Understanding Unix/Linux Files, File Types, Partitions and Filesystem Hierarchy

Unix and Linux follow the principle "everything is a file"; processes, devices, directories and even sockets are represented as files, which simplifies system design despite the existence of special file types such as pipes and sockets.

File types are categorized as regular files, directories, special device files, symbolic links, sockets and named pipes. The first character of ls -l output indicates the type (e.g., - for regular files, d for directories, l for links, c for character devices, s for sockets, p for named pipes, b for block devices). The following table summarizes the symbols:

Symbol

Meaning

-

Regular file

d

Directory

l

Link

c

Special file (character device)

s

Socket

p

Named pipe

b

Block device

Partitions divide a disk into independent sections to improve data safety and system robustness. Separate partitions isolate system files, user data and swap space, so a failure in one partition does not corrupt the whole system. Journaled file systems (e.g., ext3) maintain a log to recover from crashes.

Linux typically uses two main partition types: data partitions (type 83 ) and swap partitions (type 82 ). Additional partitions may host other file systems such as FAT, ReiserFS, JFS, NFS, etc. Common layout includes a root partition (/, 100‑500 MB), one or more data partitions, and one or more swap partitions, often sized at twice the RAM.

Mount points attach partitions to the directory tree. An empty directory (e.g., /opt/media ) is created, then the mount command links the partition to that directory. The /etc/fstab file defines automatic mounts at boot, while the df -h command shows currently mounted filesystems and their usage:

freddy:~> df -h
Filesystem          Size  Used Avail Use% Mounted on
/dev/hda8           496M  183M  288M  39% /
/dev/hda1           124M  8.4M  109M   8% /boot
/dev/hda5            19G   15G  2.7G  85% /opt
/dev/hda6           7.0G  5.4G  1.2G  81% /usr
/dev/hda7           3.7G  2.7G  867M  77% /var
fs1:/home           8.9G  3.7G  4.7G  44% /.automount/fs1/root/home

The Linux filesystem is presented as a hierarchical tree rooted at / . Standard top‑level directories include:

Directory

Purpose

/bin

Essential user commands

/boot

Boot loader and kernel files

/dev

Device files representing hardware

/etc

System configuration files

/home

Home directories for regular users

/initrd

Initial RAM disk image

/lib

Shared libraries

/lost+found

Recovered files after crashes

/mnt

Mount points for temporary mounts

/opt

Optional third‑party software

/proc

Virtual filesystem exposing kernel info

/root

Home directory of the root user

/sbin

System administration binaries

/tmp

Temporary files cleared on reboot

/usr

Secondary hierarchy for user programs and data

/var

Variable data such as logs, mail, spool

Each file is represented by an inode that stores metadata: owner, group, type, permissions, timestamps, link count, size and the disk addresses of the data blocks. The filename and directory entries are stored separately, linking names to inode numbers; the command ls -i displays inode numbers.