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🗃️ Linux File System: Server-Focused Overview

This guide explores the Linux filesystem hierarchy with a focus on what’s relevant for server environments — particularly in distributions like Debian, Ubuntu, and Alpine Linux.

Whether you’re managing containers, configuring services, or building CI/CD pipelines, this knowledge is essential.

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🗂️ Filesystem Hierarchy Overview (FHS)

Linux organizes files into a tree-like structure rooted at /. Most server-relevant data lives in well-defined subdirectories.

📂 Top-Level Directories

Path	Purpose
/	Root of the entire filesystem
/bin	Essential binaries for all users
/sbin	Essential system binaries for root/admins
/etc	System configuration files
/var	Variable data (logs, caches, spool)
/usr	Userland apps and libraries
/home	User home directories (less relevant for servers)
/root	Home directory for root user
/tmp	Temporary files, auto-cleared on reboot
/opt	Optional software packages
/lib, /lib64	Shared libraries used by binaries
/dev	Device files (disks, terminals, etc.)
/proc, /sys	Virtual files for system/kernel info
/run	Volatile runtime files (e.g., PID files)
/mnt, /media	Mount points (manual or removable devices)

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🧰 Server-Critical Directories In Depth

🔧 /etc — Configuration

• Contains system-wide config files (mostly plain text)
• Common contents:
  • /etc/hosts: Static hostname resolution
  • /etc/passwd, /etc/group: User accounts
  • /etc/fstab: Mount configuration
  • /etc/network/interfaces or /etc/netplan/: Network setup (Debian/Ubuntu)
  • /etc/hostname, /etc/hosts: Host identity
  • /etc/systemd/: Service definitions
  • /etc/nginx/, /etc/apache2/: Web server configs

# View SSH configuration
cat /etc/ssh/sshd_config

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⚙️ /var — Variable Runtime Data

• Stores logs, spools, and caches
• Server-relevant subdirs:
  • /var/log/: System logs (syslog, auth.log, nginx, etc.)
  • /var/lib/: Persistent state (e.g. MySQL, apt, Docker)
  • /var/run/ → symlink to /run: PID and runtime state
  • /var/spool/: Queued jobs (mail, print)

# Tail logs
tail -f /var/log/syslog

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📦 /usr — Installed Software and Shared Resources

• Subdirs:
  • /usr/bin/, /usr/sbin/: Most installed binaries
  • /usr/lib/: Libraries
  • /usr/share/: Architecture-independent data (man pages, locale)
  • /usr/local/: Custom-installed software

⚠️ On Debian systems, apt install places binaries in /usr/bin not /bin.

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🪪 /run — Runtime Volatile Files

• Replaces older /var/run
• Lives in memory (tmpfs)
• Contains:
  • PID files for services
  • Socket files (e.g. /run/docker.sock)
  • Network state

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🧪 /tmp — Temporary Files

• Writable by all users
• Auto-cleared on reboot (unless configured otherwise)

Use it for:

• Transient cache
• Intermediate build files

mktemp /tmp/example.XXXXXX

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🔒 /root — Root User Home

• The home directory of the root user
• Not to be confused with /, the root of the filesystem

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📁 /opt — Optional Software

• Third-party software that doesn’t fit system layout
• Useful for deploying tools manually (e.g. custom binaries, dev tools)

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🐧 Alpine-Specific Notes

Alpine follows FHS but is stripped-down for minimalism:

Debian Path	Alpine Path	Notes
/etc/init.d/	/etc/init.d/	OpenRC init system used in Alpine
/lib, /lib64	Merged into /lib	Smaller footprint
/var/lib/apk/	Alpine's package metadata

Use Alpine for:

• Containers
• Lightweight distros

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🧹 Tips for Server Environments

• Don't write to /usr or /bin directly — use package managers
• Use /etc for config, version it via git (/etc/git)
• Log to /var/log, rotate with logrotate
• Store state in /var/lib, ephemeral data in /tmp or /run

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🛠 Useful Commands

# Inspect disk usage by directory
du -sh /*

# See mounts
mount | column -t

# List services
systemctl list-units --type=service

# Find config file for a service
dpkg -L nginx | grep /etc

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📚 Further Reading

• Filesystem Hierarchy Standard (FHS)
• Debian Filesystem Overview
• Alpine Linux Wiki
• Linux File System Layout — Red Hat Docs

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