Linux Application Development For The Enterprise (Charles River Media Programming)

Today’s computers are not limited to Enterprise-level servers alone; a desktop or a laptop computer has almost become an essential piece of equipment for a normal household, very similar to a TV or a VCR. Therefore, the operating systems should be as user-friendly as possible. In fact, the operating system running on a home-based desktop computer should provide more user-friendly interface than a more robust architecture. The majority of home-based users are giving priority to more GUI-based fancy features and are not caring even if the system hangs and they have to reboot the computer several times a day. Hence, for an operating system such as Linux, which is built on a UNIX-based robust architecture, it is necessary to add some fancy features so that the common user community can accept it on their home-based desktops and laptops. For this reason, the Linux developer community has put tremendous efforts into designing state-of-the-art desktop environments, as discussed in this section.

The Desktop

Similar to the Microsoft Windows operating system, the Linux operating system also provides two ways to interact with the user—the graphical desktop and the command prompt. Because Linux was developed on the concepts of UNIX, the command line interaction with the user is very strong. Everything that you can do in the graphical desktop can also be done at the command prompt. In fact, the command line mode forms the core of Linux, over which the graphical desktop is built using the X-Windows architecture. There are different types of desktop systems developed by different organizations. However, KDE and GNOME desktops have attained a maturity level that can be compared with any commercial operating system desktops such as Microsoft Windows and MacOS. They are developed and maintained by open source developer groups. KDE stands for K Desktop Environment developed by the group www.kde.org. GNOME desktop is developed by the GNOME developer group www.gnome.org. Most of the prominent Linux distributions come with both the KDE and GNOME desktop environments. At the time of writing this book, KDE 3.1 and GNOME 2.2 are the most recent releases. These organizations are continuously working on improving the desktops and providing a set of software tools that work with their respective desktop libraries. You can build powerful GUI-based applications for these desktops, in a way similar to the traditional Windows applications. In later chapters, we will take a closer look at the tools available on the market that let us build GUI applications for the Linux platform.

Starting from Release 8.0, Red Hat provides a very similar look and feel to the desktop with their Red Hat Blue Curve desktop theme, whether you are using the GNOME or KDE desktop environment. Figure 2.5 displays a typical Red Hat Linux 8.0 or 9.0 desktop.

Figure 2.5: Typical Red Hat Linux 8.0/9.0 desktop.

The different items displayed in the figure are annotated to describe what they represent. For example, the red hat icon on the bottom panel is the starting point to launch an application and provides a start menu. Each time you log in to your desktop, you can choose either KDE or GNOME or any other desktop. You can set one of the desktops as default in the login screen. To exit one desktop and start another, just log out of the desktop session and re-log in to the desired desktop in the login screen.

File Managers and Browsers

A file manager is an application that displays a list of directories and their contents in a scrollable graphical window. The KDE desktop comes with the Konqueror file manager, and the GNOME desktop comes with the Nautilus file manager. For convenience, we will discuss the Konqueror here; you are advised to explore the Nautilus file manager on your own. Both file managers provide very similar features.

The Konqueror is an open source file manager and Web browser. Functionally, it resembles the Windows Explorer of the Microsoft Windows operating system Using Konqueror, we can see the directories and their contents in the Linux file systems. It also acts as a universal viewer; you can view the contents of the many file types without opening the file with its associated application. With the directory and file listings, you can see their properties such as size, permissions, last modified timestamp, and so on. You can see the contents in different view modes, including Icon View, MultiColumn View, Tree View, Detailed List View, and Text View. This can be done by choosing the View ® View Mode from the Konqueror menu and then selecting the appropriate view mode. Figure 2.6 displays the Konqueror file manager window in a Red Hat Linux 8.0 /9.0 desktop. Figure 2.7 illustrates how to choose the different view modes to view the contents of the file manager. However, the View Mode option is not available in the View menu if you are browsing a Web page.

As a browser, Konqueror is HTML 4.0 compliant, supports ECMA Script/JavaScript, Java applets, Cascading Style Sheets (CSS), and Secured Socket Layer (SSL) protocol. You can configure Konqueror browser settings to suit your needs. The configuration screen is displayed by choosing Settings ® Configure Konqueror… in the menu. In the displayed window, you can set your choices for settings such as appearance and HTML/Java/JavaScript/PlugIns-related choices, cookie policy and management, style sheet settings, and so on. When the Konqueror is used as the file manager, you can perform drag-and-drop operations on items from the Konqueror window to another Konqueror window, or to another application, or to the desktop. When it is used as a browser, you can drag images and HTML links and drop them to the target objects discussed earlier. You can invoke the Find Files dialog from Konqueror menu, which displays the dialog embedded within the Konqueror window. Figure 2.9 displays the Konqueror window in a SuSE Linux 8.1/8.2 desktop, and Figure 2.10 shows the embedded Find Files dialog.

As mentioned earlier, the command line is most commonly used by many UNIX/Linux veterans due to the associated ease of use and performance. A shell is an environment with which the user interacts by passing shell-specific commands, which are interpreted and executed by the associated shell program. As the name suggests, a shell is a program that hides the critical parts of the kernel and exposes the kernel’s functionality to the user through a set of user-friendly commands. Typically, shells are capable of executing (or spawning running processes of) any executable program or a shell script. There are several shells that have become popular in the UNIX environment over a period of time. Because Linux is architecturally similar to UNIX, all the better-known UNIX shells have been ported to Linux. Examples of popular Linux shells are Borne shell (sh), Korn shell (ksh), C-shell (csh), Borne-again shell (bash), and so on. There are similarities and differences between the various shell environments, which we will discuss in more detail in Chapter 4 Shell Scripting Explorer. For now, what is required is a shell prompt to execute shell commands. Every user defined in the system may have one of the shells as default, so that the user will be able to start interacting with the operating system/kernel through the shell after logging into the system. This default shell is set in the user’s profile file when the user is created and can be changed by the administrator at any time. Also, the user can switch from one shell environment to another for the currently logged session without changing the default shell. Microsoft Windows-based users may visualize the shell prompt as something similar to the DOS prompt (such as C:). However, the Linux shell (or command line) is very powerful and flexible. In fact, it is not surprising to say that every Enterprise-class application in the Linux/UNIX environment uses shell script(s). Figure 2.12 displays a typical shell prompt window in a Red Hat Linux 8.0/9.0 environment, and Figure 2.13 displays the same in a SuSE Linux 8.1 system.