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nixos/doc/manual/development/etc-overlay.section.md

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       # `/etc` via overlay filesystem {#sec-etc-overlay}

     

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       ::: {.note}

     

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       This is experimental and requires a kernel version >= 6.6 because it uses

     

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       new overlay features and relies on the new mount API.

     

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       :::

     

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       Instead of using a custom perl script to activate `/etc`, you activate it via an

     

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       overlay filesystem:

     

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       ```nix

     

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       system.etc.overlay.enable = true;

     

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       ```

     

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       Using an overlay has two benefits:

     

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       1. it removes a dependency on perl

     

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       2. it makes activation faster (up to a few seconds)

     

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       By default, the `/etc` overlay is mounted writable (i.e. there is a writable

     

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       upper layer). However, you can also mount `/etc` immutably (i.e. read-only) by

     

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       setting:

     

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       ```nix

     

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       system.etc.overlay.mutable = false;

     

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       ```

     

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       The overlay is atomically replaced during system switch. However, files that

     

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       have been modified will NOT be overwritten. This is the biggest change compared

     

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       to the perl-based system.

     

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       If you manually make changes to `/etc` on your system and then switch to a new

     

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       configuration where `system.etc.overlay.mutable = false;`, you will not be able

     

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       to see the previously made changes in `/etc` anymore. However the changes are

     

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       not completely gone, they are still in the upperdir of the previous overlay in

     

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       `/.rw-etc/upper`.

nixos/doc/manual/development/what-happens-during-a-system-switch.chapter.md

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       unit-handling.section.md

     

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       activation-script.section.md

     

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       non-switchable-systems.section.md

     

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       ```

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       unit-handling.section.md

     

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       activation-script.section.md

     

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       non-switchable-systems.section.md

     

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       etc-overlay.section.md

     

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       ```