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Hello everybody and welcome to part two of this command line essentials tutorial.
Let’s go ahead and save the file, so press ctrl + O to write the output to the file. It says File Named to Write, you can actually modify it here. So you can type in some letters, or whatever you would like. You can change the name here, or you can even change the extension if you would like, but there is no need for the time being. I’m going to go ahead and save it as test. Press Enter, there you go. It says wrote one line. Control + X to exit, there we go. Clear the screen, and now I’m going to use the cat command to get a listing of this particular file, which is far more reasonable than the previous one which we couldn’t read. The cat command was functioning properly, but it was mostly a binary file. Therefore, it gave you a listing, but you couldn’t really read anything from it. So let’s just go ahead and type in cat test, and there we go. It says, some random text goes here. That is the text that we have typed into the test file, and you can cat it like this. The cat command is especially useful if you would like to to quickly see what is located within a file.
There is another way of doing this, the command is called LESS, and let’s go ahead and navigate over /etc/apt/, ls, and here we have plenty of files to work with. Let’s take sources.list as an example here. So, if I do cat sources.list, I’m going to get everything that is within that file, as before, and it’s going to be listed in my terminal. But, there is another command which I can use, let me just go ahead and clear the screen. It is called less. If I type in less sources.list, I’m going to be prompted with a new workspace within the terminal itself, and when I press Q it’s gonna exit, and nothing will be displayed here. So, very nice in terms of being neat and organized. So less and cat are very useful commands.
Next up, we have the GREP command. With grep you grab something from something else. Here’s what I mean. You type in cat sources.list, and you type in a pipe. This symbol here is called a pipe. This means that whatever the output of this is, pipe it to whatever command comes here. I’m going type in grep, and let’s say I want to type in src. Press Enter, excellent! So, it will only print lines that have src in them. Linux is case sensitive so when you are greping, it’s going to check the case of the letters by default. You can tell it to ignore case like this, -i means it will ignore the case. You’re going to get the same output because there are not any different things within this file, but you get the general idea. So this is how you would use cat with grep in order to pull things out of a file. Very important! This is a huge function of the Linux terminal, filtering through text files. We’re going to be conducting large network scans, and we will want to create files from which we will be able to pool useful information, which we will later pass to other tools to do something with. Anyway, let me just go ahead and clear the screen.
Continuing along, we have the ECHO command. So, echo, and I give it open quotation marks. Let’s say, “I AM ALIVE.” Close the quotation marks, and it will echo these words. It literally is an echo. You type something into it and it echoes them here. Let’s go back to the home directory, get a listing, and use exactly the same command echo “I AM ALIVE.” By the way, you can scroll through the previous commands by using arrow keys. So, using the up arrow key and down arrow key you can scroll back through previously used commands, and you don’t need to retype them. So, “I AM ALIVE”, and I’m going to insert this, so I’m going to use a greater than sign, and type in test. So this will echo “I AM ALIVE” into our test file. If I type cat test, you see it has replaced the contents of test, which was some random text goes here, with I AM ALIVE. You can change variable names and values with echo, and so on and so forth. We will use this a bit more as we progress through the course. I don’t want to get in depth here because later on when we have clearer examples that directly relate to pen-testing. Then you will see more advanced usage of pretty much all of these commands, especially grep.
We have the TOUCH command. Touch is a quick way to create files. So, for example, I can type touch, and I can say file1, file2, file 3. Press ENTER, ls, and you see it has immediately created three files. Touch is a very quick way of creating any number of files that you wish, and you can specify folder paths, you can create this file in home, this one in var, and so on and so forth.
Anyway, down below you have MKDIR. So, if I go ahead and type in mkdir, it stands for make directory. Let’s name this directory placeToBe. If I ls, there we go. It states that this file is actually a directory, which is placeToBe. I can even navigate to it and say placeToBe, ls, there is nothing in it, but you get the idea.
Next up is the CHOWN command. Chown allows you to change the ownership of a particular file. Now since we only have a single user here which is root, there isn’t really any point in doing so unless we create new users. We are not going to do that as we don’t need them, but let’s do a quick example. If the owner of a file was not root, and if you wanted to change it to root, he would do the following. So, chown, and then you would type the username, colon, the user group. So the user group and the user name are usually the same on your home PCs, unless you’re a part of a larger server, or something like that. Then you just specify the name of the file. We can type in test, there we go. So it will effectively change the ownership of file test to the username root, who is from the user group root. If I type ls -la, you can see here it states who the owners are, and which groups actually own the files as well. Clear the screen.
There is another, more common, command called CHMOD, and this command allows you to change file permissions. So, this is something that you will need to use quite often. If I have an executable file in Linux, for example, let’s just go ahead and type in echo “echo hello” close quotation marks, and I want to output this to test, and I want to move test to test.sh. Sh is a BASH script for Linux. You can use BASH scripts to automate tasks. We will deal with this in greater depth as we proceed through the course. BASH scripting is very similar to the terminal itself. Almost all of the commands that you use in the terminal you can use while BASH scripting as well. Now I want execute test.sh. The way to start executables is ./, as I have stated previously, and if I type in test and I press tab, it doesn’t give me a list of possibilities. Why? Surely, now test.sh is an executable, but no. You see it doesn’t have permission to be an executable file. If we just take a look at this file, and take a look at this file, this one is green, this one is not. Now look at their permissions. So, you see it has a notation that it is executable for all groups, users, etc. While this one up here, test.sh, does not have such permissions. You need to change that. The way you change it is chmod, and you will type in +x. So if you want the file to be writable, you type in +w. If you want it to be executable, +x. If you want to be able to read the file, +r. Very simple. There is also a way of doing this with numbers. You can type in 755, don’t memorize all of those modes. Whatever you need you can go on the net and check, it can be quite complicated, but these three you need to know. It’s +w, +r, and +x, and also 777, which is a global mode. 777 is not recommended for actual usage, but you can use chmod to actually test or troubleshoot things. For example, if you want to be absolutely sure that certain actions are not being prevented due to file permissions, you change the mode of those files to 777, which is the global mode that anybody can do anything with the file. If you still have an error message you know that it is absolutely not related to anything in regards to permissions. So those four things you do need to know. So let me just go ahead and type in +x, type in test.sh, press ENTER, and if I do ls -la, you will see that test.sh is now executable. Let me just run it, and there you go. If I run this it’s going to echo hello onto my terminal screen.
There is one more command that I would like to show to you. It can be a very dangerous command, it can mess you up in ways you can’t even begin to imagine yet, and that is the command RM. So rm is remove, and once you remove things with this command it’s next to impossible to recover pretty much anything. So if I type in rm test.sh, it’s going remove it, and it’s no longer going to be there. If I navigate over to placeToBe, and touch test, it’s going to make a new file there. If I go ahead and say rm placeToBe, it’s going say rm cannot remove placeToBe because it is a directory. This is a fail-safe for the rm command so you do not accidentally delete a full directory. What you can do is type in rm -f, and then placeToBe, cannot remove directory. Okay, not a problem. I’m deliberately going to leave this part in this tutorial to troubleshoot it. So if we type in –help, and here you have recursive, remove directories and their contents recursively. Let’s go ahead and do that because this will go into the folder, and remove everything within the folder, and the folder itself. So let’s just go ahead and type in rm -r placeToBe, press Enter, there you go. It has deleted all of it, and it is no longer there. The -f function, -f that I have used, is forced. It will not ask you any questions, it will delete the folder or file which can also be very dangerous. Ideally, you want to be asked for confirmation. In any case, as I said, I have deliberately left this in order to show you that even if you get stuck, don’t worry about it. I get stuck all of the time. If you don’t know what to type in, which argument to pass, just type in –help. Take a look at what you can actually type, read a little bit, and then use the options available to you. If you can’t figure it out from the help menu just type in man rm, and you can read pretty much everything there is to know about this command here. If you still can’t perform the task, if you can’t find anything useful here, then forums are your next best bet.
In any case, this was a brief introduction to some of the basic commands that we will use throughout the course. Please make sure that you know what each one of these commands does, at least the basic functions, and then later on we will get in-depth and do more advanced stuff. In any case, I bid you all farewell, and I hope to see you in the next tutorial.
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