Script contents will go here
    sleep 10  # Wait for 10 seconds
done

Step 4: Fetching CPU Usage Data

Now, let’s write the inside body of the loop above. These commands go inside the while loop we’ve just made.

We’ll use the top command to get CPU usage data and awk for processing. This sequence of commands is slightly different for Windows/Linux and Mac Users.

Windows and Linux Users

current_cpu_usage=$(top -bn1 | grep '^%Cpu' | awk '{total += $2 + $4} END {print total/NR}')

This command does the following:

  • top -bn1: Runs top in batch mode for one iteration to get the latest CPU data.
  • grep '^%Cpu': Filters lines starting with %Cpu.
  • awk '{total += $2 + $4} END {print total/NR}': Sums the user and system usage for each CPU and calculates the average.

Mac Users

current_cpu_usage=$(top -l 1 -n 0 | awk '/CPU usage/ {user+=$3; sys+=$5} END {print (user+sys)/2}' | sed 's/%//g')

This command does the following:

  • top -l 1 -n 0: Runs top in one iteration to get the latest CPU data.
  • awk '/CPU usage/ {user+=$3; sys+=$5} END {print (user+sys)/2}': Sums the user and system usage for each CPU and calculates the average.
  • sed 's/%//g': Removes the percentage sign from the output.

Step 5: Comparing CPU Usage and Alerting

Add an if statement to compare the current CPU usage with the previous and alert if the increase is beyond the threshold (we’ll print the CPU usage otherwise):

if [ $(echo "${current_cpu_usage} > ${previous_cpu_usage} + ${threshold}" | bc) -eq 1 ]; then
    echo "Warning: Average CPU usage of ${current_cpu_usage} increased by more than $threshold% in the last minute!"
else
    echo "Average CPU Usage of ${current_cpu_usage} is below the threshold"
fi

previous_cpu_usage=${current_cpu_usage}

Here, the echo command outputs the variable current_cpu_usage > previous_cpu_usage + threshold and pipes that string to the bc tool. To get the value of a variable (rather than using the name of the variable in the string), we enclose the variable name in the ${} operator. bc is a command line calculator, so it adds the variables together, and does a greater than comparison on the result. It outputs 1 if it is true. By putting the expression inside the $() operator, we treat the result of the command as a value. With that value, we do an integer comparison to compare the result to 1 using the -eq operator.

Step 6: Running the Script

To quit nano, press Control-X on your keyboard. You’ll be prompted to save the file you’re working on, and you can say yes and hit enter to keep the same filename. Alternatively, you can press Control-O anytime to save your file.

To run the script, first make it executable:

chmod a+x cpu_monitor.sh

Then, execute it:

./cpu_monitor.sh

Submitting Your Work

You can zip up your file(s) to submit them using a zip program of your choice.

Submission

In your submission, please include answers to any questions asked on the assignment page in your README file. If you wrote code as part of this assignment, please describe your design, approach, and implementation in your README file as well. Finally, include answers to the following questions:
  • Describe what you did, how you did it, what challenges you encountered, and how you solved them.
  • Please answer any questions found throughout the narrative of this assignment.
  • If collaboration with a buddy was permitted, did you work with a buddy on this assignment? If so, who? If not, do you certify that this submission represents your own original work?
  • Please identify any and all portions of your submission that were not originally written by you (for example, code originally written by your buddy, or anything taken or adapted from a non-classroom resource). It is always OK to use your textbook and instructor notes; however, you are certifying that any portions not designated as coming from an outside person or source are your own original work.
  • Approximately how many hours it took you to finish this assignment (I will not judge you for this at all...I am simply using it to gauge if the assignments are too easy or hard)?
  • Your overall impression of the assignment. Did you love it, hate it, or were you neutral? One word answers are fine, but if you have any suggestions for the future let me know.
  • Using the grading specifications on this page, discuss briefly the grade you would give yourself and why. Discuss each item in the grading specification.
  • Any other concerns that you have. For instance, if you have a bug that you were unable to solve but you made progress, write that here. The more you articulate the problem the more partial credit you will receive (it is fine to leave this blank).

Assignment Rubric

Description Pre-Emerging (< 50%) Beginning (50%) Progressing (85%) Proficient (100%)
Algorithm Implementation (60%) The algorithm fails on the test inputs due to major issues, or the program fails to compile and/or run The algorithm fails on the test inputs due to one or more minor issues The algorithm is implemented to solve the problem correctly according to given test inputs, but would fail if executed in a general case due to a minor issue or omission in the algorithm design or implementation A reasonable algorithm is implemented to solve the problem which correctly solves the problem according to the given test inputs, and would be reasonably expected to solve the problem in the general case
Code Quality and Documentation (30%) Code commenting and structure are absent, or code structure departs significantly from best practice, and/or the code departs significantly from the style guide Code commenting and structure is limited in ways that reduce the readability of the program, and/or there are minor departures from the style guide Code documentation is present that re-states the explicit code definitions, and/or code is written that mostly adheres to the style guide Code is documented at non-trivial points in a manner that enhances the readability of the program, and code is written according to the style guide
Writeup and Submission (10%) An incomplete submission is provided The program is submitted, but not according to the directions in one or more ways (for example, because it is lacking a readme writeup or missing answers to written questions) The program is submitted according to the directions with a minor omission or correction needed, including a readme writeup describing the solution and answering nearly all questions posed in the instructions The program is submitted according to the directions, including a readme writeup describing the solution and answering all questions posed in the instructions

Please refer to the Style Guide for code quality examples and guidelines.