by Gisle Hannemyr

This chapter introduces the Unix family of operating systems and defines some of the basic components, such as users, the kernel, the shell, programs, processes, files, and the terminal. It also introduces the conventions and notations I use in this ebook.

Table of contents

The Unix familiy

Unix is a family of operating systems that share a number of similar characteristics, commands and features. There is also a UNIX™ trade mark (written with all capital letters), that is used to brand versions if the Unix family compliant with a technical specification known as SUS (Single UNIX Specification). This ebook is about using operating systems belonging to the Unix family, not about the UNIX™ specification.

When I talk about “Unix” in this ebook, I mean every current member of the Unix family, not only those that are registered as SUS compliant.

Unix first surfaced around 1969 at AT&T's Bell Labs in New Jersey, USA. Since then, Unix has evolved into a very rich computing environment suitable for many different tasks.

tipSome Unix systems come with a modern graphical user interface (GUI) similar to Microsoft Windows. A GUI is generally considered an easy to use environment, where programs ready to run presents themselves on the computer desktop and is started with the click of the mouse. However, for serious systems work, the command line interface is usually more effective than any GUI. There are also a number of Unix tools that, for a variety of reasons, does not come with a graphical interface. And there are situations when there is no GUI available, for example, in a ssh session. This ebook is all about the command line interface. If you are looking for a place to learn about Unix GUIs, this isn't it.

Characteristics shared by all members of the Unix family is that they are robust, stable, multi-user, and multi-tasking. Today, Unix is used for servers, desktops, laptops, routers, embedded systems, tablet computers, smart-phones and other mobile devices, such as digital cameras. When you know how to use the command line interface of any one of the members of the Unix family, you will also know most of what you need to use every other variant of Unix. There are difference between the family members, but they only surface for the more specialized tasks, such as system administration and system development.

Family members

Several different branches of the Unix family of operating system exist. The most popular variants are Gnu/Linux, FreeBSD and System V.

Gnu/Linux and FreeBSD are free software. This means that the software can be used, studied, distributed and modified without restrictions. The variants based upon System V are usually closed source software. This means that use requires a license from the copyright holder that imposes some restrictions on the user.

For each major variant there exists sub-branches. For example, Debian and Fedora are both based on Gnu/Linux, MacOS and Darwin are based upon FreeBSD, AIX and HP-UX are based upon System V.

Within the Gnu/Linux there are two major branches that most distributions are based upon. These are:

On distributions based upon Gnu/Linux, the following command will print all release information:

$ lsb_release -a

You may also try these commands:

$ cat /etc/redhat-release # Red hat
$ cat /etc/os-release     # Ubuntu

Users in Unix

Unix is a multi-user operating system. Traditionally, it was used mainly used to run departmental computers with dozens, and sometimes hundreds, of different users.

With multiple users comes the need to distinguish one user from another, and to make sure they do not interfere with each other's work. This means that in Unix, users are identified by a specific user name, and they need to authenticate themselves to the system as part of logging in. There are also certain rules about what a user is allowed to do.

In addition to ordinary users, there is a superuser that traditionally have the user name “root” (but there is no rule that says the superuser must be named “root”).

The root user can do many things an ordinary user cannot, such as killing any process, and manipulate any file, including changing file ownership. The root user is also the only user allowed to bind to a network port numbered below 1024.

In this day and age, Unix is also used on single user laptop machines, owned and used by people who know very little about Unix. The rule, when a Unix machine is set up, is that the first user that is created becomes the all-powerful superuser. Some individuals that set up Unix on single user machines continue to use this account as their normal user account.

It is never a good idea to use the superuser account as a normal user account. Even super-experienced system administrators don't do this. The root account is really powerful, and Unix is not forgiving. Even a simple mistake, such as typing:

# rm -rf foo *

instead of:

# rm -rf foo*

may wipe your entire system.

Also, running malware as root will totally compromise the system.

So if you've set up Unix (i.e. some variant of Gnu/Linux) on your own computer, make sure you create a normal user account and use it for your day to day work. Then learn how to use the the sudo utility to switch to temporarily become the superuser, and only use it when necessary.

In this ebook, it is assumed that you're using Unix as an ordinary user, not as root or the superuser. If you want to learn how to use Unix as root, check out a book about Unix system administration.

Operating system components

When referring to an “operating system” in this ebook, I mean a kernel, a shell and a fairly standard suite of programs.

The kernel

The kernel performs all sort of important low level tasks of the operatings system, such as allocating resources (e.g. CPU time and memory) to programs, manages the file system, and handles communications in response to system calls.

The Unix kernel will not be discussed much in this ebook (to learn about the kernel, you need to pick up a Unix book with the word “advanced” in its title). But even beginners should know about the kernel and roughly how it fits into the big scheme.

The shell

The shell is the interface between the user and the kernel. When a user logs in, the log-in program checks the user-name and password, and then starts another program called the shell. The shell is a so-called command-line interface (CLI). It interpretes the commands the user types on the command line, and arranges for them to be executed (carried out).

The shell may execute the command it itself (in the case of a command that is built into the shell), or call upon the kernel and/or some other program to do so.

Interacting with the shell is the main topic of this book.

Standard programs

There are some basic programs you'll be sure to find on every member of the Unix family, such as programs to list, copy, rename and delete files, show help, or manage processes.

In this ebook I shall explain how to use the most useful of these standard programs. But don't expect to be told about every standard program that is part of the Unix environment. This ebook is just to get you started. When you've finished, you will have learnt how to use Unix to learn about commands and other parts of the Unix environment.

Processes and files

Everything in Unix is either a process or a file.

When a program is started on a Unix system, it becomes known as a “process” on the system. Every process is assigned a unique number called its “process id” (PID), and it can be manipulated by the user that owns it (and the administrator) by referring to its PID. We'll get back to processes later, but for know, you just need to know that a process with its associated PID is one of the basic concepts of Unix.

A file is a collection of data. They are created by users using editors, word processors, compilers etc., or imported to the system from the network or some recording device.

Examples of files:

tipIn Unix, directories are also files, and can be manipulated with the same tools and commands as is used to manipulate other files.

The Unix file system

The Unix file-system is arranged in a hierarchical structure, like an inverted tree. The top of the hierarchy is called root (written as a slash – /).

A special place in the file system is your home directory. If your Unix user-name is “john” your home directory will also be named john. On most Unix systems, your home directory will be sub-directory of a directory named home, but other arrangements exists.

file system tree

The illustration above shows (parts of) a typical Unix file system. The home directory of user “john” is marked with bright yellow.

Files in the Unix files system is uniquely identified by their absolute path from the root (/). The absolute path of the file report.odt /home/john/work/report.odt. This absolute path is shown in yellow.

The terminal emulator

To access the web server using the command-line interface (CLI) from a personal computer you use a terminal emulator.

INF3272/5272To is a good introduction to the terminal emulators (and file copy programs) that are available for students at the University of Oslo on the page Fjerninnlogging on the UiO web. This information is more specific about the options available at the University of Oslo than the more generic information about terminal emulators in this section.

If your personal computer is running a version of Unix (e.g. Gnu/Linux or macOS) a terminal emulator is already installed. If you're on a system without a GUI, the terminal is probably the only “window” on the screen. If your system has a GUI such as Gnome installed, the Terminal is found under Applications » Accessories.


For CLI access on MS Windows, use Starnet's X-Win32 (commercial) or Simon Tatham's PuTTY (free software).

With some terminal emulators, you will need to configure it before you can start using it. The screen shot below is the PuTTY configuration screen. Make sure you set the connection type to “SSH”- The only other thing you need to fill in is the host name or IP-address to connect to.

Configure PuTTY to connect over ssh..

If your computer is in a secure location (e.g. at home), you may store your login credentials in X-Win32 by means of X-Config:

Using X-config do store login credentials.

Alternatively, you may configure X-Win32 to use key based authentication.

Some terminal emulators work without configuration. If you have window with the CLI prompt ($) will be there, telling you that the Unix shell is waiting for your command.


Provided your personal computer has a program named ssh installed. you can go ahead a connect to the web server with the following CLI-command (replace “username” with your own username on the web server and “” with the name of the web server).

$ ssh

The first time you connect to an unknown remote host, you may get a rather intimidating message and you will be asked if you want to continue connecting.

$ ssh
The authenticity of host ( can't be established.
RSA key fingerprint is 53:b4:ad:c8:51:17:99:4b:c9:08:ac:c1:b6:05:71:9b.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added (RSA) to the list of known hosts.

Getting the message above the first time you use ssh to access a remote host is normal, and is no cause for alarm.

INF3272/5272To access the web host set up for your use in INF3272 or INF5272, you connect to the host using ssh, using your UiO userid and password as credentials. An account has already been set up for you on your group's web host with exactly those credentials. The first time you connect, a home directory belonging to you will be automatically created. After clogging in, you should be logged in on your group web host, and placed in your newly created home directory. However, please note that the firewall on the machines used in INF3272 and INF5272 will not accept incoming connections on port 22 (ssh) unless you're connecting from the UiO network. If you try to connect from outside (e.g. from home) you need to use the UiO VPN.

INF3272/INF5272If the server you want to connect to is the webserver provided for use in the course INF3272/INF5272 at the University of Oslo, the username is your UiO-username (the username that comes before “@” in your e-mail address), and the password is your UiO-password. It is not the username and password you use to login on (unless these are the same).

The first time you connect to an unknown remote host, you'll get this rather intimidating text and you will be asked if you're sure about connecting. You get this question because ssh provides host validation. To put it simply, ssh will check to make sure that you are connecting to the host that you think you are connecting to. If someone later poisons your DNS cache to trick you into logging into their machine instead so that they can sniff your session, you will be warned that RSA key fingerprint has changed.

Getting the message above the first time you use ssh to access a remote host is normal, and is no cause for alarm.

If you respond “yes” to this question, ssh will connect to the remote system and you'll be prompted to type in your password on the remote host. Since the session is encrypted, it is safe to type the password. If the password is accepted, you will be logged in on the remote host and get the standard login message (frequently a reminder of your last login. You then will get default prompt of the remote system – in this example represented with a dollar sign.

Names for commands and files

Notice that Unix is case-sensitive. Typing “RM” is not the same as typing ”rm”. All standard commands are written with all lower case letters. The same applies to file-names, so “myfile.txt”, “MyFile.txt”, and “MYFILE.TXT” are three different files.

noteBeware if copying files from Unix to a Microsoft Windows machine. Microsoft Windows does not make this distinction, so by copying files to Microsoft Windows, you may end up overwriting files you did not intend to overwrite.


In this ebook, I shall use the following conventions:


$ ls directory

This means: At the Unix prompt “$”, type “ls” followed by a space followed by the name of some directory. Then press the [Enter] key. In this case, you must pick the name of the directory.

Usually, examples of interaction with the shell, as well as sample scripts are separated from the running text in a box. This is how a short shell script will appear:

echo "Hi, $USER!"
echo "The date and time now are: "

Final word