nixos-rebuild: add support for --file and --attr flags

adds support of building system from nix file and attrPath directly

these are --file and --attr flags in nix-build respectfully

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.8

@@ -20,6 +20,8 @@
 .Op Fl -rollback
 .Op Fl -builders Ar builder-spec
 .br
+.Op Fl -file | f Ar path
+.Op Fl -attr | A Ar attrPath
 .Op Fl -flake Ar flake-uri
 .Op Fl -no-flake
 .Op Fl -override-input Ar input-name flake-uri
@@ -45,11 +47,15 @@
 .Sh DESCRIPTION
 This command updates the system so that it corresponds to the
 configuration specified in
-.Pa /etc/nixos/configuration.nix
-or
-.Pa /etc/nixos/flake.nix Ns
-\&. Thus, every time you modify the configuration or any other NixOS module, you
-must run
+.Pa /etc/nixos/configuration.nix Ns
+,
+.Pa /etc/nixos/flake.nix
+or the file and attribute specified by the
+.Fl -file
+and/or
+.Fl -attr
+options. Thus, every time you modify the configuration or any other NixOS
+module, you must run
 .Nm
 to make the changes take effect. It builds the new system in
 .Pa /nix/store Ns
@@ -369,6 +375,32 @@ system with
 .Ic sudo Ns
 \&. Setting this option allows deploying as a non-root user.
 .
+.It Fl -file Ar path , Fl f Ar path
+Enable and build the NixOS system from the specified file. The file must
+evaluate to an attribute set, and it must contain a valid NixOS configuration
+at attribute
+.Va attrPath Ns
+\&. This is useful for building a NixOS system from a nix file that is not
+a flake or a NixOS configuration module. Attribute set a with valid NixOS
+configuration can be made using
+.Va nixos
+function in nixpkgs or importing and calling
+.Pa nixos/lib/eval-config.nix
+from nixpkgs. If specified without
+.Fl -attr
+option, builds the configuration from the top-level
+attribute of the file.
+.
+.It Fl -attr Ar attrPath , Fl A Ar attrPath
+Enable and build the NixOS system from nix file and use the specified attribute
+path from file specified by the
+.Fl -file
+option. If specified without
+.Fl -file
+option, uses
+.Pa default.nix
+in current directory.
+.
 .It Fl -flake Va flake-uri Ns Op Va #name
 Build the NixOS system from the specified flake. It defaults to the directory
 containing the target of the symlink

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -34,6 +34,9 @@ targetHost=
 remoteSudo=
 verboseScript=
 noFlake=
+attr=
+buildFile=default.nix
+buildingAttribute=1
 installBootloader=
 json=
 
@@ -58,6 +61,24 @@ while [ "$#" -gt 0 ]; do
         if [ -n "$action" ]; then showSyntax; fi
         action="$i"
         ;;
+      --file|-f)
+        if [ -z "$1" ]; then
+            log "$0: '$i' requires an argument"
+            exit 1
+        fi
+        buildFile="$1"
+        buildingAttribute=
+        shift 1
+        ;;
+      --attr|-A)
+        if [ -z "$1" ]; then
+            log "$0: '$i' requires an argument"
+            exit 1
+        fi
+        attr="$1"
+        buildingAttribute=
+        shift 1
+        ;;
       --install-grub)
         log "$0: --install-grub deprecated, use --install-bootloader instead"
         installBootloader=1
@@ -345,6 +366,12 @@ if [[ "$action" = switch || "$action" = boot || "$action" = test ]]; then
     canRun=1
 fi
 
+# Verify that user is not trying to use attribute building and flake
+# at the same time
+if [[ -z $buildingAttribute && -n $flake ]]; then
+    log "error: '--flake' cannot be used with '--file' or '--attr'"
+    exit 1
+fi
 
 # If ‘--upgrade’ or `--upgrade-all` is given,
 # run ‘nix-channel --update nixos’.
@@ -426,7 +453,10 @@ trap cleanup EXIT
 
 # Re-execute nixos-rebuild from the Nixpkgs tree.
 if [[ -z $_NIXOS_REBUILD_REEXEC && -n $canRun && -z $fast ]]; then
-    if [[ -z $flake ]]; then
+    if [[ -z $buildingAttribute ]]; then
+        p=$(runCmd nix-build --no-out-link $buildFile -A "${attr:+$attr.}config.system.build.nixos-rebuild" "${extraBuildFlags[@]}")
+        SHOULD_REEXEC=1
+    elif [[ -z $flake ]]; then
         if p=$(runCmd nix-build --no-out-link --expr 'with import <nixpkgs/nixos> {}; config.system.build.nixos-rebuild' "${extraBuildFlags[@]}"); then
             SHOULD_REEXEC=1
         fi
@@ -448,7 +478,10 @@ fi
 
 # Find configuration.nix and open editor instead of building.
 if [ "$action" = edit ]; then
-    if [[ -z $flake ]]; then
+    if [[ -n $attr || -n $buildFile ]]; then
+        log "error: '--file' and '--attr' are not supported with 'edit'"
+        exit 1
+    elif [[ -z $flake ]]; then
         NIXOS_CONFIG=${NIXOS_CONFIG:-$(runCmd nix-instantiate --find-file nixos-config)}
         if [[ -d $NIXOS_CONFIG ]]; then
             NIXOS_CONFIG=$NIXOS_CONFIG/default.nix
@@ -493,6 +526,9 @@ prebuiltNix() {
 getNixDrv() {
     nixDrv=
 
+    if [[ -z $buildingAttribute ]]; then
+        if nixDrv="$(runCmd nix-instantiate $buildFile --add-root "$tmpDir/nix.drv" --indirect -A ${attr:+$attr.}config.nix.package.out "${extraBuildFlags[@]}")"; then return; fi
+    fi
     if nixDrv="$(runCmd nix-instantiate '<nixpkgs/nixos>' --add-root "$tmpDir/nix.drv" --indirect -A config.nix.package.out "${extraBuildFlags[@]}")"; then return; fi
     if nixDrv="$(runCmd nix-instantiate '<nixpkgs>' --add-root "$tmpDir/nix.drv" --indirect -A nix "${extraBuildFlags[@]}")"; then return; fi
 
@@ -553,7 +589,9 @@ if [ "$action" = repl ]; then
     # This is a very end user command, implemented using sub-optimal means.
     # You should feel free to improve its behavior, as well as resolve tech
     # debt in "breaking" ways. Humans adapt quite well.
-    if [[ -z $flake ]]; then
+    if [[ -z $buildingAttribute ]]; then
+        exec nix repl --file $buildFile $attr "${extraBuildFlags[@]}"
+    elif [[ -z $flake ]]; then
         exec nix repl '<nixpkgs/nixos>' "${extraBuildFlags[@]}"
     else
         if [[ -n "${lockFlags[0]}" ]]; then
@@ -706,7 +744,9 @@ fi
 if [ -z "$rollback" ]; then
     log "building the system configuration..."
     if [[ "$action" = switch || "$action" = boot ]]; then
-        if [[ -z $flake ]]; then
+        if [[ -z $buildingAttribute ]]; then
+            pathToConfig="$(nixBuild $buildFile -A "${attr:+$attr.}config.system.build.toplevel" "${extraBuildFlags[@]}")"
+        elif [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' --no-out-link -A system "${extraBuildFlags[@]}")"
         else
             pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.toplevel" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
@@ -714,19 +754,25 @@ if [ -z "$rollback" ]; then
         copyToTarget "$pathToConfig"
         targetHostSudoCmd nix-env -p "$profile" --set "$pathToConfig"
     elif [[ "$action" = test || "$action" = build || "$action" = dry-build || "$action" = dry-activate ]]; then
-        if [[ -z $flake ]]; then
+        if [[ -z $buildingAttribute ]]; then
+            pathToConfig="$(nixBuild $buildFile -A "${attr:+$attr.}config.system.build.toplevel" "${extraBuildFlags[@]}")"
+        elif [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A system -k "${extraBuildFlags[@]}")"
         else
             pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.toplevel" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
         fi
     elif [ "$action" = build-vm ]; then
-        if [[ -z $flake ]]; then
+        if [[ -z $buildingAttribute ]]; then
+            pathToConfig="$(nixBuild $buildFile -A "${attr:+$attr.}config.system.build.vm" "${extraBuildFlags[@]}")"
+        elif [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vm -k "${extraBuildFlags[@]}")"
         else
             pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.vm" "${extraBuildFlags[@]}" "${lockFlags[@]}")"
         fi
     elif [ "$action" = build-vm-with-bootloader ]; then
-        if [[ -z $flake ]]; then
+        if [[ -z $buildingAttribute ]]; then
+            pathToConfig="$(nixBuild $buildFile -A "${attr:+$attr.}config.system.build.vmWithBootLoader" "${extraBuildFlags[@]}")"
+        elif [[ -z $flake ]]; then
             pathToConfig="$(nixBuild '<nixpkgs/nixos>' -A vmWithBootLoader -k "${extraBuildFlags[@]}")"
         else
             pathToConfig="$(nixFlakeBuild "$flake#$flakeAttr.config.system.build.vmWithBootLoader" "${extraBuildFlags[@]}" "${lockFlags[@]}")"