diff --git a/usr/lib/python3.8/spoc/container.py b/usr/lib/python3.8/spoc/container.py
index 1fc5f3c..023141c 100644
--- a/usr/lib/python3.8/spoc/container.py
+++ b/usr/lib/python3.8/spoc/container.py
@@ -170,7 +170,7 @@ class Container:
         subprocess.Popen(['lxc-stop', '-P', CONTAINERS_DIR, self.name])
         self.await_state(STATE_STOPPED)
 
-    def execute(self, cmd, check=False):
+    def execute(self, cmd, **kwargs):
         # TODO: Allow to pass UID/GID
         # If the container is starting or stopping, wait until the operation is finished
         state = self.get_state()
@@ -182,8 +182,17 @@ class Container:
             state = self.get_state()
         # If the container is stopped, use lxc-execute, otherwise use lxc-attach
         if state == STATE_STOPPED:
-            return subprocess.run(['lxc-execute', '-P', CONTAINERS_DIR, self.name, '--']+cmd, check=check)
+            return subprocess.run(['lxc-execute', '-P', CONTAINERS_DIR, self.name, '--']+cmd, **kwargs)
         elif state == STATE_RUNNING:
-            return subprocess.run(['lxc-attach', '-P', CONTAINERS_DIR, '--clear-env', self.name, '--']+cmd, check=check)
+            return subprocess.run(['lxc-attach', '-P', CONTAINERS_DIR, '--clear-env', self.name, '--']+cmd, **kwargs)
         else:
             raise InvalidContainerStateError(self.name, state)
+
+    def get_uidgid(self, user, group=None):
+        # Helper function to get UID/GID of an user/group from the container
+        uid_entry = self.execute(['/usr/bin/getent', 'passwd', user], capture_output=True, check=True).stdout.decode().split(':')
+        if group:
+            gid_entry = self.execute(['/usr/bin/getent', 'group', group], capture_output=True, check=True).stdout.decode().split(':')
+            return (uid_entry[2], gid_entry[2])
+        else:
+            return (uid_entry[2], uid_entry[3])
diff --git a/usr/lib/python3.8/spoc/imagebuilder.py b/usr/lib/python3.8/spoc/imagebuilder.py
index 411e9d0..16e19df 100644
--- a/usr/lib/python3.8/spoc/imagebuilder.py
+++ b/usr/lib/python3.8/spoc/imagebuilder.py
@@ -52,7 +52,7 @@ class ImageBuilder:
             self.set_env(*args.split(None, 1))
         elif 'USER' == directive:
             # Sets init UID / GID
-            self.image.uid,self.image.gid = args.split() #TODO: Get UID/GID by name + get GID automatically as primary UID group if only UID is given
+            self.set_uidgid(*args.split())
         elif 'CMD' == directive:
             # Sets init command
             self.image.cmd = args
@@ -82,11 +82,12 @@ class ImageBuilder:
         container.set_definition(self.image.get_definition())
         container.build = True
         container.create()
-        container.execute(['/bin/sh', '-lc', os.path.join('/', script_name)], True)
+        container.execute(['/bin/sh', '-lc', os.path.join('/', script_name)], check=True)
         container.destroy()
         os.unlink(script_path)
 
     def set_env(self, key, value=None):
+        # Set or unset environement variable
         if value:
             self.image.env[key] = value
         else:
@@ -95,6 +96,18 @@ class ImageBuilder:
             except KeyError:
                 pass
 
+    def set_uidgid(self, uid, gid=''):
+        # Set UID/GID for init
+        if not uid.isdigit() or not gid.isdigit():
+            # Resolve the UID/GID from container if either of them is entered as string
+            container = Container(self.image.name, False)
+            container.set_definition(self.image.get_definition())
+            container.create()
+            uid,gid = container.get_uidgid(uid, gid)
+            container.destroy()
+        self.image.uid = uid
+        self.image.gid = gid
+
     def copy_files(self, src, dst):
         # Copy files from the host or download them from a http(s) URL
         dst = os.path.join(self.image.layer_path, dst.lstrip('/'))