Georgia Downtown Association: Linux Command Line Hackery Series

Welcome back, hope you are enjoying this series, I don't know about you but I'm enjoying it a lot. This is part 3 of the series and in this article we're going to learn some new commands. Let's get started

Command: w
Syntax: w
Function: This simple function is used to see who is currently logged in and what they are doing, that is, their processes.

Command: whoami
Syntax:   whoami
Function: This is another simple command which is used to print the user name associated with the current effective user ID.

Try it and it will show up your user name.

If you want to know information about a particular user no matter whether it is you or someone else there is a command for doing that as well.

Command: finger
Syntax: finger [option] [username]
Function: finger is a user information lookup program. The [] around the arguments means that these arguments are optional this convention is used everywhere in this whole series.

In order to find information about your current user you can simply type:

finger username

Here username is your current username.
To find information about root you can type:

finger root

and it will display info about root user.

Command: uname
Syntax: uname [options]
Function:   uname is used to display information about the system.

uname is mostly used with the flag -a, which means display all information like this:

uname -a

Command: df
Syntax: df [option] [FILE ...]
Function:   df is used to display the amount of space available.
If you type df in your terminal and then hit enter you'll see the used and available space of every drive currently mounted on the system. However the information is displayed in block-size, which is not so much human friendly. But don't worry we can have a human friendly output as well using df by typing:

df -h

the -h flag is used to display the used and available space in a more user friendly format.
We can also view the info of a single drive by specifying the drive name after df like this:

df -h /dev/sda2

That's it for now about df, let's move on.

Command: free
Syntax:   free [options]
Function: free is used to display the amount of free and used physical memory and swap memory in the system.
Again the displayed information is in block-size to get a more human readable format use the -h flag like this:

free -h

Command: cal
Syntax: cal [options]
Function: cal stands for calendar. It is used to display the calendar.

If you want to display current date on the calendar you can simply type:

cal

and wohooo! you get a nice looking calendar on screen with current date marked but what if you want to display calendar of a previous month well you can do that as well. Say you want to display calendar of Jan 2010, then you'll have to type:

cal -d 2010-01

Nice little handy tool, isn't it?

Command: file
Syntax: file filename ...
Function:   file is an awesome tool, it's used to classify a file. It is used to determine the file type.

Let's demonstrate the usage of this command by solving a Noob's CTF challenge using file and base64 commands. We'll talk about base64 command in a bit. Go to InfoSecInstitute CTF Website. What you need to do here is to save the broken image file on your local computer in your home directory. After saving the file open your terminal (if it isn't already). Move to your home directory and then check what type of file it is using the file command:

cd
file image.jpg

Shocking output? The file command has identified the above file as an ASCII text file which means the above file is not an image file rather it is a text file now it's time to see it's contents so we'll type:

cat image.jpg

What is that? It's some kind of gibberish. Well it's base64 encoded text. We need to decode it. Let's learn how to do that.

Command: base64
Syntax:   base64 [option] FILE ...
Function: base64 command is used to encode/decode data and then print it to stdout.

If we're to encode some text in base64 format we'd simply type base64 hit enter and then start typing the text in the terminal after you're done hit enter again and then press CTRL+D like this:

base64
some text here
<CTRL+D>
c29tZSB0ZXh0IGhlcmUK # output - the encoded string

But in the above CTF we've got base64 encoded data we need to decode it, how are we going to do that? It's simple:

base64 -d image.jpg

There you go you've captured the flag.
The -d flag here specifies that we want to decode instead of encode and after it is the name of file we want to decode.

Voila!
So now you're officially a Hacker! Sorry no certificates available here :)

That's it for this article meet ya soon in the upcoming article.

Georgia Downtown Association

Monday, August 24, 2020

Linux Command Line Hackery Series - Part 3

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