diff --git a/packages/index.json b/packages/index.json
index 17cb9fb..df8bbdf 100644
--- a/packages/index.json
+++ b/packages/index.json
@@ -1,6 +1,6 @@
 {
   "packages": {
-    "trapos-core": "0.6.2",
+    "trapos-core": "0.6.3",
     "trapos-test": "0.2.1",
     "trapos-boot": "0.3.3",
     "trapos-net": "0.3.0",
diff --git a/packages/trapos-core/ccpm.json b/packages/trapos-core/ccpm.json
index 8782fa2..45cef32 100644
--- a/packages/trapos-core/ccpm.json
+++ b/packages/trapos-core/ccpm.json
@@ -1,6 +1,6 @@
 {
   "name": "trapos-core",
-  "version": "0.6.2",
+  "version": "0.6.3",
   "description": "TrapOS base: package manager, event loop, upgrade and event tools",
   "dependencies": [],
   "files": [
diff --git a/programs/trapos-postinstall.lua b/programs/trapos-postinstall.lua
index 9c3bf26..cf9a74a 100644
--- a/programs/trapos-postinstall.lua
+++ b/programs/trapos-postinstall.lua
@@ -47,8 +47,7 @@ local version = readManifestVersion();
 local configuredSandboxUrl = false;
 
 if isBlank(settings.get(SANDBOX_URL_SETTING)) then
-  settings.set(SANDBOX_URL_SETTING, DEFAULT_SANDBOX_URL);
-  settings.save();
+  shell.run('set', SANDBOX_URL_SETTING, DEFAULT_SANDBOX_URL);
   configuredSandboxUrl = true;
 end
 
diff --git a/tests/postinstall.lua b/tests/postinstall.lua
index b42eaa5..dedfc30 100644
--- a/tests/postinstall.lua
+++ b/tests/postinstall.lua
@@ -7,27 +7,29 @@ local DEFAULT_SANDBOX_URL = 'wss://trapos.trapcloud.fr';
 local function fakeSettings(values)
   return {
     values = values or {},
-    saved = 0,
     get = function(self, key)
       return self.values[key];
     end,
-    set = function(self, key, value)
-      self.values[key] = value;
-    end,
-    save = function(self)
-      self.saved = self.saved + 1;
-    end,
   };
 end
 
 local function runPostinstall(settingsLib)
   local lines = {};
+  local shellRuns = {};
   local env = setmetatable({
     require = require,
     settings = {
       get = function(key) return settingsLib:get(key); end,
-      set = function(key, value) settingsLib:set(key, value); end,
-      save = function() settingsLib:save(); end,
+    },
+    shell = {
+      run = function(...)
+        local args = { ... };
+        shellRuns[#shellRuns + 1] = args;
+        if args[1] == 'set' and args[2] then
+          settingsLib.values[args[2]] = args[3];
+        end
+        return true;
+      end,
     },
     print = function(line)
       lines[#lines + 1] = line or '';
@@ -38,7 +40,7 @@ local function runPostinstall(settingsLib)
   if not chunk then error(loadErr, 0); end
   local ok, err = pcall(chunk);
   if not ok then error(err, 0); end
-  return lines;
+  return lines, shellRuns;
 end
 
 local function containsLine(lines, expected)
@@ -50,20 +52,23 @@ end
 
 testlib.test('postinstall defaults missing sandbox url', function()
   local settingsLib = fakeSettings();
-  local lines = runPostinstall(settingsLib);
+  local lines, shellRuns = runPostinstall(settingsLib);
 
   testlib.assertEquals(settingsLib.values['sandbox.url'], DEFAULT_SANDBOX_URL);
-  testlib.assertEquals(settingsLib.saved, 1);
+  testlib.assertEquals(#shellRuns, 1);
+  testlib.assertEquals(shellRuns[1][1], 'set');
+  testlib.assertEquals(shellRuns[1][2], 'sandbox.url');
+  testlib.assertEquals(shellRuns[1][3], DEFAULT_SANDBOX_URL);
   testlib.assertTrue(containsLine(lines, '=> Configured sandbox.url to ' .. DEFAULT_SANDBOX_URL .. '.'));
   testlib.assertTrue(containsLine(lines, '=> Sandbox password not set.'));
 end);
 
 testlib.test('postinstall treats empty sandbox url as missing', function()
   local settingsLib = fakeSettings({ ['sandbox.url'] = '' });
-  runPostinstall(settingsLib);
+  local _, shellRuns = runPostinstall(settingsLib);
 
   testlib.assertEquals(settingsLib.values['sandbox.url'], DEFAULT_SANDBOX_URL);
-  testlib.assertEquals(settingsLib.saved, 1);
+  testlib.assertEquals(#shellRuns, 1);
 end);
 
 testlib.test('postinstall preserves existing sandbox url and password', function()
@@ -71,10 +76,10 @@ testlib.test('postinstall preserves existing sandbox url and password', function
     ['sandbox.url'] = 'wss://custom.example',
     ['sandbox.password'] = 'secret',
   });
-  local lines = runPostinstall(settingsLib);
+  local lines, shellRuns = runPostinstall(settingsLib);
 
   testlib.assertEquals(settingsLib.values['sandbox.url'], 'wss://custom.example');
-  testlib.assertEquals(settingsLib.saved, 0);
+  testlib.assertEquals(#shellRuns, 0);
   testlib.assertTrue(containsLine(lines, '=> Keeping existing sandbox.url.'));
   testlib.assertTrue(containsLine(lines, '=> Sandbox password already configured.'));
 end);