diff --git a/flake.nix b/flake.nix
index 9e176d1b..20f5f94c 100644
--- a/flake.nix
+++ b/flake.nix
@@ -82,25 +82,6 @@
       in {
         inherit lib;
 
-        apps =
-          computedOutputs.apps.${system}
-          // {
-            run-test = let
-              testScript = pkgs.writeShellApplication {
-                name = "test-templates";
-                runtimeInputs = [
-                  inputs.nickel.packages."${system}".nickel-lang-cli
-                  pkgs.parallel
-                  pkgs.gnused
-                ];
-                text = builtins.readFile ./run-test.sh;
-              };
-            in {
-              type = "app";
-              program = pkgs.lib.getExe testScript;
-            };
-          };
-
         checks.alejandra = pkgs.runCommand "check-alejandra" {} ''
           ${pkgs.lib.getExe pkgs.alejandra} --check ${self}
           touch $out
diff --git a/lib/builders.ncl b/lib/builders.ncl
index ce90c208..ad3aa96e 100644
--- a/lib/builders.ncl
+++ b/lib/builders.ncl
@@ -1,4 +1,4 @@
-let { NickelDerivation, Derivation, NixString, .. } = import "contracts.ncl" in
+let { NickelDerivation, Derivation, NixString, NullOr, .. } = import "contracts.ncl" in
 
 let lib = import "lib.ncl" in
 
@@ -8,6 +8,15 @@ let concat_strings_sep = fun sep values =>
   else
     std.array.reduce_left (fun acc value => nix-s%"%{acc}%{sep}%{value}"%) values
 in
+
+let MutExclusiveWith = fun other name other_name label value =>
+  if value == null && other == null then
+    std.fail_with "You must specify either %{name} or %{other_name} field"
+  else if value != null && other != null then
+    std.fail_with "You can set only one of %{name} or %{other_name} fields"
+  else
+    value
+in
 {
   NickelPkg
   # we should only need two '%%', but a current Nickel bug (#XXX) bug makes the example being
@@ -84,4 +93,75 @@ in
         structured_env.buildInputs = packages,
       }
         | (NickelPkg & { packages | { _ : Derivation } }),
+
+  ShellApplication
+    | doc m%"
+    "%
+    =
+      {
+        name,
+        version | default = "0.0.0",
+        text | NullOr NixString | MutExclusiveWith file "text" "file" | default = null,
+        file | NullOr String | default = null,
+        runtime_inputs | { _ : Derivation } = {},
+        shell_binary | NixString | default = nix-s%"%{lib.import_nix "nixpkgs#bash"}/bin/bash"%,
+
+        build_command = {
+          cmd = nix-s%"%{lib.import_nix "nixpkgs#bash"}/bin/bash"%,
+          args = [
+            "-c",
+            nix-s%"
+              set -euo pipefail
+              source .attrs.sh
+              PATH="%{lib.import_nix "nixpkgs#coreutils"}/bin:$PATH"
+              mkdir -p "${outputs[out]}/bin"
+              target="${outputs[out]}/bin/${name}"
+              %{if text != null then
+                m%"cat > "$target" <<<"$script""%
+                else
+                m%"(cat <<<"$scriptHeader"; cat "$scriptPath") > "$target""%
+              }
+              chmod +x "$target"
+              "%{shell_binary}" -n -O extglob "$target"
+              "%{lib.import_nix "nixpkgs#shellcheck"}/bin/shellcheck" "$target"
+            "%
+          ],
+        },
+        nix_drv =
+          let path_line | NixString =
+              if std.record.is_empty runtime_inputs then
+                ""
+              else
+                let paths =
+                  runtime_inputs
+                  |> std.record.values
+                  |> std.array.map (fun s => nix-s%"%{s}/bin"%)
+                  |> concat_strings_sep ":"
+                in
+                nix-s%"export PATH="%{paths}:$PATH""%
+            in
+          let script_header | NixString = nix-s%"
+            #!%{shell_binary}
+            set -o errexit
+            set -o nounset
+            set -o pipefail
+            %{path_line}
+          "%
+            in
+          if text != null then
+            # text != null && file == null
+            {
+              script | NixString = nix-s%"
+                    %{script_header}
+                    %{text}
+                  "%,
+            }
+          else
+            # file != null && text == null
+            {
+              scriptPath | NixString = lib.import_file file,
+              scriptHeader = script_header,
+            },
+      }
+        | NickelPkg,
 }
diff --git a/project.ncl b/project.ncl
index bdffa328..d8b609f8 100644
--- a/project.ncl
+++ b/project.ncl
@@ -12,5 +12,21 @@ let import_nix = organist.lib.import_nix in
         gnused = import_nix "nixpkgs#gnused",
       },
     },
+
+  flake.apps.run-test =
+    let testScript | organist.builders.ShellApplication = {
+        name = "run-test.sh",
+        file = "./run-test.sh",
+        runtime_inputs = {
+          nickel = organist.lib.import_nix "nickel#nickel-lang-cli",
+          parallel = import_nix "nixpkgs#parallel",
+          gnused = import_nix "nixpkgs#gnused",
+        },
+      }
+      in
+    {
+      type = "app",
+      program | organist.contracts.NixString = nix-s%"%{testScript}/bin/run-test.sh"%
+    },
 }
   | organist.contracts.OrganistExpression