| Use the deregisterNode command to unregister a node from an administrative agent so that you can use the node stand-alone, register the node with another administrative agent, or federate the node with the deployment manager. The Linux command line is a text interface to your computer. Often referred to as the shell, terminal, console, prompt or various other names, it can give the appearance of being complex and confusing to use. Yet the ability to copy and paste commands from a website, combined with the power and flexibility the command line offers, means that using it may be essential when trying to follow instructions online, including many on this very website! Show This tutorial will teach you a little of the history of the command line, then walk you through some practical exercises to become familiar with a few basic commands and concepts. We’ll assume no prior knowledge, but by the end we hope you’ll feel a bit more comfortable the next time you’re faced with some instructions that begin “Open a terminal”. Originally authored by peppertop. What you’ll learn
What you’ll need
Every Linux system includes a command line of one sort or another. This tutorial includes some specfic steps for Ubuntu 18.04 but most of the content should work regardless of your Linux distribution. During the formative years of the computer industry, one of the early operating systems was called Unix. It was designed to run as a multi-user system on mainframe computers, with users connecting to it remotely via individual terminals. These terminals were pretty basic by modern standards: just a keyboard and screen, with no power to run programs locally. Instead they would just send keystrokes to the server and display any data they received on the screen. There was no mouse, no fancy graphics, not even any choice of colour. Everything was sent as text, and received as text. Obviously, therefore, any programs that ran on the mainframe had to produce text as an output and accept text as an input. Compared with graphics, text is very light on resources. Even on machines from the 1970s, running hundreds of terminals across glacially slow network connections (by today’s standards), users were still able to interact with programs quickly and efficiently. The commands were also kept very terse to reduce the number of keystrokes needed, speeding up people’s use of the terminal even more. This speed and efficiency is one reason why this text interface is still widely used today. When logged into a Unix mainframe via a terminal users still had to manage the sort of file management tasks that you might now perform with a mouse and a couple of windows. Whether creating files, renaming them, putting them into subdirectories or moving them around on disk, users in the 70s could do everything entirely with a textual interface. Each of these tasks required its own program or command: one to change directories ( Linux is a sort-of-descendent of Unix. The core part of Linux is designed to behave similarly to a Unix system, such that most of the old shells and other text-based programs run on it quite happily. In theory you could even hook up one of those old 1970s terminals to a modern Linux box, and access the shell through that. But these days it’s far more common to use a software terminal: that same old Unix-style text interface, but running in a window alongside your graphical programs. Let’s see how you can do that yourself! On a Ubuntu 18.04 system you can find a launcher for the terminal by clicking on the Activities item at the top left of the screen, then typing the first few letters of “terminal”, “command”, “prompt” or “shell”. Yes, the developers have set up the launcher with all the most common synonyms, so you should have no problems finding it.  Other versions of Linux, or other flavours of Ubuntu, will usually have a terminal launcher located in the same place as your other application launchers. It might be hidden away in a submenu or you might have to search for it from within your launcher, but it’s likely to be there somewhere. If you can’t find a launcher, or if you just want a faster way to bring up the terminal, most Linux systems use the same default keyboard shortcut to start it: Ctrl-Alt-T. However you launch your terminal, you should end up with a rather dull looking window with an odd bit of text at the top, much like the image below. Depending on your Linux system the colours may not be the same, and the text will likely say something different, but the general layout of a window with a large (mostly empty) text area should be similar. Let’s run our first command. Click the mouse into the window to make sure that’s where your keystrokes will go, then type the following command, all in lower case, before pressing the Enter or Return key to run it. You should see a directory path printed out (probably something like There are a couple of basics to understand here, before we get into the detail of what the command actually did. First is that when you type a command it appears on the same line as the odd text. That text is there to tell you the computer is ready to accept a command, it’s the computer’s way of prompting you. In fact it’s usually referred to as the prompt, and you might sometimes see instructions that say “bring up a prompt”, “open a command prompt”, “at the bash prompt” or similar. They’re all just different ways of asking you to open a terminal to get to a shell. On the subject of synonyms, another way of looking at the prompt is to say that there’s a line in the terminal into which you type commands. A command line, if you will. Again, if you see mention of “command line”, including in the title of this very tutorial, it’s just another way of talking about a shell running in a terminal. The second thing to understand is that when you run a command any output it produces will usually be printed directly in the terminal, then you’ll be shown another prompt once it’s finished. Some commands can output a lot of text, others will operate silently and won’t output anything at all. Don’t be alarmed if you run a command and another prompt immediately appears, as that usually means the command succeeded. If you think back to the slow network connections of our 1970s terminals, those early programmers decided that if everything went okay they may as well save a few precious bytes of data transfer by not saying anything at all. The importance of case A sense of locationNow to the command itself. One important concept to understand is that the shell has a notion of a default location in which any file operations will take place. This is its working directory. If you try to create new files or directories, view existing files, or even delete them, the shell will assume you’re looking for them in the current working directory unless you take steps to specify otherwise. So it’s quite important to keep an idea of what directory the shell is “in” at any given time, after all, deleting files from the wrong directory could be disastrous. If you’re ever in any doubt, the You can change the working directory using the Note that the directory separator is a forward slash ("/"), not the backslash that you may be used to from Windows or DOS systems Now your working directory is “/”. If you’re coming from a Windows background you’re probably used to each drive having its own letter, with your main hard drive typically being “C:”. Unix-like systems don’t split up the drives like that. Instead they have a single unified file system, and individual drives can be attached (“mounted”) to whatever location in the file system makes most sense. The “/” directory, often referred to as the root directory, is the base of that unified file system. From there everything else branches out to form a tree of directories and subdirectories. Too many roots From the root directory, the following command will move you into the “home” directory (which is an immediate subdirectory of “/”): To go up to the parent directory, in this case back to “/”, use the special syntax of two dots ( Typing You can also use Notice that in the previous example we described a route to take through the directories. The path we used means “starting from the working directory, move to the parent / from that new location move to the parent again”. So if we wanted to go straight from our home directory to the “etc” directory (which is directly inside the root of the file system), we could use this approach: Relative and absolute pathsMost of the examples we’ve looked at so far use relative paths. That is, the place you end up at depends on your current working directory. Consider trying to But what if you’re in your home directory? You’ll see an error saying “No such file or directory” before you even get to run the last But we have seen two commands that are absolute. No matter what your current working directory is, they’ll have the same effect. The first is when you run It also gives us another way to get back to your home directory, and even to the folders within it. Suppose you want to go straight to your “Desktop” folder from anywhere on the disk (note the upper-case “D”). In the following command you’ll need to replace USERNAME with your own username, the There’s one other handy shortcut which works as an absolute path. As you’ve seen, using “/” at the start of your path means “starting from the root directory”. Using the tilde character ("~") at the start of your path similarly means “starting from my home directory”. Now that odd text in the prompt might make a bit of sense. Have you noticed it changing as you move around the file system? On a Ubuntu system it shows your username, your computer’s network name and the current working directory. But if you’re somewhere inside your home directory, it will use “~” as an abbreviation. Let’s wander around the file system a little, and keep an eye on the prompt as you do so: You must be bored with just moving around the file system by now, but a good understanding of absolute and relative paths will be invaluable as we move on to create some new folders and files! In this section we’re going to create some real files to work with. To avoid accidentally trampling over any of your real files, we’re going to start by creating a new directory, well away from your home folder, which will serve as a safer environment in which to experiment: Notice the use of an absolute path, to make sure that we create the tutorial directory inside /tmp. Without the forward slash at the start the In case you hadn’t guessed, There’s something a little different about that command. So far we’ve only seen commands that work on their own ( Back to our new directories. The command above will have created three new subdirectories inside our folder. Let’s take a look at them with the If you’ve followed the last few commands, your terminal should be looking something like this: Notice that This time you’ll see that only dir4 has been added to the list, because dir5 is inside it, and dir6 is inside that. Later we’ll install a useful tool to visualise the structure, but you’ve already got enough knowledge to confirm it: The “-p” that we used is called an option or a switch (in this case it means “create the parent directories, too”). Options are used to modify the way in which a command operates, allowing a single command to behave in a variety of different ways. Unfortunately, due to quirks of history and human nature, options can take different forms in different commands. You’ll often see them as single characters preceded by a hyphen (as in this case), or as longer words preceded by two hyphens. The single character form allows for multiple options to be combined, though not all commands will accept that. And to confuse matters further, some commands don’t clearly identify their options at all, whether or not something is an option is dictated purely by the order of the arguments! You don’t need to worry about all the possibilities, just know that options exist and they can take several different forms. For example the following all mean exactly the same thing: Now we know how to create multiple directories just by passing them as separare arguments to the You probably didn’t even need to type that one in to guess what would happen: two new folders, one called another and the other called folder. If you want to work with spaces in directory or file names, you need to escape them. Don’t worry, nobody’s breaking out of prison; escaping is a computing term that refers to using special codes to tell the computer to treat particular characters differently to normal. Enter the following commands to try out different ways to create folders with spaces in the name: Although the command line can be used to work with files and folders with spaces in their names, the need to escape them with quote marks or backslashes makes things a little more difficult. You can often tell a person who uses the command line a lot just from their file names: they’ll tend to stick to letters and numbers, and use underscores ("_") or hyphens ("-") instead of spaces. Creating files using redirectionOur demonstration folder is starting to look rather full of directories, but is somewhat lacking in files. Let’s remedy that by redirecting the output from a command so that, instead of being printed to the screen, it ends up in a new file. First, remind yourself what the Suppose we wanted to capture the output of that command as a text file that we can look at or manipulate further. All we need to do is to add the greater-than character (">") to the end of our command line, followed by the name of the file to write to: This time there’s nothing printed to the screen, because the output is being redirected to our file instead. If you just run Okay, so it’s not exactly what was displayed on the screen previously, but it contains all the same data, and it’s in a more useful format for further processing. Let’s look at another command, Yes, You should Where you want to pass multiple file names to a single command, there are some useful shortcuts that can save you a lot of typing if the files have similar names. A question mark ("?") can be used to indicate “any single character” within the file name. An asterisk ("*") can be used to indicate “zero or more characters”. These are sometimes referred to as “wildcard” characters. A couple of examples might help, the following commands all do the same thing: More escaping required If you look at the output of What do you think will happen if we run those two commands a second time? Will the computer complain, because the file already exists? Will it append the text to the file, so it contains two copies? Or will it replace it entirely? Give it a try to see what happens, but to avoid typing the commands again you can use the Up Arrow and Down Arrow keys to move back and forth through the history of commands you’ve used. Press the Up Arrow a couple of times to get to the first As you can see, the file looks the same. That’s not because it’s been left untouched, but because the shell clears out all the content of the file before it writes the output of your Repeat the first When viewing a file through A note about caseUnix systems are case-sensitive, that is, they consider “A.txt” and “a.txt” to be two different files. If you were to run the following lines you would end up with three files: Generally you should try to avoid creating files and folders whose name only varies by case. Not only will it help to avoid confusion, but it will also prevent problems when working with different operating systems. Windows, for example, is case-insensitive, so it would treat all three of the file names above as being a single file, potentially causing data loss or other problems. You might be tempted to just hit the Caps Lock key and use upper case for all your file names. But the vast majority of shell commands are lower case, so you would end up frequently having to turn it on and off as you type. Most seasoned command line users tend to stick primarily to lower case names for their files and directories so that they rarely have to worry about file name clashes, or which case to use for each letter in the name. Good naming practice Now that we’ve got a few files, let’s look at the sort of day-to-day tasks you might need to perform on them. In practice you’ll still most likely use a graphical program when you want to move, rename or delete one or two files, but knowing how to do this using the command line can be useful for bulk changes, or when the files are spread amongst different folders. Plus, you’ll learn a few more things about the command line along the way. Let’s begin by putting our combined.txt file into our dir1 directory, using the You can confirm that the job has been done by using Now suppose it turns out that file shouldn’t be in dir1 after all. Let’s move it back to the working directory. We could The With combined.txt now moved into dir2, what happens if we decide it’s in the wrong place again? Instead of dir2 it should have been put in dir6, which is the one that’s inside dir5, which is in dir4. With what we now know about paths, that’s no problem either: Notice how our Since we seem to be using (and moving) that file a lot, perhaps we should keep a copy of it in our working directory. Much as the Great! Now let’s create another copy of the file, in our working directory but with a different name. We can use the That’s good, but perhaps the choice of backup name could be better. Why not rename it so that it will always appear next to the original file in a sorted list. The traditional Unix command line handles a rename as though you’re moving the file from one name to another, so our old friend This also works on directories, giving us a way to sort out those difficult ones with spaces in the name that we created earlier. To avoid re-typing each command after the first, use the Up Arrow to pull up the previous command in the history. You can then edit the command before you run it by moving the cursor left and right with the arrow keys, and removing the character to the left with Backspace or the one the cursor is on with Delete. Finally, type the new character in place, and press Enter or Return to run the command once you’re finished. Make sure you change both appearances of the number in each of these lines. Deleting files and foldersWarning Now we know how to move, copy and rename files and directories. Given that these are just test files, however, perhaps we don’t really need three different copies of combined.txt after all. Let’s tidy up a bit, using the Perhaps we should remove some of those excess directories as well: What happened there? Well, it turns out that Well that’s a little better, but there’s still an error. If you run In this case, however, we do mean to. The addition of options to our A more common approach, when you’re really, really, really sure you want to delete a whole directory and anything within it, is to tell Remember: although Important Warning Today’s computers and phones have the sort of graphical and audio capabilities that our 70s terminal users couldn’t even begin to imagine. Yet still text prevails as a means to organise and categorise files. Whether it’s the file name itself, GPS coordintates embedded in photos you take on your phone, or the metadata stored in an audio file, text still plays a vital role in every aspect of computing. It’s fortunate for us that the Linux command line includes some powerful tools for manipulating text content, and ways to join those tools together to create something more capable still. Let’s start with a simple question. How many lines are there in your combined.txt file? The Similarly, if you wanted to know how many files and folders are in your home directory, and then tidy up after yourself, you could do this: That method works, but creating a temporary file to hold the output from Notice that there’s no temporary file created, and no file name needed. Pipes operate entirely in memory, and most Unix command line tools will expect to receive input from a pipe if you don’t specify a file for them to work on. Looking at the line above, you can see that it’s two commands, Note that the spaces around the pipe character aren’t important, we’ve used them for clarity, but the following command works just as well, this time for telling us how many items are in the /etc directory: Phew! That’s quite a few files. If we wanted to list them all it would clearly fill up more than a single screen. As we discovered earlier, when a command produces a lot of output, it’s better to use Going back to our own files, we know how to get the number of lines in combined.txt, but given that it was created by concatenating the same files multiple times, I wonder how many unique lines there are? Unix has a command, That line probably resulted in a count that’s pretty close to the total number of lines in the file, if not exactly the same. Surely that can’t be right? Lop off the last pipe to see the output of the command for a better idea of what’s happening. If your file is very long, you might want to pipe it through It appears that very few, if any, of our duplicate lines are being removed. To understand why, we need to look at the documentation for the Because this type of documentation is accessed via the The The question, then, is how to rearrange the lines in our file so that duplicate entries are on adjacent lines. If we were to sort the contents of the file alphabetically, that would do the trick. Unix offers a You should be able to see that the lines have been reordered, and it’s now suitable for piping straight into As you can see, the ability to pipe data from one command to another, building up long chains to manipulate your data, is a powerful tool, as well as reducing the need for temporary files, and saving you a lot of typing. For this reason you’ll see it used quite often in command lines. A long chain of commands might look intimidating at first, but remember that you can break even the longest chain down into individual commands (and look at their man pages) to get a better understanding of what it’s doing. Many manuals One good reason for learning some command line basics is that instructions online will often favour the use of shell commands over a graphical interface. Where those instructions require changes to your machine that go beyond modifying a few files in your home directory, you’ll inevitably be faced with commands that need to be run as the machine’s administrator (or superuser in Unix parlance). Before you start running arbitrary commands you find in some dark corner of the internet, it’s worth understanding the implications of running as an administrator, and how to spot those instructions that require it, so you can better gauge whether they’re safe to run or not. The superuser is, as the name suggests, a user with super powers. In older systems it was a real user, with a real username (almost always “root”) that you could log in as if you had the password. As for those super powers: root can modify or delete any file in any directory on the system, regardless of who owns them; root can rewrite firewall rules or start network services that could potentially open the machine up to an attack; root can shutdown the machine even if other people are still using it. In short, root can do just about anything, skipping easily round the safeguards that are usually put in place to stop users from overstepping their bounds. Of course a person logged in as root is just as capable of making mistakes as anyone else. The annals of computing history are filled with tales of a mistyped command deleting the entire file system or killing a vital server. Then there’s the possibility of a malicious attack: if a user is logged in as root and leaves their desk then it’s not too tricky for a disgruntled colleague to hop on their machine and wreak havoc. Despite that, human nature being what it is, many administrators over the years have been guilty of using root as their main, or only, account. Don’t use the root account In an effort to reduce these problems many Linux distributions started to encourage the use of the By minimising the amount of time spent logged in as root, the use of Don’t use su When using Better to disable the root account entirely and then, instead of allowing long-lived terminal sessions with dangerous powers, require the user to specifically request superuser rights on a per-command basis. The key to this approach is a command called On a Ubuntu system the first user created when the system is installed is considered to be the superuser. When adding a new user there is an option to create them as an administrator, in which case they will also be able to run superuser commands with Assuming you’re on a Linux system that uses If you enter your password when prompted you should see the contents of the Be careful with sudo For instructions targeting Ubuntu, a common appearance of Installing new software Increasingly, Ubuntu is making use of “snaps”, a new package format which offers some security improvements by more closely confining programs to stop them accessing parts of the system they don’t need to. But some options can reduce the security level so, if you’re asked to run Let’s install a new command line program from the standard Ubuntu repositories to illustrate this use of Once you’ve provided your password the Going back to the command that actually installed the new program (
You can put Beware of sudo su In this section you’ve learnt about the dangers of the root account, and how modern Linux systems like Ubuntu try to reduce the risk of danger by using Before we conclude this tutorial it’s worth mentioning hidden files (and folders). These are commonly used on Linux systems to store settings and configuration data, and are typically hidden simply so that they don’t clutter the view of your own files. There’s nothing special about a hidden file or folder, other than it’s name: simply starting a name with a dot (".") is enough to make it disappear. You can still work with the hidden file by making sure you include the dot when you specify its file name: If you run Notice that the shortcuts we used earlier, As for our recently installed Switch back to your home directory ( You shouldn’t usually need to deal with hidden files, but occasionally instructions might require you to Cleaning upWe’ve reached the end of this tutorial, and you should be back in your home directory now (use As a last step, let’s close the terminal. You can just close the window, but it’s better practice to log out of the shell. You can either use the This tutorial has only been a brief introduction to the Linux command line. We’ve looked at a few common commands for moving around the file system and manipulating files, but no tutorial could hope to provide a comprehensive guide to every available command. What’s more important is that you’ve learnt the key aspects of working with the shell. You’ve been introduced to some widely used terminology (and synonyms) that you might come across online, and have gained an insight into some of the key parts of a typical shell command. You’ve learnt about absolute and relative paths, arguments, options, man pages, With these key concepts you should be able to make more sense of any command line instructions you come across. Even if you don’t understand every single command, you should at least have an idea of where one command stops and the next begins. You should more easily be able to tell what files they’re manipulating, or what other switches and parameters are being used. With reference to the man pages you might even be able to glean exactly what the command is doing, or at least get a general idea. There’s little we’ve covered here that is likely to make you abandon your graphical file manager in favour of a prompt, but file manipulation wasn’t really the main goal. If, however, you’re intrigued by the ability to affect files in disparate parts of your hard drive with just a few keypresses, there’s still a lot more for you to learn. Further readingThere are many online tutorials and commercially published books about the command line, but if you do want to go deeper into the subject a good starting point might be the following book:
The reason for recommending this book in particular is that it has been released under a Creative Commons licence, and is available to download free of charge as a PDF file, making it ideal for the beginner who isn’t sure just how much they want to commit to the command line. It’s also available as a printed volume, should you find yourself caught by the command line bug and wanting a paper reference. What is command line utilities in Linux?The Linux command is a utility of the Linux operating system. All basic and advanced tasks can be done by executing commands. The commands are executed on the Linux terminal. The terminal is a command-line interface to interact with the system, which is similar to the command prompt in the Windows OS.
What is a command line utility?Command line utilities are tools that you can run on the command line of a computer. We most often see these on Linux and MacOS computers using the 'bash' shell, but Windows users have options like CMD, git-bash and powershell too. These tools allow you to instruct the computer to do things using text alone.
What is CLI and why it is used?CLI is a command line program that accepts text input to execute operating system functions. In the 1960s, using only computer terminals, this was the only way to interact with computers. In the 1970s an 1980s, command line input was commonly used by Unix systems and PC systems like MS-DOS and Apple DOS.
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