Homework 2

Due Date: 2025-10-27 23:59:59

Homework 2
- Introduction
- Submission

Introduction

This lab is all about Linux. We’ll start with some basic tutorials on TryHackMe to get you familiar with the Linux command line. Then we’ll move on to some more advanced topics.

Download the following free ebooks:
Join the following room on TryHackMe: https://tryhackme.com/room/linuxfundamentalspart1 . The room covers basic Linux commands for navigating directories and files (echo, whoami, cd, ls, cat, pwd, find, grep, wc, as well as operators&, &&, >, and >>). Complete the exercise.

Take a screenshot showing completion of the room
Join the following room on TryHackMe: https://tryhackme.com/r/room/linuxmodules. This covers lots of useful linux commands! Complete the exercise.

Take a screenshot showing completion of the room.
grep is a tool which allows for regular expression matching of contents of a line or file. It is commonly known as a filter – it removes unwanted cruft and only allows the display or further processing of data that matches (or doesn’t match, in the case of an inverted search).

grep has multiple modes, which can be invoked with an alternative name or a specific flag. For instance, to use extended regular expression syntax, you can invoke as egrep or call as grep -E. Both operate identically. The different modes are
- Extended regular expressions: -E
- PERL compatible regular expressions: -P
- Fixed string matching: -F
- Default behavior, if you want to specify it as a flag (scripts and the like): -G
grep is a tool you will likely make a ton of use of. That will require both a knowledge of regular expressions and an understanding of how grep works and is controlled by its flags. See the man page for grep for the latter, and you’re on your own for anything beyond very basic regular expressions. I’m a firm believer that if you try to solve a problem with regular expressions, you now have many problems, rather than just the one.

Use grep to find the following things in the /usr/share/wordlists/rockyou.txt file (you may have to decompress it or use zgrep):
- All lines that contain the word “password” (case-sensitive)
- All lines that contain “password” (case-insensitive)
- All lines that end with exactly 3 numerical digits
  - with an anchor
  - without an anchor
- All lines that contain exactly 3 numerical digits, in any position (do not have to be adjacent, just 3 total)
For each of the above, also provide the number of lines that match the criteria.
sed is the stream editor. Please look up the sed manpage, and provide a command to do the following:
1. Replace all instances of from this.that import something to from that import something in all python files in the current directory tree. Remember that sed can be used to edit files in place, so you don’t need to worry about redirection. Remember the find tool I mentioned in class?
2. Print lines 312-345 of a file (inclusive). This is a common use of sed to print a range of lines from a file. Use any file you like for this, as long as it has enough lines.
3. In the /etc/ssh/sshd_config file, replace the line #PasswordAuthentication yes with PasswordAuthentication no. This is a common security practice to disable password authentication for SSH. Use sed to do this in place, as well.
Run each of objdump, nm, and readelf on /bin/ls.
- What do each of these tools do?
- What information do they provide?
- How do they differ from each other?
Give examples of each tool’s output.
Virtual environments are a common feature in today’s development ecosystem. Each environment must be activated in order to be used, by sourcing the activate script in the environment’s bin directory. Typically, this file exists in a .env subdirectory, but not always. Write a command which will activate the virtual environment in the current directory, regardless of where the activate script is located. Ensure that it is in bin directory (immediately), and it is named (exactly) activate.

Now write a shell function you can use to perform this action from anywhere in the filesystem. This should be a zsh function (as that’s the default on Kali).
Take a look at the following pipeline (you can run this on ada for real results)
```
$ ps -efH --no-header | awk '{print $1}' | grep -Ev $(python3 -c 'import sys; print("|"s.join(sys.argv[1:]))' $(cut -f1 -d':' /etc/passwd)) | sort | uniq -c | sort -n
```
No, I don’t type that every time. I have it as an alias in my startup script, since it’s how I print out a list of users who are causing excessive load on the server. Play with it, see if you can improve on it. You can also see the use of immediate python combined with the use of awk, cut, and grep.

Questions to answer:
1. What do you think the grep portion does? Why is that useful?
2. What is an alias?
3. Can you replace the awk pipeline component with a cut? What about the reverse?
4. Take a look at a tool called paste. How might you use it to replace the use of python in this pipeline?

Submission

Once you have completed the above, you should have a markdown file in your repo called hw2/hw2.md that contains all the requested information. Commit and push this to your repo. Also commit and include any requested screenshots. Once you have done this, you can consider the assignment submitted.

In order to include output from your shell, see the technical writing page for some basic instructions. For instance:

❯ ps -efH --no-header | awk '{print $1}' | grep -Ev $(python3 -c 'import sys; print("|".join(sys.argv[1:]))' $(cut -f1 -d':' /etc/passwd)) | sort | uniq -c | sort -rn | head -n 11
USER01
USER02
USER03
USER04
USER05
USER06
USER07
USER08
USER09
USER10
USER11

Could be created with the following markdown:

```sh
❯ ps -efH --no-header | awk '{print $1}' | grep -Ev $(python3 -c 'import sys; print("|".join(sys.argv[1:]))' $(cut -f1 -d':' /etc/passwd)) | sort | uniq -c | sort -rn | head -n 11
USER01
USER02
USER03
USER04
USER05
USER06
USER07
USER08
USER09
USER10
USER11
```