Introduction to Linux - Cowsay Mini-adventure

Attempt this miniadventure after going through the content of the “intro to linux” main tutorial.

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This tutorial adopted from one written by Brianna Huynh, Class of 2019, in January 2019

Welcome to Linux! The Kali terminal is powerful and can be used for many things, including making the cow above. But before any of that happens, we need to learn some basic commands that’ll help us move around the terminal.

  1. Log in to Kali
  2. Open up a terminal window

  3. For this tutorial, get a shell running as user root by running this within your terminal:

    sudo -s
    

    This launches a shell (-s) as root via the sudo command.

  4. Let’s see where we are right now. Enter the following command to show our current directory pwd: print working directory

    We see that we are in the root directory. This is the “home directory” for the root user. There also exists a root user on linux systems. root is the root admin for an operating system. In other words, it has the highest clearance in the system and is able to perform all commands.
    Other users may not have permissions to run certain actions, but root can do all.

  5. Now that we know which directory we’re working in, what kind of files do we have in here? Type in the following command to list directory contents.

    ls
    
  6. Sometimes, files can be hidden from the ls command. Like ninjas. We get around this by adding a flag to the command which tells it to do something specific.

     ls -a
    

    -a: display all, including hidden files

  7. Many commands have useful flags that tailor it to do exactly what you need it to do. Where do we find out all the options of the command? We can use man!

    Well, specifically the man command. Use man <name of the command you want to check> to see all the documentation available.

    Let’s try man ls

  8. There are a variety of different applications that can be installed into Linux. For example, the cow at the top of the lab can be made in the terminal and is, arguably, the most important function that can be performed in Linux.

    Sudo apt install cowsay 
    

    sudo: allows the user to run commands as root

    apt install: installs an application using kali’s aptitude package manage.r

    cowsay: the best application

  9. After the installation is complete, try the command out.

    cowsay <something you want the cow to say>
    
  10. Oh dear, that didn’t work. When applications are installed via apt, they often install themselves into a location that makes them discover-able and therefore run-able via just their standalone names. But this doesn’t work when using user root (it does work for non-root users). We’ll have to do it ourselves.

    First, find where the cowsay application was installed on the filesystem.

    find / -name 'cowsay'
    
  11. Show off your navigational prowess by changing directories to where cowsay said it was located.

    You could do:

    cd /usr/games
    

    Or you could do:

    cd ..
    cd usr
    cd games
    

    Run pwd to see that where you are located matches the handy prompt info:

    pwd
    

    See what is in your current directory. Do you see cowsay?

    ls -a
    
  12. We’ve found cowsay! To run an application, you either (1) specify its full path (e.g., /usr/games/cowsay), or you put it on your $PATH environment variable, so that you do not have to specify its path.

    Verify that you can run cowsay via its full path:

    /usr/games/cowsay <thing to say>
    
    Warning: If you run cowsay without any arguments, it will sit and wait, listening for standard input, for you to type what you want it to say. Finish your message with ctrl+d, or kill the process with ctrl+c.

    But let’s update our PATH env variable to include /usr/games/ on it.

    First, look at your current path:

    echo $PATH
    

    Do you see /usr/games there? You do not. Update your PATH:

    PATH="$PATH:/usr/games/"
    

    This will set PATH to be the current (expanded) value of PATH, plus the /usr/games/ directory. (The : delimits path for the $PATH variable.)

    Now check your PATH again:

    echo $PATH
    

    Check that we can now run cowsay without specifying the full path:

    `cowsay <thing to say>`
    

    This is nice, but we would have to run this every time we opened a new shell. Let’s make our updated PATH permanent. We can do that for our shell (terminal) by entering commands that will be run on each launch, in the .bashrc file located in our home directory.

    Move back to the home folder

    cd
    
    • Recall that cd without any arguments will shortcut-navigate you to your home directory. You could also have run:

       cd ~
      

    Your .bashrc file is located here. Verify that this is so:

    ls -a
    

    Open up a text editor with the .bashrc

    nano .bashrc
    

    Add this line to the bottom of the file:

    export PATH="$PATH:/usr/games"
    

    … where export will make the variable stick outside of the process that is reading the .bashrc file (otherwise, the update would only persist within the scope of the shell initialization process).

    Save the file

    ctrl+o
    <enter>
    

    Exit the file

    ctrl+x
    

    1. Tell your shell to read .bashrc so that the path for your current shell gets read.

      source ~/.bashrc
      

      alternatively,

      . ~/.bashrc
      

      alternatively, close this terminal and open a new one.

    2. Let’s try cowsay again:

      cowsay <something you want to say>
      

      Did it work? If so, yay!

Proof of cowsay mastery