Command Line Interface

A shell is a program that accepts commands and instructs the operating system to execute those commands. When you connect to a Linux system over the network, for example, a shell is started and acts as your interface to the system. The shell in this particular case is a command line interpreter. The command-line interface is often referred to by its abbreviation, CLI.

When you connect to a Linux system directly via an attached keyboard and display, you will either be presented with a textual interface or a graphical interface, depending on how that system is configured. In the case of a textual interface, you will have a very similar experience as if you had connected to that system over the network. When you log in, a command line shell is started and you are presented with a prompt.

If you connect to a system that is in graphical mode, you will be interacting with a graphical user interface (GUI). In order to access the command line while logged into a GUI, you will need to start a terminal emulator application. Common terminal emulators include xterm, GNOME Terminal, Konsole, rxvt, and Eterm. The one you choose depends on personal preference and availability, but they all provide the same basic functionality­—access to the command line.

The following demonstrates logging into an Ubuntu Linux server at the command line.

The line jason@linuxsvr:~$ is the command prompt. The default prompt varies from distribution to distribution and shell to shell. There are a variety of shells with the most common and popular one being Bash. All users can customize their shell prompt to their liking. The information provided in this shell prompt includes the username, the server name, and the current directory.

The tilde represents the home directory of the current user which is /home/jason in this example. You can also specify a username after the tilde, in which case it will expand to the home directory of that user. For example, ~john expands to /home/john. No matter where the user's home directory is, ~username will be translated to that directory. In the case of an application user such as www-data, ~www-data expands to /var/www.

The following are examples of various shell prompts.

Shell prompts are not limited to a single line. The following example shell prompts span multiple lines.

In the remainder of examples in this book, the shell prompt will be abbreviated to just the dollar sign, unless displaying the entire prompt provides additional clarity.

Basic Commands

In Linux, commands are case-sensitive and are typically lowercase. Note that items surrounded by square brackets are optional. Let's start with two commands that will allow you to navigate around the system at the command line. They are pwd and cd.

pwd   The pwd command displays the present working directory to your screen. This command allows you to keep track of where you are in the file system hierarchy.

cd [directory]   The cd command changes the current directory to the supplied directory. If cd is executed without specifying a directory, it changes the current directory to your home directory. This is how you navigate around the system.

The following is an example of how the pwd and cd commands can be used. Remember that case matters.

The most common way to see the contents of a directory is to use the ls command. To view the contents of a file, use the cat command.

ls   The ls command lists directory contents. Additionally, ls can be used to display information about files and directories including permissions, size, and type.

cat [file]   The cat command concatenates, or displays, files.

To end your command line session, type exit, logout, or Ctrl-d.

exit, logout, or Ctrl-d   Exits the shell or your current session.

Command Line Help

The Linux operating system provides built-in documentation. To access these online manuals, also called man pages, use the man command. Man pages are great for looking up the available options for a given command or even finding a command that will help you accomplish the task at hand.

man [command]   The man command displays the online manual for a given command.

Once you have executed the man command, you can navigate through the man page with the arrow keys, as well as the Page Up and Page Down keys. You can also use Enter to move down a line, the Spacebar to move down a page, g to move to the beginning, and capital G to move to the end of the man page. To quit, type q. To learn about even more commands available while viewing man pages, type h for help.

1. Navigating Man Pages

To search the man pages, supply a keyword to the -k option of the man command. If you are looking for a command that will reboot the system, you could search for "reboot."  Once you have a list of man pages that contain that keyword, you can read the documentation for the most promising ones.

Some commands will print a help message when -h or --help is supplied as an argument. Even the man command follows this convention.

Given what you know about the Linux directory structure and the documentation that comes with the Linux operating system, you can start exploring commands on your own. For example, list the directory contents of /bin and /usr/bin. Pick out some commands that grab your attention and use the man command to find out what each one of them does.

The output of the preceding ls commands was truncated. In reality, you will likely find dozens of commands in /bin and hundreds in /usr/bin.

Before we move on, I want to share one last basic command that you might find useful. It is the clear command. If you want to a fresh screen to work with, issue the clear command to clear the contents of your screen.