first commit

.gitignore

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+result

default.nix

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+{ substituteAll, lib, coreutils, jq, kexec-tools, runtimeShell, installShellFiles, ... }: substituteAll {
+  name = "nixos-kexec-profile";
+  src = ./nixos-kexec-profile.sh;
+  dir = "bin";
+  isExecutable = true;
+
+  path = lib.makeBinPath [ coreutils jq kexec-tools ];
+  inherit runtimeShell;
+
+  nativeBuildInputs = [
+    installShellFiles
+  ];
+
+  meta = {
+    description = "kexec into a NixOS profile";
+    homepage = "https://github.com/jeffguorg/nixos-kexec-profile";
+    licenses = lib.license.gpl;
+    mainProgram = "nixos-kexec-profile";
+  };
+}

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1698553279,
+        "narHash": "sha256-T/9P8yBSLcqo/v+FTOBK+0rjzjPMctVymZydbvR/Fak=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "90e85bc7c1a6fc0760a94ace129d3a1c61c3d035",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "kexec into nixos profile";
+
+  outputs = { self, nixpkgs }: {
+    packages.x86_64-linux = rec {
+      nixos-kexec-profile = nixpkgs.legacyPackages.x86_64-linux.pkgs.callPackage ./default.nix {};
+      default = nixos-kexec-profile;
+    };
+  };
+}

nixos-kexec-profile.sh

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+#!@runtimeShell@
+# shellcheck shell=bash
+
+if [ -x "@runtimeShell@" ]; then export SHELL="@runtimeShell@"; fi;
+
+set -euo pipefail
+
+export PATH=@path@:$PATH
+
+build-configuration-legacy() {
+    run nix-build '<nixpkgs/nixos>' -A system -I nixos-config="$1"
+}
+
+build-configuration-flake() {
+    run nix build "$1.out" --no-link --print-out-paths
+}
+
+type nix > /dev/null
+type nix-build > /dev/null
+type jq > /dev/null
+type kexec > /dev/null
+type id > /dev/null
+
+DEFAULT_TARGET="${BUILD:-/nix/var/nix/profiles/system}"
+
+TYPE=""
+BUILD=""
+BOOT=n
+
+msg() {
+    echo "=====> msg:  $@" >&2
+}
+
+fail() {
+    echo "=====> fail: $@" >&2
+    exit 1
+}
+
+run() {
+    echo "=====> run:  $@" >&2
+    "$@"
+}
+
+set-target() {
+    if ! [ -z "$BUILD" ]; then
+        fail "Target is already set once"
+    fi
+
+    if [[ ( "$1" =~ '.*#.*' ) || ( "${2:-}" = flake ) ]]; then
+        BUILD="$1"
+        TYPE=flake
+    elif [  -r "$1/flake.nix" ]; then
+        BUILD="$1#nixosConfigurations."$HOSTNAME".config.system.build.toplevel"
+        TYPE=flake
+    elif [ -r "$1/configuration.nix" ]; then
+        BUILD="$1/configuration.nix"
+        TYPE=legacy
+    elif [ -r "$1/boot.json" ]; then
+        TARGET=$1
+        TYPE=built
+    elif [ ! -e "$BUILD" ]; then
+        fail "nothing is found $BUILD"
+    elif [ -d "$BUILD" ]; then
+        fail "not a valid directory. it should either contains a flake.nix to be a flake build, a configuration.nix to be a legacy build, or a boot.json to be a nixos top level directory"
+    else
+        return 1
+    fi
+}
+
+while [[ $# -gt 0 ]]; do
+    opt="$1"
+    shift
+    case "$opt" in
+        --flake)
+            set-target "$1" flake
+            shift
+            ;;
+        -b|--boot)
+            BOOT=y
+            ;;
+        *)
+            if set-target "$opt"; then
+                :
+            else
+                fail "unrecognized argument: $opt"
+            fi
+            ;;
+    esac
+done
+
+if [ -z "$BUILD" ]; then
+    msg "Target not set. using $DEFAULT_TARGET"
+    msg
+    if set-target "$DEFAULT_TARGET"; then
+        :
+    else
+        fail "weird...how am i getting this? $opt"
+    fi
+fi
+
+if [[ ( ! -z "$BUILD" ) && (( $TYPE = flake ) ||  ( $TYPE = legacy ) ) ]]; then
+    if [ $TYPE = flake ]; then
+        TARGET=$(build-configuration-flake "$BUILD")
+    else
+        TARGET=$(build-configuration-legacy "$BUILD")
+    fi
+    RET=$?
+    if [ $RET -ne 0 ]; then
+        exit $RET
+    fi
+fi
+
+if [ -z "$TARGET" ]; then
+    fail "this is unreachable"
+fi
+
+LABEL="$(jq '."org.nixos.bootspec.v1".label' /nix/var/nix/profiles/system/boot.json -r)"
+
+INITRD="$(jq '."org.nixos.bootspec.v1".initrd' /nix/var/nix/profiles/system/boot.json -r)"
+KERNEL="$(jq '."org.nixos.bootspec.v1".kernel' /nix/var/nix/profiles/system/boot.json -r)"
+INIT="$(jq '."org.nixos.bootspec.v1".init' /nix/var/nix/profiles/system/boot.json -r)"
+
+CMDLINE="init=${INIT} $(jq '."org.nixos.bootspec.v1".kernelParams | join(" ")' /nix/var/nix/profiles/system/boot.json -r)"
+
+msg "Loading:  $LABEL"
+msg "Toplevel: $TARGET"
+msg
+msg "Kernel:   $KERNEL"
+msg "Initrd:   $INITRD"
+msg "Cmdline:  $CMDLINE"
+msg
+
+if [ $(id -u) -ne 0 ]; then
+    msg "Warn: You are not running as root. kexec might fail. it is normal."
+fi
+
+run kexec -l --initrd=$INITRD --command-line="$CMDLINE" $KERNEL
+
+if [ $BOOT == y ]; then
+    run kexec -e
+fi