The Shell Game, Part 1

Kevin O'BrienWashtenaw Linux Users Group

http://www.lugwash.org

What is a Shell?

“A shell is a program that provides the traditional, text-only user interface for Linux and other Unix-like operating systems. Its primary function is to read commands that are typed into a console (i.e., an all-text display mode) or terminal window (an all-text window) in a GUI (graphical user interface) and then execute (i.e., run) them.”

--http://www.linfo.org/shell.html

Why “Shell”?

● At its most fundamental, the OS consists of a kernel, which is the heart of the OS

● The kernel keeps track of different processes running, gives them CPU time as needed, handles interaction with peripheral devices, etc.

● The Shell surrounds all of this, and creates a way to interact with it.

So what is a shell?

● It is the basic way for a person to interact with a computer

● This is done by typing in commands● If you open a terminal window, you are using

the shell● If you cannot boot into a GUI interface, you

can usually boot into a shell and fix things

Servers

● Servers commonly do not even bother to install a GUI. Everything is done via the shell.

● This saves on resources, since you do not have to devote RAM and CPU cycles to the GUI, and can instead devote them to serving files, or whatever.

Headless Servers

● Sometimes a server is set up without a monitor even attached. This is called “Headless”.

● Server administration is done via a terminal connection of some kind, commonly via ssh

ssh

● Ssh stands for Secure Shell● This is the replacement for telnet, which was

used in the old days● Telnet is not secure, that is why it was

replaced

Server Administration

● Pretty much all server administration in Linux or indeed any Unix-like operating system is done via the shell

● This is not just because the shell saves on resources. It is also the case that the shell is more efficient, and more powerful, than any GUI.

Tip for new users

● Buy a box of 3X5 index cards, and keep the box by your computer

● Every time you find a useful command, write it down. You may want to use this command again at a later time.

● Sometimes you will want to group related commands

Example Card – Fixing an Install

● Here is a set of commands I put on one card to help me when I have a problem with an install on my Kubuntu machine

– Sudo dpkg --configure -a

– Sudo apt-get -f install

– Sudo fuser -vik /var/lib/dpkg/lock

Why you need to know this

● Jaunty upgrade → No video!● No way to launch a GUI● But I could go into failsafe mode, and get a

shell prompt● With the help of a command I found online, I

got my video up and running again

Coming attractions

● This is the first in a series of short presentations designed for the newcomer to Linux

● In future installments we will look at choices among shells, techniques for the most common shell (bash), and some tips and tricks to help move you to being an expert