Domain No. 1 - Essential Commands

6. File Permissions in Linux

In this lecture we're going to talk about Linux file permissions. In the file permissions, the first ten characters in the output of LS and LS, by the way, is the file listing. If you are in a directory and you just type LS,you'll get all the files that are under that directory. It's similar to Dosas or Windows Dirty. So LSAL gives you a long listing that gives you a little bit more information instead of just the file names. So the files, the first of these characteristics, are used to indicate the file type. So the file type could be just a hyphen, which is a regular file. It could start with a D, which is a directory. It could start with an L, which is a symbolic link; it could start with a C, which is a character device; it could also start with a dash, B, or block device. So the next nine characters of the file attributes are called a file mode and represent the read, write, and execute permissions of the file owner, the file group owner, and the rest of the users, commonly referred to as the world. Whereas the read permission on a file allows the same to be opened and read, the same permission on a directory allows its contents to be listed if the execute command is also set. And here I have an example. This is what reading, writing, and executing look like. So, if you're familiar with binary numbers, you'll have a two to the power of zero, which means one two to the power of one, which equals two, and two to the power of two, which equals four. You add them up, and that's number seven here. We have nothing here. Nothing here. There is no binary number here, but R has a two to the power of two. So that means there's a 4. So the addition is four. The same is true here. So the file permission, if this is given to a particular file, there will be seven four four.That means that the owner has all the permission to read, write, and execute. The group owner only has read permission. The rest of the world has only read permission as well, and we'll look at it in a minute on the system. So we are logged in to our machine right now. I'm going to do an LS, and this is what we're talking about. If you see in the beginning, if you have a D, that's a directory. If you have a dash only,that means it's a regular file. I don't have an example of a block or character device in it, but it will be represented by an aB or a C lower case and it's a symbolic link, which we're going to talk about later in the course, which will be represented by Dashl. So we're going to, for example, we're going to take one of our files, say takefile number two, this one right here. So far, the user or owner has read and written but has not executed, the group has read and written but has not executed, and the rest of the world has read, but has not written or executed. I can make it chmod seven, four-four file file two, and let's see what the permission looks like now. alfile two, as you can see, the permission has changed. Now the owner has all three permissions where you try to execute. The group still has the previous permissions only read and the rest of the world can only read it if you further change it. Let's see PH mod seven seven file two and let's do an LS file two. Now everybody has all the permissions, so this is something you'll only do in a situation where you do want everybody in the group and the rest of the world to have read. Write and execute to this particular file; otherwise, it is probably not a good idea to give seven seven seven to any file. Okay? Besides using numbers, you can also use characters. For instance, Let's see, we have LS all right, let's go with file five this time, OK? So chmog and I want all of the users to haveexecute permission on this file file five. Let's do an LSal file five and as you can see now the owner has read. Write. Access. Read. Write. The Execute group has read and executed, and the rest of the world has read and executed. Okay? And suppose if I want to take away the executepermission for everybody a minus x file five and another trick, if you just do an up arrow in Linux, then it'll bring back the command, so you don't have to retype the whole thing again. So I'm just going to do an up arrow and up arrow again and enter, as you can see. My execute permission is gone from all three different kinds of users, so you can play around with this and execute as well as write and read. Subtract them. Add them to see how the file is changing and also the Octal number 744-6447. Try all these different combinations to see what changes you see when you do it LS.

7. Input-output redirects

Hello, students, and welcome. In this short lecture, we're going to talk about input output redirection. This is also one of the competencies in your domain, the first domain of this test. So what that is actually is that when we run a command from the CLI in Linux, sometimes you want to send the output somewhere else or you want to manipulate the output. You don't want to see it the way it's coming because it's either not readable or you want to save it because you sent it or you want to send it over somewhere or because you want to get the output so you can archive it for later on. And here are some examples that the domain mentions. This is a redirect STD out to a file. The second one is a redirect and a pen. The third one is called a pipe. It gets used a lot, especially when we get into processes and how to manage processes. I think it's the third or fourth domain. We are going to talk about it even more. And this is a redirect error. The redirect standard error is used specifically in shell scripting. So you have a shell script that you have created and you can send the redirect to a file which only has content when your script has some error in it. So these are some additional definitions I wanted to highlight. redirects STD out to a file. This one redirects STD out and appends it to a file. Meaning not only does it get the output of the command you're about to run, but it can also append to it. And we'll talk about it in a minute when we actually run. And these are pipes, of course, in the redirect STD, or standard error. So currently we are logged into the user Larry, and right now he only has one file on it. So let's create some more files. File one. Sorry, touch file one. File two. File three. Okay, so now we've got four files all together in there. One is the script, which we're going to use later on, and the other three I just touched. So I want to do an LS. So far you have seen me using just LS forlisting, but this time instead of seeing it on the screen, I wanted to go to so you do ashift and then this greater sign, and this is the direct and I want to send it to file listing. So you see that it didn't display anything on the screen this time. Why? Because I have redirected it to a file listing and, as you can see, it has automatically the file listing wasn't even there before it created that file and put the listing in there. And to make sure that we actually did that, we'll do a more detailed look at a file. Okay, so let's do a cat instead. File listing and guess what? It's got everything that LS was supposed to show you on the screen. Instead of showing you on the screen, hit enter to put it in that file. Instead of using the same command, we're going to do the redirect in the pen. Let's see what happens in this one. So we do LS and instead of one greater design, we can have two file listings. Okay? So we still have the file listing file, but look at the content now file system, which has more than doubled. It had whatever it previously had and then appended it again because we are giving it two of these greater than signs. So we are saying that redirect and append as well. Now we're going to use the more or the pipe command. So LS, etc., etc. It is a very important directory where there are a lot of configuration files that reside there, and we want to see a few lines at a time, OK? because it might scroll off the screen. So we'll do a pipe and more and then enter. So as you can see, this goes all the way to the bottom and then there's more at the bottom. And when you have this, you can either hit Enter to see one line at a time, or if there are way too many lines and you want to see the next page, you can hit the space bar. There you go. And it keeps giving me page after page of what is listed under etc. Many files are listed under etc. Okay? And our last bit of this is a redirect standard error. So you probably saw my file. It's called "my script." I have intentionally created the script, if I'm mistaken it.The script is complex. We're not getting into scripting right now, but asking to remove a system file that doesn't exist. So on purpose we made this error and we'll see how the redirect takes care of it. So we're going to say this is how you run a script, linux my script, and also you can type the first couple of characters and hit the tab key and it'll complete your command for you. Unless there is more than one command in that subdirectory that is identical. Suppose if I have file one, file two, and file three,then if you type Fi and try to tap it,the system doesn't know which file you're talking about. File one, file two, and file three, right? In that case, you have to type the whole thing. But since the script, my script, is the only file with that name, I just typed in My and just tapped it and it finished the whole thing. Now I'm going to use the redirect and I'm going to send it to a file called "Error File." So the error file doesn't exist right now. It's going to send the error to this file, create that file, and send the error to that file. Okay, so it's showing you on the screen aswell that this is the error, error code 256. And let's see, this is our error file. And now I'm going to do more on the error file. And this is the actual error. It's telling you that you have a command in there to remove something, and that file or directory doesn't exist, so we can't remove it. So that's how the redirect works, and as with any other topic, it's fairly broad. There are a lot of ways you can use it, but as far as your exam is concerned, this should be sufficient. If you want to research it more, you can definitely do the man pages or search for it online and get more information. Or if you want me to elaborate further,please make sure to leave a comment and any other confusion about or something. I'll definitely explain it to you.

8. Hard and Soft links

Hello, In this lecture we're going to talk about links, hard links, and soft links in Linux. So in Linux, everything is a file, which meansfile is a file, and it treats directories as files as well, although we know the difference. But as far as Linux is concerned, it treats you pretty much all the commands that you can do on a file and you can do on a directory as well. And a file is fundamentally a link to an INOTE. An inode is a data structure that stores everything about a file apart from its name and actual content. It's a very important interview question. Also, what is an inode? So an inode is a piece of metadata or a structure that stores everything about a file except its name and content. So a hard link is a file that points to the same underlying INOTE as another file. If you delete one file, it will remove one link to the underlying I note, whereas a symbolic link or a soft link is a link to another file in the file system. Another important difference between the two types of links is that hard links can only work within the same file system, while symbolic links can go across different file systems. In order to create hard links in Linux, we will use the Ln utility. For example, the following command creates a hard link. We'll do LSL to see what we have in the filesystem right now, and I have a script called My Script. So what I'm going to do is I'll doAllen MyScript and I'll call it test. Okay? And I'll do LSDL, and you can see that it has been created right here as a link. Right now, looking at the output using the LS command, the new file is not indicated as a link,it is shown as a regular file. It means that the test file that we just created through the link is just another regular executable file that points to the same underlying INOTE as our original file, which was my script. Okay? So now we're going to deal with how to create a symbolic link. So in order to create a symbolic link, symbolic link will use the same Ln utility with the S switch. So, for example, what I have done is to remember that we had a script called My Script. I have moved it to slash TMP just to show you that it goes across file systems. So we're going to create a link here, but actually the link is created for the file which resides inflash timing and right now we are in Larry's homedirectory, so we're going to do S and then Tmpmyescript. Then you're going to call it "test two." Our link is created and here you can see the bottom of the screen showing an L. Remember the file types that we discussed file permissions in one of the previous lectures? So here. It tells you when you do an LS L that the bottom file is a link and it's linked. Test two is actually linked to a file called Slash Tenscript. So now you can access it anywhere within this system, within this server, and you can go if you want to CD it. You can CD to test two or you can CD to TMPmy script and you'll end up at the same point. And suppose you are done with the link, you don't need it anymore. Instead of it being just a stale linksomewhere, you can go ahead and remove it just like you would remove any other file. So remember, our link is test two, and I'm just going to use the remove command, which is RM test two. And when we do LS all, it's gone. Test two is gone now. So that link that we created, it doesn't show us there and it's gone.

9. Linux File System

Hello and welcome again. In this lecture we're going to talk about the Linux file system. So the top of the Linux file system is the theroot file system, which is just the flash and is the logical beginning of the Linux file system structure. Every single file path in Linux starts with root, in some way, and Slash contains your entire operating system. The next one is Slash Bin, which is where most of your binary files are stored, typically for the Linux terminal commands and core utilities such as CD, PWD, and MV. These are some of the commands we have already used in this course and some we are going to use in the future. But this is where they are kept. The next one is boots. This is where all the needed files for Linux to boot are kept. The next one is depth. This is where your physical devices are mounted, such as hard drives, USB drives, optical drives, and your CD Rom. You have already explored that. Typically, your system hard drive is mounted under SDA, whereas your USB thumb drive might be mounted under SDE. So it's different from different computers' having it differently, but it's always bounced in development. The next one is the etc, which is R etc. Also, this is one of the most important file systems because this is where the configuration files are stored. And for example, one of the most important files is the isetsy host file, which is where the host information, host name, and host IP address are located. Etsy password file: this is where all the users' passwords, including the root user, are located. So it's a very important file system. The next one is live, and this is where libraries are kept. You'll notice that many times wheninstalling Linux software packages, additional libraries are also automatically downloaded and they almost always start with Live something.These are basically the files needed for your programmes on Linux to work. You can think of this folder as somewhat equivalent to the programme file folder on Windows. The next one is media, or it could be CDROM as well, another place where external drives such as optical devices and USB drives can be mounted. The next one is MNT, and as you can guess, this is also for mount. It's a placeholder, it's an empty file. But what you do is suppose if you have to,if you want to use the network file system NFS,you would mount your file system or slash Mt. I mean, you can mount it anywhere you want,but it's usually typically mounted on MNT and then shared so that people who are on the same network can access that file system. The next one is opt or opt for optional software for a system that is not already managed by your distressed package manager. So the stuff that's not installed by default can be stored here and it depends. You can install it anywhere on your system. You can even install it under route if you want to and if you have the space. But good practise is to put it in opt if you have any additional optional software that you want to use. OK, the next one is the slash root, which is the home directory for the user root, so like if you remember, in our example we had flash home Larry, which was Larry's home directory, all the non rootusers will have the home directories inhomesomething their username butfor root is just flash. This is where root files are kept The next one is Fbin. It's similar to Bin except that it is dedicated to certain commands that can only be run by the root user or the superuser. The next one is TMP or temporary files. This is also something we have used in one of our modules before and this is where temporary files are stored and they are usually deleted upon shutdown, which saves you from having to manually delete them. They are not there permanently, like is required in Windows, so you only put stuff that you don't want to be stored permanently. The next three file systems are: the first one is User, which contains files and utilities that are shared between users. The next one is Slash Bar, This is also very important and you're going to be using it a lot This is where variable data is kept, usually system logs but can also include other types of data as well, so system logs are VAR log messages. or one of the files that you access a lot, If there is any kind of crash or anything like that, look at the VAR log messages The last one is the VAR log, followed by the frock or processes. This is where a lot of system information is represented as files. It is basically a way for the Linux kernel, the core of the operating system, to send and receive information from various processes running in the analytic environment.

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