1. Add Kali Linux Repositories to Ubuntu:

echo "deb http://http.kali.org/kali kali-rolling main contrib non-free" | sudo tee /etc/apt/sources.list.d/kali.list

2.Add Kali GPG Key:

wget -q -O - https://archive.kali.org/archive-key.asc | sudo apt-key add -

3.Update APT package list: sudo apt update

4.Install Specific Kali Tools: sudo apt install amass

If you only want to install a few tools and avoid potential future conflicts, you can remove the Kali repositories after installing the tools:

Remove Kali Repositories After Installation (Optional):

sudo rm /etc/apt/sources.list.d/kali.list

Update APT: sudo apt update

I will also provide some additional options:

1)Using snap to Install Tools

Some tools can be installed via snap, which is a universal package manager:

Install using snap: `sudo snap install amass --classic`

2)Use Katoolin:

Katoolin is a script that simplifies the process of installing Kali Linux tools on Ubuntu: Install Katoolin: sudo apt install python3 python3-pip

sudo pip3 install katoolin

Run Katoolin: sudo katoolin

Follow the on-screen instructions to install categories of tools.

Introduction

Katoolin is a collection of scripts for automating the installation of Kali linux tools in different platforms other than Kali linux, and installing Kali linux tools with Katoolin in other OSs or Windows Subsystem for Linux is always less secure than installing the same tools in Kali Linux. Kali Linux is named after the Hindu goddess of destruction which should give the user some indication that even Kali Linux itself is risky and difficult to manage, especially for new users.

Katoolin draws its tools from a variety of sources including apt and GitHub. Katoolin doesn't know anything about the platform that it is running on, so the safest way to use Katoolin is to install each tool the hard way one tool at a time, checking each time to see if that tool is included in the default Ubuntu repositories and if so installing it that way instead. When the installation is finished, the software sources that were added by Katoolin should be immediately disabled in order to prevent Katoolin from replacing packages that were installed from the default Ubuntu repositories with its own updated packages whenever Ubuntu software is updated.

Installation of Kali linux tools

(Re)install Katoolin as follows:

1. Open the terminal and type:

   sudo apt install git  
   git clone https://github.com/LionSec/katoolin.git && sudo cp katoolin/katoolin.py /usr/bin/katoolin  
   

2. Make /usr/bin/katoolin executable.

   sudo chmod +x /usr/bin/katoolin  
   

3. Run Katoolin as follows and you will see the Katoolin interface:

   cd /usr/bin/
   sudo ./katoolin   
   

   
   ^{View the categories of available software in katoolin interface}

Press 0 to install all Kali linux tools.

When you go to View Categories and then enter 0 for all, the installer will immediately return to where you were. Scroll up, and you'll see that a number of the packages can't be found. Those packages are no longer maintained or have been removed from the Kali repositories. Even with this issue, you'll still find a lot of the Kali software available for installation.

So the best option is to install Kali linux tools in a series of distinct steps. Just like if you want to install Forensic tools on your system, then choose its option from the category and then type 0 to install all forensics tools. If this still doesn't work, then you have to install whatever forensic tools that can be installed one package at a time.

This is why the Katoolin interface sometimes hangs when you try to install all of the Kali linux tools at once. Take the time of waiting for the server to respond to a request for a package that is not available and multiply it by how many unavailable Kali linux tools there are in the Katoolin repositories.

If you wish to go back to the main menu, type gohome in your katoolin interface. Now select the option 3 or 4 to install either the classicmenu indicator or the Kali menu. Press the y key to continue and then press Enter to start its setup.

Once you are done with installation of Kali tools using katoolin, you can quit by pressing the keyboard combination Ctrl+C and you will see the goodbye message.

shutdown requested....Goodbye...

Credit for the above text block goes to Kashif at How To Install Kali Linux Tools On Ubuntu 16 And CentOS 7 with my additions italicized.

Warning about updates after installing Kali linux tools

The LionSec Katoolin GitHub webpage clearly warns Katoolin users to be careful when updating software.

Before updating your system, please remove all Kali-linux repositories to avoid any kind of problem.