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nixpkgs/nixos/modules/installer/cd-dvd/channel.nix

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Nix

# Provide an initial copy of the NixOS channel so that the user
# doesn't need to run "nix-channel --update" first.
{ config, lib, pkgs, ... }:
with lib;
let
nixpkgs = lib.cleanSource pkgs.path;
# We need a copy of the Nix expressions for Nixpkgs and NixOS on the
# CD. These are installed into the "nixos" channel of the root
# user, as expected by nixos-rebuild/nixos-install. FIXME: merge
# with make-channel.nix.
channelSources = pkgs.runCommand "nixos-${config.system.nixos.version}"
{ preferLocalBuild = true; }
''
mkdir -p $out
cp -prd ${nixpkgs.outPath} $out/nixos
chmod -R u+w $out/nixos
if [ ! -e $out/nixos/nixpkgs ]; then
ln -s . $out/nixos/nixpkgs
fi
${optionalString (config.system.nixos.revision != null) ''
echo -n ${config.system.nixos.revision} > $out/nixos/.git-revision
''}
echo -n ${config.system.nixos.versionSuffix} > $out/nixos/.version-suffix
echo ${config.system.nixos.versionSuffix} | sed -e s/pre// > $out/nixos/svn-revision
'';
in
{
# Provide the NixOS/Nixpkgs sources in /etc/nixos. This is required
# for nixos-install.
boot.postBootCommands = mkAfter
''
if ! [ -e /var/lib/nixos/did-channel-init ]; then
echo "unpacking the NixOS/Nixpkgs sources..."
mkdir -p /nix/var/nix/profiles/per-user/root
${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/per-user/root/channels \
-i ${channelSources} --quiet --option build-use-substitutes false
mkdir -m 0700 -p /root/.nix-defexpr
ln -s /nix/var/nix/profiles/per-user/root/channels /root/.nix-defexpr/channels
mkdir -m 0755 -p /var/lib/nixos
touch /var/lib/nixos/did-channel-init
fi
'';
}