Learn Linux 01: What Is Shell?

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Contents

Introduction

When we speak of the command line, we are really referring to the shell. The shell is a program that takes keyboard commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU Project called bash. The name is an acronym for bourne-again shell, a reference to the fact that bash is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.

Terminal Emulators

When using a graphical user interface (GUI), we need another program called a terminal emulator to interact with the shell. If we look through our desktop menus, we will probably find one. KDE uses konsole, and GNOME uses gnome-terminal, though it’s likely called simply Terminal on your menu. A number of other terminal emulators are available for Linux, but they all basically do the same thing: give us access to the shell.

Some Simple Commands

Let’s get started. Launch the terminal emulator. Once it comes up, we should see something like this.

[user@linux ~]$

This is called a shell prompt, and it will appear whenever the shell is ready to accept input. While it might vary in appearance somewhat depending on the distribution, it will typically include your username@machinename, followed by the current working directory (more about that in a little bit) and a dollar sign.

If the last character of the prompt is a hash mark (#) rather than a dollar sign, the terminal session has superuser privileges. This means either we are logged in as the root user or we selected a terminal emulator that provides superuser (administrative) privileges.

So let’s try some typing. Type some text at the prompt and press enter.

[user@linux ~]$ blabla
bash: blabla: command not found

Because this command makes no sense, the shell tells us so. Now that we have learned to enter text in the terminal emulator, let’s try a few simple commands. Let’s begin with the date command, which displays the current time and date.

[user@linux ~]$ date
Sun Jan 1 05:09:00 EST 2017

To display the current amount of the free disk space, enter df.

[user@linux ~]$ df
Filesystem      1K-blocks    Used Available Use% Mounted on
/dev/sda2        12116452 5012392  49484528  34% /
/dev/sda5         4070524 6545424   4070524  44% /home
/dev/sda1            5120   13370      5120  12% /boot
tmpfs              814104       0    814100   0% /dev/shm

Likewise, to display the current amount of free memory, enter free.

[user@linux ~]$ free
               total        used        free      shared  buff/cache   available
Mem:         8141048      308324     7077816        1204      754908     7569828
Swap:        4194300           0     4194300

To end a terminal session we can close the terminal emulator window, enter the exit command at the shell prompt, or press ctrl-D.

[user@linux ~]$ exit

The Console Behind The Scene

In Linux even if we have no terminal emulator running, several terminal sessions continue to run behind the graphical desktop. We can access these sessions, called virtual consoles, by pressing ctrl-alt-F1 through ctrl-alt-F6 on most Linux distributions. When a session is accessed, it presents a login prompt into which we can enter our username and password. To switch from one virtual console to another, press alt-F1 through alt-F6. On most systems, we can return to the graphical desktop by pressing alt-F7.

Summary

In this chapter we began our adventure into the Linux command line world. We have learned what the shell is along with the terminal emulator. We also learned how to start and end a terminal session and how to perform simple commands.