Linux Filesystem

An FHS-compliant file system looks like,

/
│
├── /boot -> Boot loader files
│   ├── kernels
│   ├── initrd
│   ├── vmlinuz
│   ... 
│   
├── /bin -> OS essential command binaries (single user mode)
│   ├── cd
│   ├── ls
│   ├── cat
│   ... 
│   
├── /sbin -> OS essential system binaries (for root users)
│   ├── init
│   ├── fdisk
│   ├── mkfs
│   ├── ifconfig
│   ... 
│   
├── /lib -> Library files essential for bin and sbin
│   ├── /libc.so.x -> C library
│   ├── /libpthread.so.x
│   ... 
│   
├── /etc -> System-wide config files
│   ├── /fstab
│   ├── /shells
│   ├── /opt
│   ├── /X11
│   ... 
│    
├── /dev -> Device files
│   ├── MAKEDEV (command)
│   ... 
│   
├── /media -> Mount point for removable media
│   ├── floppy
│   ├── cdrom
│   ... 
│   
├── /mnt -> Mount point for temp mounted filesystems
│   ... 
│   
├── /opt -> Directory for optional third-party apps
│   ├── /eclipse
│   ├── /chrome
│   ...
│   
├── /root -> Root user directory (media files, docs, etc.)
│   ... 
│   
├── /home -> User directory (media files, docs, etc.)
│   ... 
│   
├── /var -> Fast changing variable data
│   ├── /log
│   ├── /cache
│   ... 
│   
├── /run -> Volatile runtime info
│   ├── User sessions
│   ├── systemd details
│   ... 
│   
├── /srv -> Data for services provided by the system (via ftp, rsync, etc)
│   ├── /physics
│   ├── /compsci
│   ... 
│   
├── /tmp -> For programs that use temporary files
│   ... 
│   
├── /proc -> Virtual filesystem (process and kernel info)
│   ├── cpu info
│   ├── mount info
│   ... 
│   
├── /sys -> Virtual filesystem (devices, drivers, and kernel info)
│   ├── network stacks
│   ├── keyboard
│   ... 
│   
├── /usr (Unix System Resources)
│   ├── /bin -> user command binaries (managed by system's package manager)
│       ├── bash
│       ├── gcc
│       ├── ...
│   ├── /sbin -> user system binaries
│   ├── /lib -> Library files for user binaries
│       ├── qt 
│	    ├── gtk 
│	    ├── ... 
│   └── /local
│       ├── /bin -> user command binaries (managed manually)
│           ├── my-script
│           ...
│  

Note

• Single user mode: No user filesystems are mounted

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Refs

1. https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html
2. https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
3. https://www.youtube.com/watch?v=42iQKuQodW4