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Sometimes when I go through sections of Linux books about filesystem, it may turn out that it is a chapter talking about the directory structure of a standard Linux system, like /proc, /dev, /etc, /usr directories, with a brief explanation. Sometimes it may be about filesystem types like ext3, ext4 and nfs, dos, etc. I feel quite confused about that. Which one is correct? Or did I miss something?

In short, my question is:

The word filesystem means directory structure of Linux system or things like ext3, ext4, fat32, etc.?

A filesystem is both a reference to a tree structure of directories and files as well as a overarching structure that can be placed on a physical medium such as a hard drive or other similar types of medium.

At the end of of the day they're both layers of abstraction that people create so that things are standardized.

The directories + files analogy used is to mimic how people think, with respect to the physical world for storing items (files) inside of something (folders).

So too is a filesystem such as ext4 of fat32. Here it might not be so obvious, but the structure that this type of filesystem provides serves the same purpose, just at a lower level.

For example, a raw disk is just a sequence of bits. By creating a structure on top of it using inodes, we're able to access sections of the disk in an organized methodical fashion.

Notice the image of the inode structure (from wikipedia article titled: inode pointer structure

The structure of a filesystem representing files + directories

computer architectures

One thing you'll notice as you continue to study computer architectures is that the same concepts are used over and over again. The notion of hostnames is nested too.

                                   .---> <-----.

                                   |           |

                               .------.    .------.

                 ^------------>| .com |    | .net |

                 |             '------'    '------'

                 |                 ^

                 |                 |

             .--------.    .---------------.

             | google |    | stackexchange |

             '--------'    '---------------'

                  ^                ^

                  |                |

               .-----.          .------.

               | www |          | unix |

               '-----'          '------'

Or in programming, class inheritance (Ruby):

class Mammal  

  def breathe  

    puts "inhale and exhale"  

  end  

end  





class Cat < Mammal  

  def speak  

    puts "Meow"  

  end  

end  



jake = Cat.new  

jake.breathe  

jake.speak  

As is usual in natural languages, the word filesystem has several related meanings. A filesystem relates to how files are organized; this can be interpreted at various levels.

The filesystem of a unix system is the set of all files that are accessible at a given time. For example, a system call that accesses a file can be classified as a filesystem system call. A process's filesystem credentials are the credentials that determine what files it can access.

The filesystem can also refer to the structure of that filesystem: what directories and files exist on a unix system and what they're used for. On Unix & Linux, we use the tag directory-structure for that meaning.

A filesystem is a way through which files can be accessed on a given system. The filesystem (in the first sense above) of a unix systems consists of several filesystems (in this sense), each mounted at a particular directory. Filesystems in this sense are usually refered to by their mount point: the root filesystem (mounted on /), the proc filesystem (normally mounted on /proc), the or a home filesystem (normally mounted on /home or a subdirectory, containing users' home directories), etc.

A filesystem type (or filesystem format) is a layout format for filesystems in the previous sense. Ext2, tmpfs, proc, sshfs, etc. are filesystem types. Filesystem type can be abbreviated to filesystem by metonymy. In fact, filesystem or filesystem is strictly speaking not just the layout format, but also the rules for modifying it.

A filesystem can also be a specific driver for a filesystem type.