[Linux] Add file system overview

Author: shorodilov

src/linux/filesys.txt

@@ -1,3 +1,42 @@
 *******************************************************************************
                         File system and file management
 *******************************************************************************
+
+A simple description of the UNIX system, also applicable to Linux, is this:
+
+    "On a UNIX system, everything is a file; if something is not a file,
+    it is a process"
+
+This statement is true because there are special files that are more that just
+files (named pipes and sockets for instance), but to keep things simple, saying
+that everything is a file is an acceptable generalization. A Linux system, just
+like UNIX, makes no difference between a file and a directory, since
+a directory is just a file containing names of other files. Programs, services,
+texts, images, and so forth, are all files. Input and output devices, and
+generally all devices, are considered to be files, according to the system.
+
+.. rubric:: Sorts of files
+
+:regular files:
+    most files are just *files*, called *regular* files; they contain normal
+    data, for example text files, executable files or programs, input or
+    output from a program and so on.
+
+:directories:
+    files that are lists of other files.
+
+:special files:
+    the mechanism used for input and output.
+    Most special files are in ``/dev``.
+
+:links:
+    a system to make a file or directory visible in multiple parts of
+    the system's file tree.
+
+:sockets:
+    a special file type, similar to TCP/IP sockets, providing inter-process
+    networking protected by the file system's access control.
+
+:named pipes:
+    act more or less like sockets and form a way for processes to communicate
+    with each other, without using network socket semantic.