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Add a couple of AWS questions
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</tr>
<tr>
<td align="center"><a href="#programming"><img src="images/programming.png" width="75px;" height="75px;" alt="programming"/><br /><b>Programming</b></a></td>
<td align="center"><a href="exercises/software_development/README.md"><img src="images/programming.png" width="75px;" height="75px;" alt="programming"/><br /><b>Software Development</b></a></td>
<td align="center"><a href="#python"><img src="images/python.png" width="80px;" height="75px;" alt="Python"/><br /><b>Python</b></a></td>
<td align="center"><a href="#go"><img src="images/Go.png" width="75px;" height="75px;" alt="go"/><br /><b>Go</b></a></td>
<td align="center"><a href="exercises/shell/README.md"><img src="images/bash.png" width="70px;" height="75px;" alt="Bash"/><br /><b>Shell Scripting</b></a></td>
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</tr>
<tr>
<td align="center"><a href="exercises/cloud/README.md"><img src="images/cloud.png" width="110px;" height="75px;" alt="Cloud"/><br /><b>Cloud</b></a></td>
<td align="center"><a href="#aws"><img src="images/aws.png" width="75px;" height="75px;" alt="aws"/><br /><b>AWS</b></a></td>
<td align="center"><a href="exercises/aws/README.md"><img src="images/aws.png" width="75px;" height="75px;" alt="aws"/><br /><b>AWS</b></a></td>
<td align="center"><a href="#azure"><img src="images/azure.png" width="75px;" height="75px;" alt="azure"/><br /><b>Azure</b></a></td>
<td align="center"><a href="#gcp"><img src="images/googlecloud.png" width="80px;" height="75px;" alt="Google Cloud Platform"/><br /><b>Google Cloud Platform</b></a></td>
<td align="center"><a href="#openstack"><img src="images/openstack.png" width="75px;" height="75px;" alt="openstack"/><br /><b>OpenStack</b></a></td>
Expand Down Expand Up @@ -828,7 +828,19 @@ The root of the filesystem. The beginning of the tree.
</b></details>

<details>
<summary>Can you create files in /proc?</summary><br><b>
<summary>What makes /proc different from other filesystems?</summary><br><b>
</b></details>

<details>
<summary>True or False? only root can create files in /proc</summary><br><b>

False. No one can create file in /proc directly (certain operations can lead to files being created in /proc by the kernel).
</b></details>

<details>
<summary>What can be found in /proc/cmdline?</summary><br><b>

The command passed to the boot loader to run the kernel
</b></details>

<details>
Expand Down Expand Up @@ -1023,7 +1035,7 @@ tail -f <file_name>
<summary>How you measure time execution of a program?</summary><br><b>
</b></details>

#### Linux Kernel
#### Linux - Kernel

<details>
<summary>What is a kernel, and what does it do?</summary><br><b>
Expand Down Expand Up @@ -1054,6 +1066,68 @@ The operating system executes the kernel in protected memory to prevent anyone f
Applications can access system resources and indirectly the kernel space by making what is called "system calls".
</b></details>

<details>
<summary>In what phases of kernel lifecycle, can you change its configuration?</summary><br><b>

* Build time (when it's compiled)
* Boot time (when it starts)
* Runtime (once it's already running)
</b></details>

<details>
<summary>Where can you find kernel's configuration?</summary><br><b>

Usually it will reside in `/boot/config-<kernel version>.<os release>.<arch>`
</b></details>

<details>
<summary>Where can you find the file that contains the command passed to the boot loader to run the kernel?</summary><br><b>

`/proc/cmdline`
</b></details>

<details>
<summary>How to list kernel's runtime parameters?</summary><br><b>

`sysctl -a`
</b></details>

<details>
<summary>Will running <code>sysctl -a</code> as a regular user vs. root, produce different result?</summary><br><b>

Yes, you might notice that in most systems, when running `systctl -a` with root, you'll get more runtime parameters compared to executing the same command with a regular user.
</b></details>

<details>
<summary>You would like to enable IPv4 forwarding in the kernel, how would you do it?</summary><br><b>

`sudo sysctl net.ipv4.ip_forward=1`

To make it persistent (applied after reboot for example): insert `net.ipv4.ip_forward = 1` into `/etc/sysctl.conf`

Another way to is to run `echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward`
</b></details>

<details>
<summary>How <code>sysctl</code> applies the changes to kernel's runtime parameters the moment you run sysctl command?</summary><br><b>

If you `strace` the sysctl command you can see it does it by changing the file under /proc/sys/...

In the past it was done with sysctl system call, but it was deprecated at some point.
</b></details>

<details>
<summary>How changes to kernel runtime parameters persist? (applied even after reboot to the system for example)</summary><br><b>

There is a service called `systemd-sysctl` that takes the content of /etc/sysctl.conf and applies it. This is how changes persist, even after reboot, when they are written in /etc/sysctl.conf
</b></details>

<details>
<summary>Are the changes you make to kernel parameters in a container, affects also the kernel parameters of the host on which the container runs?</summary><br><b>

No. Containers have their own /proc filesystem so any change to kernel parameters inside a container, are not affecting the host or other containers running on that host.
</b></details>

#### Linux - SSH

<details>
Expand Down Expand Up @@ -2342,6 +2416,21 @@ MemAvailable - The amount of available memory for new workloads (without pushing

#### Linux - Misc

<details>
<summary>How to generate a random string?</summary><br><b>

One way is to run the following: `cat /proc/sys/kernel/random/uuid`
</b></details>

<details>
<summary>What is a Linux distribution?</summary><br><b>

* A collection of packages - kernel, GNU, third party apps, ...
* Sometimes distributions store some information on the distribution in `/etc/*-release` file
* For example for Red Hat distribution it will be `/etc/redhat-release` and for Amazon it will be `/etc/os-release`
* `lsb_release` is a common command you can use in multiple different distributions
</b></details>

<details>
<summary>Name 5 commands which are two letters long</summary><br><b>

Expand Down Expand Up @@ -2615,242 +2704,6 @@ Yes, it's a operating-system-level virtualization, where the kernel is shared an
The introduction of virtual machines allowed companies to deploy multiple business applications on the same hardware while each application is separated from each other in secured way, where each is running on its own separate operating system.
</b></details>

## Programming

<details>
<summary>What programming language do you prefer to use for DevOps related tasks? Why specifically this one?</summary><br><b>
</b></details>

<details>
<summary>What are static typed (or simply typed) languages?</summary><br><b>

In static typed languages the variable type is known at compile-time instead of at run-time.
Such languages are: C, C++ and Java
</b></details>

<details>
<summary>Explain expressions and statements</summary><br><b>

An expression is anything that results in a value (even if the value is None). Basically, any sequence of literals so, you can say that a string, integer, list, ... are all expressions.

Statements are instructions executed by the interpreter like variable assignments, for loops and conditionals (if-else).
</b></details>

<details>
<summary>What is Object Oriented Programming? Why is it important?</summary><br><b>
</b></details>

<details>
<summary>Explain Composition</summary><br><b>
</b></details>

<details>
<summary>What is a compiler?</summary><br><b>
</b></details>

<details>
<summary>What is an interpreter?</summary><br><b>
</b></details>

<details>
<summary>Are you familiar with SOLID design principles?</summary><br><b>

SOLID design principles are about:

* Make it easier to extend the functionality of the system
* Make the code more readable and easier to maintain

SOLID is:

* Single Responsibility - A class should only have a single responsibility
* Open-Closed - An entity should be open for extension, but closed for modification. What this practically means is that you should extend functionality by adding a new code and not by modifying it. Your system should be separated into components so it can be easily extended without breaking everything.
* Liskov Substitution - Any derived class should be able to substitute the its parent without altering its corrections. Practically, every part of the code will get the expected result no matter which part is using it
* Interface segregation - A client should never depend on anything it doesn't uses
* Dependency Inversion - High level modules should depend on abstractions, not low level modules
</b></details>

<details>
<summary>What is YAGNI? What is your opinion on it?</summary><br><b>
</b></details>

<details>
<summary>What is DRY? What is your opinion on it?</summary><br><b>
</b></details>

<details>
<summary>What are the four pillars of object oriented programming?</summary><br><b>
</b></details>

<details>
<summary>Explain recursion</summary><br><b>
</b></details>

<details>
<summary>Explain Inversion of Control</summary><br><b>
</b></details>

<details>
<summary>Explain Dependency Injection</summary><br><b>
</b></details>

<details>
<summary>True or False? In Dynamically typed languages the variable type is known at run-time instead of at compile-time</summary><br><b>

True
</b></details>

<details>
<summary>Explain what are design patterns and describe three of them in detail</summary><br><b>
</b></details>

<details>
<summary>Explain big O notation</summary><br><b>
</b></details>

<details>
<summary>What is "Duck Typing"?</summary><br><b>
</b></details>

<details>
<summary>Explain string interpolation</summary><br><b>
</b></details>

##### Common algorithms

<details>
<summary>Binary search:

* How does it works?
* Can you implement it? (in any language you prefer)
* What is the average performance of the algorithm you wrote?</summary><br><b>

It's a search algorithm used with sorted arrays/lists to find a target value by dividing the array each iteration and comparing the middle value to the target value. If the middle value is smaller than target value, then the target value is searched in the right part of the divided array, else in the left side. This continues until the value is found (or the array divided max times)

[python implementation](coding/python/binary_search.py)

The average performance of the above algorithm is O(log n). Best performance can be O(1) and worst O(log n).
</b></details>

##### Code Review

<details>
<summary>What are your code-review best practices?</summary><br><b>
</b></details>

<details>
<summary>Do you agree/disagree with each of the following statements and why?:

* The commit message is not important. When reviewing a change/patch one should focus on the actual change
* You shouldn't test your code before submitting it. This is what CI/CD exists for.</summary><br><b>
</b></details>

#### Strings

<details>
<summary>In any language you want, write a function to determine if a given string is a palindrome</summary><br><b>
</b></details>

<details>
<summary>In any language you want, write a function to determine if two strings are Anagrams </summary><br><b>
</b></details>

#### Integers

<details>
<summary>In any language you would like, print the numbers from 1 to a given integer. For example for input: 5, the output is: 12345</summary><br><b>
</b></details>

#### Time Complexity

<details>
<summary>Describe what would be the time complexity of the operations <code>access</code>, <code>search</code> <code>insert</code> and <code>remove</code> for the following data structures:</summary><br><b>

* Stack
* Queue
* Linked List
* Binary Search Tree
</b></details>

<details>
<summary>What is the complexity for the best, worst and average cases of each of the following algorithms?:

* Quick sort
* Merge sort
* Bucket Sort
* Radix Sort</summary><br><b>
</b></details>

#### Data Structures & Types

<details>
<summary>Implement Stack in any language you would like</summary><br><b>
</b></details>

<details>
<summary>Tell me everything you know about Linked Lists</summary><br><b>

* A linked list is a data structure
* It consists of a collection of nodes. Together these nodes represent a sequence
* Useful for use cases where you need to insert or remove an element from any position of the linked list
* Some programming languages don't have linked lists as a built-in data type (like Python for example) but it can be easily implemented
</b></details>

<details>
<summary>Describe (no need to implement) how to detect a loop in a Linked List</summary><br><b>

There are multiple ways to detect a loop in a linked list. I'll mention three here:

Worst solution:<br>
Two pointers where one points to the head and one points to the last node. Each time you advance the last pointer by one and check whether the distance between head pointer to the moved pointer is bigger than the last time you measured the same distance (if not, you have a loop).<br>
The reason it's probably the worst solution, is because time complexity here is O(n^2)

Decent solution:<br>

Create an hash table and start traversing the linked list. Every time you move, check whether the node you moved to is in the hash table. If it isn't, insert it to the hash table. If you do find at any point the node in the hash table, it means you have a loop. When you reach None/Null, it's the end and you can return "no loop" value.
This one is very easy to implement (just create a hash table, update it and check whether the node is in the hash table every time you move to the next node) but since the auxiliary space is O(n) because you create a hash table then, it's not the best solution

Good solution:<br>
Instead of creating a hash table to document which nodes in the linked list you have visited, as in the previous solution, you can modify the Linked List (or the Node to be precise) to have a "visited" attribute. Every time you visit a node, you set "visited" to True.<br>
Time compleixty is O(n) and Auxiliary space is O(1), so it's a good solution but the only problem, is that you have to modify the Linked List.

Best solution:<br>
You set two pointers to traverse the linked list from the beginning. You move one pointer by one each time and the other pointer by two. If at any point they meet, you have a loop. This solution is also called "Floyd's Cycle-Finding"<br>
Time complexity is O(n) and auxiliary space is O(1). Perfect :)
</b></details>

<details>
<summary>Implement Hash table in any language you would like</summary><br><b>
</b></details>

<details>
<summary>What is Integer Overflow? How is it handled?</summary><br><b>
</b></details>

<details>
<summary>Name 3 design patterns. Do you know how to implement (= provide an example) these design pattern in any language you'll choose?</summary><br><b>
</b></details>

<details>
<summary>Given an array/list of integers, find 3 integers which are adding up to 0 (in any language you would like)</summary><br><b>

```
def find_triplets_sum_to_zero(li):
li = sorted(li)
for i, val in enumerate(li):
low, up = 0, len(li)-1
while low < i < up:
tmp = var + li[low] + li[up]
if tmp > 0:
up -= 1
elif tmp < 0:
low += 1
else:
yield li[low], val, li[up]
low += 1
up -= 1
```
</b></details>

## Python

### Python Exercises
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