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Learn Ethical Hacking Episode #13: Command Line Essentials (Part 1)


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Hello everybody and welcome to this tutorial. Today, I will go over a few basic commands which you will need in order to follow this course through. These basic commands are used in order to navigate within the terminal, or through the file system, to figure out where you are at the moment, to copy things, move things, get listings of files, folders, see the contents, remove them, change the ownership of files, or change the modes of files.

So, first off, we have CD which is change directory. If I type in cd /home/ I am going to navigate over to my home directory. CD stands for change directory, as I said before, you are literally changing your working directory. If you type in cd .. you will always go one step back, so double dot is always the previous folder.

Next up we have LS. LS shows you the listing of the current folders contents, but you can also use ls /, for example home, and you get the listing of a specified folder as well. Typing LS in will give you the listing of the current folder, as opposed to typing LS and then a path to a folder which will give you a listing of a specified folder. In addition to that, you also have ls -l which will give you a long listing. It will tell you who the owners are, the size, the date, the type, the permissions, and so on and so forth. However, one of the more common usages of ls is -la to show the hidden files as well.

Next up is PWD, which prints the working directory, and you might find this a bit confusing, but slash in all Linux/UNIX-like systems simply refers to the root directory, to the beginning, to the root directory. I’m going to navigate over to home and then to PWD. You can see that I am in home at the moment. So print working directory does exactly what the name itself says, it prints your current working directory.

Next up, we have the CP command, which we already have used in order to copy VirtualBox guest additions from one place to another. Fairly simple. So if I, for example, go ahead and type in cp, and if I want to copy it somewhere, let’s say that I wish to copy it to var. I will delete this of course, and I will show you how to do that as well, but it’s fairly simple. So with cp you specify what you want to copy and where you want to copy it to. Keep in mind that you could have actually typed in here a folder, a full path to, you didn’t actually need to be in the home directory. The -v flag gives us this, so it tells us what was copied where. Let me show you what would happen if I didn’t have that, just like this. Nothing. There is no output. Now this is fairly simple when you have one very small file, but when you have a large file, let’s say 20 gigabytes or so, and you are copying it, you’re going have a blank screen below. Nothing will be happening. You will not be able to figure out where the copying process is going on, or whether it has actually crashed, bugged, or something of a kind. We’re passing the -v flag because the machine actually tells you what it is doing.

Now you might have noticed that all of these commands, more or less, have their own arguments which can be passed to them in order to modify what the command does. You can view a listing of these arguments, you do not need to memorize them all. So let’s use cp in this example. I’m going to type –help, so this is a universal way of getting help on a particular subject within the terminal. You type in the command, –help, press Enter, and you get a listing of possibilities. So I know it looks a bit messy and a bit difficult to see, but you don’t actually need to look at all of these things here. You can just scroll upwards and see all the arguments, they are listed clearly here. You have the argument here, and then you have an explanation of what the argument does. For the time being, you’re not going to be able to memorize all of these. But as you practice more and more, and as you start actually using these commands, you will begin to remember more and more. Basically, you’ll begin caching the information. Anyway, over here, in the upper part of this help menu, you’ll get usage. So in the usage you get the format, or the syntax, of the command. So with CP you pass an option, and then you specify your source and destination. Very simple. This is universal for all of the commands. You can always use the help menu.

In addition to the help menu, you also have the man pages. So if I type in man, and I’m going to use the command PWD, the man page on PWD gives me a ton of information. It gives you the name, it’s called synopsis here, but basically the syntax of the command. You get a full description of what the command does. Full description of the arguments. You even get the author who actually wrote it, some notes, copyright, etc. This is a very short man page because PWD is a very short command, and a very basic command. But, for example, if I was to type in man grep, you can see that the man page is a lot. Grep is a multi-purpose command which I will show to you in a moment, but I just wanted to show you the difference between a basic and a more complex command. Let me just go ahead and clear the screen.

Next up we have move, so MV. That will cause a folder to move. This is how you rename folders too, or files. So let’s just go ahead and navigate over to var where I have actually copied this file, ls, excellent! So if I say mv, I can either move it to a different folder, under a different name, or I can rename it within my current folder. So if I just type it in, I don’t know, let’s say RandomIsNotSoRandom. I’m just going to rename it to that and say .run. You can put whatever extension you want, of course it functions the same, but I’m just showing you the way of renaming a file. If I type ls again, you see that has been successfully renamed to I can also move to my home directory. And if I type ls, and then specify a path to my home directory, and press enter, you can see that I have actually moved it here. But,
unlike the copy command, it doesn’t retain the original file, within the original folder. So, once you move it it will be deleted from the original folder. Also, be very careful with deleting files in Linux, because once you delete them from the terminal you will not be able to recover them. There are a few complex procedures that might give you a fraction of what you wanted to recover, but if you remove it from the terminal it’s very difficult. It’s not like Windows when you delete something. In Windows you can always recover a file, you basically didn’t even delete it. You just don’t see it. When you delete something on Linux it’s gone, permanently. So take great caution, take great care. So, I’ll just go ahead and clear the screen.

Next up we have CAT. Now cat gives us, well let me show you what cat does. Let’s navigate over to home, and let’s cat It shows you the the entire contents of that file, whatever that might be. This content is not pretty at all, it is difficult to read like this. Let’s create another file, and I’m going to cat it. Nano is my favorite text editor from the terminal. You will need to learn how to use terminal text editors, because you don’t want to be switching back and forth between GUI text editors and working on something in terminal. It’s extremely time consuming, and plus here, you can actually edit things with root. However, if you are, for example, using another Linux distribution, most of the GUI text editors will not allow you to run them as root. You will receive an error message. So that can be a bit problematic. So just type in nano, and type in the name of the file that you wish to create. So, for example, I’m going to go ahead and type in test. This is going to be a test file, no need to give it any extensions. Now we are in the Nano text editor. So here I can, for example, write some text. Some random text goes here. Let’s just do that. Okay, so you have a wide variety of options down here. Well, maybe not a wide variety, but certainly a good amount. So this ^X simply means control + X. So if you press control + X you’re going to exit. Control + O is WriteOut, that’s basically save. Where Is is basically the way to search things, so control + V, and let’s say random, press enter, and there you go. It’s going to point to random. Cut text is also very nice because it allows you to remove large portions of text at a relatively fast rate. So if I press ctrl + K, it’s going to remove the current line where the cursor is, not what you have selected with your mouse. I will continue discussing basic terminal commands in the follow up tutorial. Until then, I bid you farewell.

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