feature: added support for substituting secrets into files

modules/age-home.nix

@@ -65,6 +65,12 @@ with lib; let
     ''}
   '';
 
+  substituteSecret = secretType:
+    builtins.concatStringsSep "\n" (builtins.map (file: ''
+        ${pkgs.gnused}/bin/sed -i "s#@${secretType.name}@#$(cat ${secretType.path})#" ${file}
+      '')
+      secretType.substitutions);
+
   testIdentities =
     map
     (path: ''
@@ -91,6 +97,11 @@ with lib; let
     ++ [cleanupAndLink]
   );
 
+  substituteSecrets = builtins.concatStringsSep "\n" (
+    ["echo '[agenix] substituting secrets...'"]
+    ++ (map substituteSecret (builtins.attrValues cfg.secrets))
+  );
+
   secretType = types.submodule ({
     config,
     name,
@@ -117,6 +128,14 @@ with lib; let
           Path where the decrypted secret is installed.
         '';
       };
+      substitutions = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          List of files to substitute the secret into.
+          WARNING: It is recommended to set `force = true` for files managed through home-manager.
+        '';
+      };
       mode = mkOption {
         type = types.str;
         default = "0400";
@@ -135,6 +154,7 @@ with lib; let
       text = ''
         ${newGeneration}
         ${installSecrets}
+        ${substituteSecrets}
         exit 0
       '';
     };

modules/age.nix

@@ -129,6 +129,17 @@ with lib; let
     ++ (map chownSecret (builtins.attrValues cfg.secrets))
   );
 
+  substituteSecret = secretType:
+    builtins.concatStringsSep "\n" (builtins.map (file: ''
+        ${pkgs.gnused}/bin/sed -i "s#@${secretType.name}@#$(cat ${secretType.path})#" ${file}
+      '')
+      secretType.substitutions);
+
+  substituteSecrets = builtins.concatStringsSep "\n" (
+    ["echo '[agenix] substituting secrets...'"]
+    ++ (map substituteSecret (builtins.attrValues cfg.secrets))
+  );
+
   secretType = types.submodule ({config, ...}: {
     options = {
       name = mkOption {
@@ -155,6 +166,13 @@ with lib; let
           Path where the decrypted secret is installed.
         '';
       };
+      substitutions = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        description = ''
+          List of files to substitute the secret into.
+        '';
+      };
       mode = mkOption {
         type = types.str;
         default = "0400";
@@ -298,6 +316,12 @@ in {
         text = "";
         deps = ["agenixChown"];
       };
+
+      # Substitute secrets into files.
+      system.activationScripts.agenixSubstitute = {
+        text = substituteSecrets;
+        deps = ["agenix" "etc"];
+      };
     })
     (optionalAttrs isDarwin {
       launchd.daemons.activate-agenix = {