Introduction to Linux

History of Unix

Unix is an operating system developed in 1969 at AT&T Bell Labs by Ken Thompson, Dennis Ritchie, and Brian Kernighan. Bell Labs had originally been developing a multitasking operating system called “Multics”, but the system became too large. To play a game called “Space Travel”, which had been developed as a side project on Multics, they built a smaller operating system on a dusty DEC PDP-7 computer sitting in a corner of the lab. That was the beginning of Unix.

Various UNIX Systems

Unix-like operating systems have been independently extended by companies and organizations, resulting in many types and versions such as System V (AT&T), BSD (University of California, Berkeley), Solaris (Sun Microsystems), HP-UX (HP), AIX (IBM), XENIX (Microsoft), UnixWare (Novell), FreeBSD, NetBSD, and OpenBSD. Several Unix-like OS standardization groups, including UNIX International, OSF, and X/Open, were also founded, merged, and reorganized. Today, The Open Group handles this standardization.

UNIX and Unix

The name “UNIX” is a trademark managed by The Open Group. Only operating systems that satisfy the “Single UNIX Specification”, an extension of the IEEE POSIX standard for Unix-like operating systems, may call themselves “UNIX”. However, not many systems satisfy this specification. Operating systems that do not meet it are usually referred to as “Unix-like operating systems” or written as “Unix” with mixed case rather than “UNIX”.

The Emergence of Linux

Linux is one of the Unix-like operating systems developed in November 1991 by Linus Torvalds of the University of Helsinki in Finland. Because it was released as open source and could run on PCs, it spread rapidly. Strictly speaking, Linux is the name of the OS kernel. When combined with basic software such as the C compiler (gcc) developed by the GNU Project, it is called GNU/Linux. There are also many “Linux distributions” based on GNU/Linux that add installers, package management systems, and other tools.

Operating System An interface that lets users make practical use of a computer. Examples: Windows, Mac, Linux, Unix

Linux Distributions

In the early days, many distributions appeared, including TurboLinux, Miracle Linux, Debian, and Red Hat Linux. Today, Debian-based and Red Hat-based distributions are the mainstream. Among Debian-based systems, Ubuntu is widely used and has also been adopted as an operating system for low-cost PCs. Red Hat Linux split into Red Hat Enterprise Linux (RHEL), which improves commercial stability, and Fedora, which experimentally integrates the latest technologies. CentOS, a free distribution compatible with RHEL, has also been widely used.

Debian-based distributions

Red Hat-based distributions

Slackware-based distributions

Other distributions

Where Unix/Linux Is Used

Early Unix-like operating systems were mainly used in engineering work, but today they are used for many purposes, including web servers, mail servers, routers and other network devices, televisions, and car navigation systems. Mac was also rebuilt on BSD Unix starting with Mac OS X. Google’s Android is also based on Linux. Microsoft Windows, which can be considered a major counterpart to Unix/Linux, is not called a Unix-like operating system, but it has incorporated some Unix-like technologies and ideas.

Characteristics of Linux

• It has permissions.
  • Linux systems use two types of permissions: ownership and access permissions.
• It supports a multi-user environment.
  • Linux systems support multi-user and multitasking environments, so many users can work at the same time.
• It distinguishes uppercase and lowercase letters.
  • Check whether the Caps Lock key is on or off.
• It does not use the Windows-style concept of file extensions.

TUI and GUI

• TUI (Text User Interface)
  • An environment where input and output are handled as text.
• GUI (Graphical User Interface)
  • A graphical environment.

Difference Between Directories and Folders

In a TUI environment, they are called directories. In a GUI environment, they are called folders.

Users

In UNIX, users are identified by user names such as devkuma or kimkc. User information is recorded in the /etc/passwd file.

Groups

A user can belong to multiple groups. For example, suppose two users, devkuma and kimkc, belong to a group called adm. Group information is recorded in the /etc/groups file.

Permissions

Nine characters such as rwxr-xr-x represent permissions.

Shell

The UNIX equivalent of the Windows command prompt, formerly called the MS-DOS prompt, is the shell. When commands are entered in text mode, the results are also displayed as text.

Prompt Definition

A prompt is a waiting state in which commands can be entered and executed.

Prompt Structure

Windows (MS-DOS)

  drive-name//path>

  C://windows>

Linux (Bash Shell)

[root@localhost ~]#
[devkuma@localhost ~]$

Environment Variables

Environment variables are special variables that programs can refer to when exchanging information with each other. They are similar to the PATH setting in Windows AUTOEXEC.BAT. CGI exchanges many pieces of information between the web server and CGI scripts through environment variables. The main environment variables are as follows.

To check the value of an environment variable, use the printenv command in the shell (or set if printenv is unavailable). In Perl, use $ENV{'environment variable name'}.

Differences Between UNIX and Windows

POSIX

When reading Linux or Unix-related materials, you will often see the word POSIX. POSIX stands for Portable Operating System Interface with Unix. It is a standard adopted by the International Organization for Standardization (ISO) to improve Unix compatibility and portability. In the early 1990s, several companies began independently developing their own Unix systems. Even though these were all Unix operating systems, each company’s unique customizations caused compatibility and portability to drop significantly. As a result, users had difficulty running and moving programs even between Unix systems, and system administration and usage methods differed by vendor, creating major inconvenience for users. For this reason, ISO adopted POSIX as a standard to improve Unix compatibility and portability. Therefore, most Unix versions released since the late 1990s have tried to satisfy the POSIX standard. The POSIX standard covers a very broad range.