Questions about UAPI.9 Linux File System Hierarchy

[rostidev]

Hello there,

• Is this specification tightly coupled with systemd or it is good when used with any Linux init system? Even with SysV init?
• How does UAPI.9 differ from the FHS 3.0 specification?
• The current version of UAPI.9 is 0.1, and work on it is still ongoing. When is the first version of this specification scheduled or planned for release?

[keszybz]

On Mon, Dec 15, 2025 at 09:25:00AM -0800, Rostislav Krasny wrote: Hello there, * Is this specification tightly coupled with `systemd` or it is good when used with any Linux init system? Even with SysV init?

It's not tightly coupled to systemd. The general layout makes sense also on installations without an init system.

* How does UAPI.9 differ from the FHS 3.0 specification?

FHS 3.0 was released 15 years ago, and arguably wasn't keeping up with the changes that were happening in the decade before that. The UAPI doc is also more opionated, and describes the layout that is considered the modern standard. In particular, it incorporates the usr merge, the bin-sbin merge, tries is clear that libexec and /opt may be used, but are not particularly useful and are deprecated.

* The current version of UAPI.9 is 0.1, and work on it is still ongoing. When is the first version of this specification scheduled or planned for release?

This tag is made this way to reserve the right to make changes before 1.0 is tagged. But no big changes are planned. Zbyszek

[rostidev]

It's not tightly coupled to systemd. The general layout makes
sense also on installations without an init system.

Interesting. I've never heard of Linux without an init system. I know the Linux kernel can simply run /sbin/init , which can contain any ELF file or shell script, but where is that used in practice?

This tag is made this way to reserve the right to make changes
before 1.0 is tagged. But no big changes are planned.

It sounds like the release of version 1.0 may happen in the very near future, doesn't it?

[keszybz]

On Tue, Dec 16, 2025 at 05:45:55AM -0800, Rostislav Krasny wrote: Interesting. I've never heard of Linux without an init system. I know the Linux kernel can simply run `/sbin/init` , which can contain any ELF file or shell script, but where is that used in practice?

Containers, portable services, chroots, etc.

It sounds like the release of version 1.0 may happen in the very near future, doesn't it?

Yes.