7. Remote execution of applications
One of the most amazing features of Unix (yet one of the most unknown to new users) is its great support for remote and distributed execution of applications.
7.1 Telnet
Telnet is a program that allows a person to use a remote computer as if that person were actually at the remote site. Telnet is one of the most powerful tools for Unix, allowing for true remote administration. It is also an interesting program from the point of view of users, because it allows remote access to all their files and programs from anywhere in the Internet. Combined with an X server, there is no difference (apart from the delay) between being at the console or on the other side of the planet. Telnet daemons and clients are available with most Linux distributions.
Encrypted remote shell sessions are available through SSH ( http://www.ssh.fi/sshprotocols2/index.html) thus effectively allowing secure remote administration.
7.2 Remote commands
In Unix, and in particular in Linux, remote commands exist that allow for interaction with other computers from the shell prompt. Examples are: rlogin, which allows for login in a remote machine in a similar way to telnet, rcp, which allows for the remote transfer of files among machines, etc. Finally, the remote shell command rsh allows the execution of a command on a remote machine without actually logging onto that machine.
7.3 The X Window System
The X Window System was developed at MIT in the late 1980s, rapidly becoming the industry standard windowing system for Unix graphics workstations. The software is freely available, very versatile, and is suitable for a wide range of hardware platforms. Any X environment consists of two distinct parts, the X server and one or more X clients. It is important to realise the distinction between the server and the client. The server controls the display directly and is responsible for all input/output via the keyboard, mouse or display. The clients, on the other hand, do not access the screen directly - they communicate with the server, which handles all input and output. It is the clients which do the "real" computing work - running applications or whatever. The clients communicate with the server, causing the server to open one or more windows to handle input and output for that client.
In short, the X Window System allows a user to log in into a remote machine, execute a process (for example, open a web browser) and have the output displayed on his own machine. Because the process is actually being executed on the remote system, very little CPU power is needed in the local one. Indeed, computers exist whose primary purpose is to act as pure X servers. Such systems are called X terminals.
A free port of the X Window System exists for Linux and can be found at: Xfree. It is included in most Linux distributions.
Related HOWTO:
7.4 VNC
VNC stands for Virtual Network Computing. It is, in essence, a remote display system which allows one to view a computing 'desktop' environment not only on the machine where it is running, but from anywhere on the Internet and from a wide variety of machine architectures. Both clients and servers exist for Linux as well as for many other platforms. It is possible to execute MS-Word in a Windows NT or 95 machine and have the output displayed in a Linux machine. The opposite is also true; it is possible to execute an application in a Linux machine and have the output displayed in any other Linux or Windows machine. One of the available clients is a Java applet, allowing the remote display to be run inside a web browser. Another client is a port for Linux using the SVGAlib graphics library, allowing 386s with as little as 4 MB of RAM to become fully functional X-Terminals.
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