Linux File System Hierarchy

Also Known As

TL;DR

Explanation

FHS (Filesystem Hierarchy Standard) defines where things go: /etc (config files), /var (variable data — logs, spool, databases), /var/log (logs), /var/www (web content), /tmp (temp files, cleared on reboot), /usr/local (locally installed software), /home (user home dirs), /root (root home), /proc (virtual — kernel and process info), /sys (virtual — hardware info), /run (runtime data, cleared on reboot), /opt (optional/third-party software). PHP apps live in /var/www; logs go to /var/log; runtime files (PID, sockets) go to /run.

Common Misconception

Why It Matters

Common Mistakes

• Writing temporary files to the app directory — use sys_get_temp_dir() or /tmp; app directory may not be writable.
• Storing uploaded files in /tmp — /tmp is cleared on reboot; use /var/www/uploads or object storage.
• Log files in the application root — they should go to /var/log/appname/ for log rotation and access control.
• Hardcoding /home/username paths — breaks when deployed as a different user or on a different server.

Code Examples

// Files in wrong locations:
define('LOG_FILE',    '/tmp/app.log');         // Cleared on reboot!
define('UPLOAD_DIR',  '/tmp/uploads/');         // Cleared on reboot!
define('CONFIG_FILE', '/home/paul/app.conf');   // User-specific path
define('PID_FILE',    '/var/www/html/app.pid'); // Wrong — should be /run/

// Correct FHS locations:
define('LOG_FILE',    '/var/log/myapp/app.log');
define('UPLOAD_DIR',  '/var/www/myapp/uploads/');  // Persistent
define('CONFIG_FILE', '/etc/myapp/config.php');     // Standard config location
define('PID_FILE',    '/run/myapp/app.pid');        // Standard runtime location
define('CACHE_DIR',   sys_get_temp_dir() . '/myapp_cache/'); // Portable temp

References

• ↗ https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html

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