home-manager: add support for custom backup command (#6424) (#7153)

This adds a new backupCommand option to allow users to specify a
custom command to run on existing files during activation, as an
alternative to the existing backupFileExtension mechanism.

Adds backupCommand option to NixOS and nix-darwin modules.

Exports HOME_MANAGER_BACKUP_COMMAND environment variable when set.

Updates file activation logic to use the custom backup command if
provided, falling back to the existing backup extension logic.

Updates collision checking and user-facing instructions to
mention the new option.

This enables advanced backup workflows, such as moving files to
trash or archiving with custom tools, before managing them with
Home Manager.

home-manager/home-manager

@@ -1132,6 +1132,11 @@ while [[ $# -gt 0 ]]; do
             EXTRA_NIX_PATH+=("$1")
             shift
             ;;
+        -B)
+            [[ -v 1 && $1 != -* ]] || errMissingOptArg "$opt"
+            export HOME_MANAGER_BACKUP_COMMAND="$1"
+            shift
+            ;;
         -b)
             [[ -v 1 && $1 != -* ]] || errMissingOptArg "$opt"
             export HOME_MANAGER_BACKUP_EXT="$1"

modules/files.nix

@@ -146,13 +146,18 @@ in
           for sourcePath in "$@" ; do
             relativePath="''${sourcePath#$newGenFiles/}"
             targetPath="$HOME/$relativePath"
-            if [[ -e "$targetPath" && ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
-              # The target exists, back it up
-              backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
-              if [[ -e "$backup" && -n "$HOME_MANAGER_BACKUP_OVERWRITE" ]]; then
-                run rm $VERBOSE_ARG "$backup"
+            if [[ -e "$targetPath" && ! -L "$targetPath" ]] ; then
+              if [[ -n "$HOME_MANAGER_BACKUP_COMMAND" ]] ; then
+                verboseEcho "Running $HOME_MANAGER_BACKUP_COMMAND $targetPath."
+                run $HOME_MANAGER_BACKUP_COMMAND "$targetPath" || errorEcho "Running `$HOME_MANAGER_BACKUP_COMMAND` on '$targetPath' failed."
+              elif [[ -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
+                # The target exists, back it up
+                backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
+                if [[ -e "$backup" && -n "$HOME_MANAGER_BACKUP_OVERWRITE" ]]; then
+                  run rm $VERBOSE_ARG "$backup"
+                fi
+                run mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
               fi
-              run mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
             fi
 
             if [[ -e "$targetPath" && ! -L "$targetPath" ]] && cmp -s "$sourcePath" "$targetPath" ; then

modules/files/check-link-targets.sh

@@ -30,6 +30,9 @@ for sourcePath in "$@" ; do
     if cmp -s "$sourcePath" "$targetPath"; then
       # First compare the files' content. If they're equal, we're fine.
       warnEcho "Existing file '$targetPath' is in the way of '$sourcePath', will be skipped since they are the same"
+    elif [[ ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_COMMAND" ]] ; then
+      # Next, try to run the custom backup command. Assume this always succeeds.
+      verboseEcho "Existing file '$targetPath' exists and differs from '$sourcePath'. `$HOME_MANAGER_BACKUP_COMMAND` will be used to backup the file."
     elif [[ ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
       # Next, try to move the file to a backup location if configured and possible
       backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
@@ -49,12 +52,13 @@ done
 
 if [[ ${#collisionErrors[@]} -gt 0 ]] ; then
   errorEcho "Please do one of the following:
-- Move or remove the files below and try again.
 - In standalone mode, use 'home-manager switch -b backup' to back up"\
 " files automatically.
-- When used as a NixOS or nix-darwin module, set"\
-" 'home-manager.backupFileExtension'"\
-" to, for example, 'backup' and rebuild.
+- When used as a NixOS or nix-darwin module, set either
+  - 'home-manager.backupFileExtension', or
+  - 'home-manager.backupCommand',
+  to move the file to a new location in the same directory, or run a"\
+" custom command.
 - Set 'force = true' on the related file options to forcefully overwrite"\
 " the files below. eg. 'xdg.configFile.\"mimeapps.list\".force = true'"
 

nix-darwin/default.nix

@@ -28,6 +28,9 @@ in
               ${lib.optionalString (
                 cfg.backupFileExtension != null
               ) "export HOME_MANAGER_BACKUP_EXT=${lib.escapeShellArg cfg.backupFileExtension}"}
+              ${lib.optionalString (
+                cfg.backupCommand != null
+              ) "export HOME_MANAGER_BACKUP_COMMAND=${lib.escapeShellArg cfg.backupCommand}"}
               ${lib.optionalString cfg.overwriteBackup "export HOME_MANAGER_BACKUP_OVERWRITE=1"}
               ${lib.optionalString cfg.verbose "export VERBOSE=1"}
               exec ${usercfg.home.activationPackage}/activate --driver-version ${driverVersion} >&2

nixos/common.nix

@@ -81,6 +81,16 @@ in
       argument in Home Manager. This disables the Home Manager
       options {option}`nixpkgs.*`'';
 
+    backupCommand = mkOption {
+      type = types.nullOr (types.either types.str types.path);
+      default = null;
+      example = lib.literalExpression "''${pkgs.trash-cli}/bin/trash";
+      description = ''
+        On activation run this command on each existing file
+        rather than exiting with an error.
+      '';
+    };
+
     backupFileExtension = mkOption {
       type = types.nullOr types.str;
       default = null;

nixos/default.nix

@@ -12,7 +12,9 @@ let
 
   serviceEnvironment = lib.mkMerge [
     (mkIf cfg.verbose { VERBOSE = "1"; })
-
+    (mkIf (cfg.backupCommand != null) {
+      HOME_MANAGER_BACKUP_COMMAND = cfg.backupCommand;
+    })
     (mkIf (cfg.backupFileExtension != null) {
       HOME_MANAGER_BACKUP_EXT = cfg.backupFileExtension;
     })