Overview of Linux File System Hierarchy

The Linux file system is hierarchical, starting from the root directory /. Every file and directory stems from /.

The structure is standardized by the Filesystem Hierarchy Standard (FHS).

Top-Level Directories in RHEL

Directory	Description
`/`	Root directory. The top of the file system tree.
`/bin`	Essential command binaries (e.g., `ls`, `cp`, `mv`, `bash`) needed for single-user mode.
`/boot`	Static files for boot loader (e.g., GRUB), kernel files (`vmlinuz`, `initrd.img`).
`/dev`	Device files (e.g., `/dev/sda`, `/dev/null`).
`/etc`	System-wide configuration files (e.g., `/etc/fstab`, `/etc/hostname`).
`/home`	Default location for user home directories (`/home/user`).
`/lib`	Essential shared libraries for binaries in `/bin` and `/sbin`.
`/lib64`	64-bit system libraries (used on 64-bit systems).
`/media`	Temporary mount point for removable media (CD-ROM, USB).
`/mnt`	Temporarily mounted file systems (admin-defined).
`/opt`	Optional or third-party software packages.
`/proc`	Virtual file system exposing kernel and process information.
`/root`	Home directory of the `root` user.
`/run`	Temporary runtime data (since boot).
`/sbin`	System binaries used for system administration.
`/srv`	Data for services provided by the system (e.g., HTTP, FTP).
`/sys`	Virtual filesystem exposing devices and kernel info (sysfs).
`/tmp`	Temporary files (cleared on reboot).
`/usr`	Secondary hierarchy for user applications and files.
`/var`	Variable data like logs, mail, spool files, caches, etc.

Important Subdirectories

`/etc` – Configuration Files

Contains static system configuration. Examples:

/etc/hosts: Static hostname-to-IP mappings.
/etc/passwd: User account info.
/etc/systemd/: Systemd service units.

`/var` – Variable Files

Stores logs, mail, and spools.

/var/log/: System logs like messages, secure, dmesg.
/var/spool/: Tasks waiting for processing (e.g., cron, mail).
/var/tmp/: Persistent temporary files.

`/proc` – Kernel and Process Info

Dynamic, virtual files. Examples:

/proc/cpuinfo: CPU info.
/proc/meminfo: Memory usage.
/proc/[pid]/: Process-specific info.

`/sys` – Kernel Device Tree

Exposes devices and kernel attributes. Examples:

/sys/class/: Device classes like network or block.
/sys/block/: Block devices (e.g., sda, loop0).

`ls -l /` Sample Output Explanation (Red Hat)


$ ls -l /
drwxr-xr-x.   2 root root  4096 Jul 21  /bin
drwxr-xr-x.   4 root root  4096 Jul 21  /boot
drwxr-xr-x.  19 root root  3920 Jul 22  /dev
drwxr-xr-x. 122 root root 12288 Jul 22  /etc

Explanation:

drwxr-xr-x.: d is directory; permission bits follow.
2: Number of hard links.
root root: Owner and group.
4096: File size in bytes.
Jul 21: Last modified date.
/bin: File or directory name.

Exploring with Commands

List all top-level directories:


ls -l /

Show disk usage:


df -h

Show memory usage:


free -h

Show partition layout:


lsblk

Show mount points:


mount | column -t

Identify filesystem type:


df -T

Logical Volume Manager (LVM) – RHEL Feature

Red Hat often uses LVM for flexible disk management.

View volumes:


lvs
vgs
pvs

Configuration files:
- /etc/lvm/lvm.conf

SELinux and Red Hat

RHEL enforces SELinux (Security-Enhanced Linux), which adds security labels to the filesystem.

Check status:


sestatus

Contexts:


ls -Z /

File System Tips for Red Hat Admins

Regularly check /var/log/ for troubleshooting.
Clean up /tmp and /var/tmp to save space.
Keep /boot clean (old kernels can fill space).
Use du -sh * to check directory sizes.
Monitor disk usage with df -h and inode usage with df -i.