diff --git a/integration/buildfile_test.go b/integration/buildfile_test.go
index 0a8479e50a..2165fe3b06 100644
--- a/integration/buildfile_test.go
+++ b/integration/buildfile_test.go
@@ -154,12 +154,11 @@ RUN [ "$(/hello.sh)" = "hello world" ]
 FROM {IMAGE}
 
 # Make sure our defaults work
-RUN [ "$(id -u):$(id -g)" = '0:0' ]
-RUN [ "$(id -un):$(id -gn)" = 'root:root' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)" = '0:0/root:root' ]
 
 # TODO decide if "args.user = strconv.Itoa(syscall.Getuid())" is acceptable behavior for changeUser in sysvinit instead of "return nil" when "USER" isn't specified (so that we get the proper group list even if that is the empty list, even in the default case of not supplying an explicit USER to run as, which implies USER 0)
 USER root
-RUN [ "$(id -G) -- $(id -Gn)" = '0 -- root' ]
+RUN [ "$(id -G):$(id -Gn)" = '0:root' ]
 
 # Setup dockerio user and group
 RUN echo 'dockerio:x:1000:1000::/bin:/bin/false' >> /etc/passwd
@@ -167,62 +166,42 @@ RUN echo 'dockerio:x:1000:' >> /etc/group
 
 # Make sure we can switch to our user and all the information is exactly as we expect it to be
 USER dockerio
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 -- dockerio' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000:dockerio' ]
 
 # Switch back to root and double check that worked exactly as we might expect it to
 USER root
-RUN [ "$(id -u):$(id -g)" = '0:0' ]
-RUN [ "$(id -un):$(id -gn)" = 'root:root' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '0 -- root' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '0:0/root:root/0:root' ]
 
 # Add a "supplementary" group for our dockerio user
 RUN echo 'supplementary:x:1001:dockerio' >> /etc/group
 
 # ... and then go verify that we get it like we expect
 USER dockerio
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 1001 -- dockerio supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000 1001:dockerio supplementary' ]
 USER 1000
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 1001 -- dockerio supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000 1001:dockerio supplementary' ]
 
-# and finally, super test the new "user:group" syntax
+# super test the new "user:group" syntax
 USER dockerio:dockerio
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 -- dockerio' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000:dockerio' ]
 USER 1000:dockerio
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 -- dockerio' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000:dockerio' ]
 USER dockerio:1000
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 -- dockerio' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000:dockerio' ]
 USER 1000:1000
-RUN [ "$(id -u):$(id -g)" = '1000:1000' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:dockerio' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1000 -- dockerio' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1000/dockerio:dockerio/1000:dockerio' ]
 USER dockerio:supplementary
-RUN [ "$(id -u):$(id -g)" = '1000:1001' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:supplementary' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1001 -- supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1001/dockerio:supplementary/1001:supplementary' ]
 USER dockerio:1001
-RUN [ "$(id -u):$(id -g)" = '1000:1001' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:supplementary' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1001 -- supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1001/dockerio:supplementary/1001:supplementary' ]
 USER 1000:supplementary
-RUN [ "$(id -u):$(id -g)" = '1000:1001' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:supplementary' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1001 -- supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1001/dockerio:supplementary/1001:supplementary' ]
 USER 1000:1001
-RUN [ "$(id -u):$(id -g)" = '1000:1001' ]
-RUN [ "$(id -un):$(id -gn)" = 'dockerio:supplementary' ]
-RUN [ "$(id -G) -- $(id -Gn)" = '1001 -- supplementary' ]
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1000:1001/dockerio:supplementary/1001:supplementary' ]
+
+# make sure unknown uid/gid still works properly
+USER 1042:1043
+RUN [ "$(id -u):$(id -g)/$(id -un):$(id -gn)/$(id -G):$(id -Gn)" = '1042:1043/1042:1043/1043:1043' ]
 `,
 		nil,
 		nil,