While Windows may “dominate the game on home computers,” (Galov, 2021) the Linux operating system has over 95% of the market share for the world’s top 1 million web servers (Vaughan-Nichols, 2015). Linux is a popular operating system most often used in servers, and as the OS for Android phones. Like any OS, Linux has a command line interface, or CLI, that is usually used by system administrators. A CLI works by inputting commands and arguments. For Linux, the syntax of this input is command [arg1] [arg2].... This paper will go over some of the most important commands to know when using Linux. I will stay away from some of the more powerful commands, and will stick to beginner-friendly commands to get you more comfortable with Linux CLI.
Let’s begin by grouping and summarizing the commands I will explain in this paper. First, we have the get-out-of-jail-free commands. These include help, and man. These commands will assist users in figuring out how specific commands work, or just present a shortened list of commands that can be used at the present moment in the OS. Next is the traversal commands: pwd, cd. These commands are used to traverse your file directory. Finally, I will touch on file manipulation commands, including mkdir, rmdir, touch, vim/nano, and cat.
Let’s start with help. While help isn’t always available on every distribution of Linux, if you find yourself in a place with no idea what command to use, it is a good idea to try typing either help or ?. These commands on their own will sometimes give you a list of some basic commands you can use within that workspace. This is good if you are pentesting and you gain access to a device and have no idea what commands can be used. Additionally, sometimes using help [command] will give you a shortened description of how to use the command. The man command is arguably one of the most important commands, because it will reference the manual page for a given program or command. Whether you completely forget how to use a command, or want to figure out how to use a specific feature of that command, typing man [command] is always a good bet. Be sure to understand that man doesn’t have entries for every single command possible. If you install something from GitHub, it may not have a manual entry.
Once you know what commands you can use and how to use them, you may want to explore the system directory. This is where the following commands come in handy. First is pwd. This command stands for print working directory. It essentially just prints the full filename of your current working directory, or where you are in the filesystem. One thing to note is that everything is a file in Linux. Directories (the equivalent of folders in Windows OS) are files, just like how a text file is a file. That’s why the pwd command will print the full filename of where you are. If you ever find yourself lost in the filesystem, use this command to regain your bearings. After you know where you are, you can navigate to another file with cd. This stands for change directory. It allows you to navigate to another file, or directory, within the system. You can navigate with an explicit file, or implicitly. To navigate to an explicit file, you must type the file filename and location for that file, from the highest level all the way down to the actual file. This may look like /home/kali/Desktop/Important Documents. It is called explicit because you are explicitly referencing the directory you want to travel to, with no regard for your current location. Implicit file traversal uses your current location to find where you want to go. You can reference parent directories with <../> and the current directory with <./>. If you want to travel to a directory called “Dir” that is two parent directories above your current location, you can do so with cd ../../Dir.
Now, I expect that you have found the directory you wished to visit. How can you create a new directory to store files in? Of course, with the mkdir command, which stands for make directories. This command does exactly that, it makes a directory either in your current location, or with the given explicit/implicit location. It only creates the directory if one by that name and in that location does not already exist. You can also set the mode for the created directory, which affects who can read, write, and execute files within the directory. Further, you can remove directories with rmdir, however they must be empty. There are certain flags within the rm command that allows you to recursively delete a non-empty directory, but I will not be writing about that in this paper.
You can create empty files with the touch command. The official use of this command is to change file timestamps, but the average Linux user will use this to create empty files. To edit a file on a Linux command line, you will likely use either Vim, or Nano. These are two text editors that will come preinstalled on most versions of Linux. To run either, simply type Vim/nano [filename]. This will launch the editor and allow you to edit the provided file. If the file does not exist, these programs will open an empty file that will be saved as the provided filename upon a save command being used. Nano is considered a lot more beginner friendly, as Vim is known for having a pretty steep learning floor, however the skill ceiling for Vim is a lot higher, and becoming proficient in it’s keybindings will allow you to edit files incredibly fast.
The last command I will write about is cat. The official use is to concatenate files, and print to the command line, but like the touch command it is typically used to display the contents of various types of files. The syntax is cat [filename]. Running this command will display all the contents of the file. Be cautious when using this on non-text files or compressed/encrypted files. It may display characters that can confuse your shell, and then you may need to launch a new shell to get it to work properly again.
References
Galov, N. (2021, August 9). 111+ mind-boggling linux statistics and facts for 2021 - linux rocks! HostingTribunal. Retrieved January 22, 2022, from https://hostingtribunal.com/blog/linux-statistics/#gref
Vaughan-Nichols, S. (2015, October 15). Can the internet exist without linux? ZDNet. Retrieved January 22, 2022, from https://www.zdnet.com/article/can-the-internet-exist-without-linux/