nixpkgs/nixos/doc/manual/administration/rollback.xml

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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id="sec-rollback">
<title>Rolling Back Configuration Changes</title>
<para>After running <command>nixos-rebuild</command> to switch to a
new configuration, you may find that the new configuration doesnt
work very well. In that case, there are several ways to return to a
previous configuration.</para>
<para>First, the GRUB boot manager allows you to boot into any
previous configuration that hasnt been garbage-collected. These
configurations can be found under the GRUB submenu “NixOS - All
configurations”. This is especially useful if the new configuration
fails to boot. After the system has booted, you can make the selected
configuration the default for subsequent boots:
<screen>
$ /run/current-system/bin/switch-to-configuration boot</screen>
</para>
<para>Second, you can switch to the previous configuration in a running
system:
<screen>
$ nixos-rebuild switch --rollback</screen>
This is equivalent to running:
<screen>
$ /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>
where <replaceable>N</replaceable> is the number of the NixOS system
configuration. To get a list of the available configurations, do:
<screen>
$ ls -l /nix/var/nix/profiles/system-*-link
<replaceable>...</replaceable>
lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
</screen>
</para>
</section>