Filesystem Hierarchy Standard¶

Linux follows the Filesystem Hierarchy Standard (FHS). There are no drive letters - everything starts from a single root /. All devices, network filesystems, and special interfaces are attached via mounting. This entry covers the directory layout and the "everything is a file" philosophy.

Key Facts¶

Single root / - no drive letters (C:, D:)
Case-sensitive filesystem
All physical devices, network mounts, virtual interfaces appear as files under the tree
Config files live in /etc, logs in /var/log, user data in /home

Standard Directory Layout¶

Directory	Purpose
`/`	Root of filesystem tree
`/bin`	Essential user binaries (ls, cp, mv, bash)
`/sbin`	System binaries (iptables, reboot, fdisk)
`/boot`	Kernel, initrd, GRUB bootloader
`/dev`	Device files (hardware interfaces)
`/etc`	System-wide configuration files
`/home`	User home directories
`/lib`, `/lib64`	Shared libraries for `/bin` and `/sbin`
`/media`	Auto-mounted removable devices (USB, CD)
`/mnt`	Manual/temporary mount point
`/opt`	Optional/third-party software
`/proc`	Virtual FS: process and kernel info
`/root`	Root user's home directory
`/run`	Runtime data (PIDs, sockets) - cleared on reboot
`/srv`	Service data (web, FTP)
`/sys`	Kernel device/driver info
`/tmp`	Temporary files - cleared on reboot
`/usr`	User programs, libraries, documentation
`/var`	Variable data: logs, mail, caches

Everything Is a File¶

Unix philosophy: devices, directories, pipes, sockets - all represented as files.

Symbol in `ls -l`	Type
`-`	Regular file
`d`	Directory
`l`	Symbolic link
`b`	Block device (random access: HDD, SSD)
`c`	Character device (sequential: terminal, keyboard)
`p`	Named pipe (FIFO)
`s`	Socket

Special Filesystems¶

Type	Mount	Purpose
procfs	`/proc`	Process info, kernel parameters (required by `ps`, `top`)
sysfs	`/sys`	Kernel device/driver info
tmpfs	`/tmp`, `/run`	RAM-backed, fast, cleared on reboot
devfs	`/dev`	Device files (managed by `udev` at boot)

Filesystem Types for Disk Partitions¶

ext4 - standard Linux default
ext3 - older journaling FS
XFS - high-performance, default on RHEL/CentOS
btrfs - modern: snapshots, compression, subvolumes
NTFS/FAT32 - Windows filesystems (read/write supported)

Virtual/Overlay Filesystems¶

VFS - kernel abstraction for uniform FS access
NFS - mount remote filesystem over network
AUFS - merge multiple filesystems (used by Docker)
EncFS - transparent file encryption

Disk Usage Commands¶

df -h                    # mounted filesystems and space usage
df -h /home              # specific mount
du -sh /var/log          # total size of directory
du -h --max-depth=1 /    # size per top-level directory
lsblk                    # list block devices

Archive Utilities¶

# tar
tar -czf archive.tar.gz dir/         # create gzip archive
tar -xzf archive.tar.gz              # extract
tar -tf archive.tar.gz               # list contents

# zip
zip -r archive.zip dir/
unzip archive.zip
unzip -l archive.zip                  # list contents

Gotchas¶

/tmp has sticky bit by default - everyone can write, but only file owners can delete their own files
/proc files have size 0 in ls but contain data when read (generated on the fly by kernel)
/run vs /var/run: /run is tmpfs (cleared on reboot), /var/run may persist
/home is often a separate partition - reinstalling OS preserves user data