# Linux Root Filesystem

The root filesystem is the last component of embedded Linux.

## Requirements

To make a minimal rootfs, these components are essential:
- **init**: The init script is an executable binary which kicks everything off.
More about it, in the [Initramfs section](./Initramfs.md).
- **Shell**: A shell is needed for command prompts and running functions in
the init script.
- **Daemons**: Background programs providing services to others. E.g. dockerd,
syslogd, sshd, ...
- **Shared libraries**: Most programs depend on shared libraries
- **Config files**: Series of files needed to configure systemd, init, etc.
Usually present in */etc*
- **Device nodes**: Special files giving access to device drivers.
- **proc, sys fs**: Pseudo filesystem running in RAM and representing the
Kernel structure as a hierarchy of files
- **Kernel modules**: If kernel modules are used, they are (most of the time)
stored in */lib/modules/[kernel version]*
- **Device specific applications**: Apps that make the device do the job it is
intended to do

:::{note}
The init file doesn't have to be the initial script. In case a
developer has created his very own init script (e.g. /custom-init), it can
replace the default init script by passing it to the kernel command line:
**init=/custom-init**
:::

## Base structure

The rootfs usually has the following base structure:
- **/bin** => Symlink to /usr/bin: executable binaries.
- **/dev** => Devices (Disks, tty, ...).
- **/etc** => Static system configs.
- **/home** => Home directory for system users.
- **/lib** => Symlink to /usr/lib: Libraries, shared objects (.so files, ...).
- **/proc** => Pseudo filesystem for kernel status and hardware info.
- **/sbin** => symlinks /usr/sbin: System executables.
- **/sys** => RAM-based pseudo filesystem to get info about the system.
- **/tmp** => Temporary files. Gets wiped after reboot.
- **/usr** => UnixSystemResources. Manpages, scripts, executables, ...
- **/var** => Changing variables: Log-files, Game progress, printer queue, ...

## Additional:
- **/boot** => Everything needed for booting, mounted on another partition.
- **/lost+found** => Lost files and fragments that have been found by e.g.
recovering from system damage.
- **/opt** => Usually used for additional, non-free licensed programms.
- **/root** => Home directory for root user.
- **/run** => /var/run symlinks this. Used for daemons in early rootfs.

More base information about the Linux rootfs
[here](https://www.youtube.com/watch?v=rrB13utjYV4)!


*sources:*
- Mastering Embedded Linux Programming - Third Edition By Frank Vasquez, Chris
Simmonds
- [Linux in 100 seconds](https://www.youtube.com/watch?v=rrB13utjYV4)