Linux Architecture
Layering of files in the Linux OS
More than one file may be linked to a single inode; this means that two filenames are used to refer to the same physical file. Changes to one file will be reflected in the other as both point to the same location. Files that have different names and exist on different paths but point to the same inode are connected by a hard link. Sometimes, files are set up to be 'shortcuts' to other files on other paths. Such shortcuts are referred to as symbolic or soft links as they are not actually files but just links to other files on the OS level. Information about the number of hard links of a file is part of the file's inode entry.
Linux has a layered architecture. The central part of the OS is known as the kernel. The kernel's core interfaces with hardware through device drivers and configuration files. The memory management and time slicing functions construe the next level above the core. Above these are the shells through which commands are issued to the OS. Executing processes exist above this level in appropriate layers.
Figure 1d: Linux OS Layers
Programs that perform functions that are the same irrespective of the user (printing, mail etc.) are called System services while individual programs carrying out custom user tasks such as emacs (a file editing tool) and others are called utilities. Basic programs that carry out OS directives like 'cd', 'ls' and so on are called commands.
