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33 lines
1.9 KiB
Markdown
33 lines
1.9 KiB
Markdown
---
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title: systemd File Hierarchy Requirements
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category: Documentation for Developers
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# systemd File Hierarchy Requirements
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There are various attempts to standardize the file system hierarchy of Linux systems.
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In systemd we leave much of the file system layout open to the operating system, but here's what systemd strictly requires:
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- `/`, `/usr`, `/etc` must be mounted when the host systemd is first invoked.
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This may be achieved either by using the kernel's built-in root disk mounting (in which case `/`, `/usr` and `/etc` need to be on the same file system), or via an initrd, which could mount the three directories from different sources.
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- `/bin`, `/sbin`, `/lib` (and `/lib64` if applicable) should reside on `/`, or be symlinks to the `/usr` file system (recommended).
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All of them must be available before the host systemd is first executed.
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- `/var` does not have to be mounted when the host systemd is first invoked, however,
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it must be configured so that it is mounted writable before local-fs.target is reached (for example, by simply listing it in` /etc/fstab`).
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- `/tmp` is recommended to be a tmpfs (default), but doesn't have to.
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If configured, it must be mounted before local-fs.target is reached (for example, by listing it in `/etc/fstab`).
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- `/dev` must exist as an empty mount point and will automatically be mounted by systemd with a devtmpfs. Non-devtmpfs boots are not supported.
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- `/proc` and `/sys` must exist as empty mount points and will automatically be mounted by systemd with procfs and sysfs.
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- `/run` must exist as an empty mount point and will automatically be mounted by systemd with a tmpfs.
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The other directories usually found in the root directory (such as `/home`, `/boot`, `/opt`) are irrelevant to systemd.
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If they are defined they may be mounted from any source and at any time, though it is a good idea to mount them also before local-fs.target is reached.
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