diff --git a/contrib/check-code.py b/contrib/check-code.py
--- a/contrib/check-code.py
+++ b/contrib/check-code.py
@@ -94,6 +94,8 @@ utestpats = [
      "explicit exit code checks unnecessary"),
     (uprefix + r'set -e', "don't use set -e"),
     (uprefixc + r'( *)\t', "don't use tabs to indent"),
+    (uprefixc + r'.*do\s*true;\s*done',
+     "don't use true as loop body, use sleep 0"),
   ],
   # warnings
   []
diff --git a/tests/test-hup.t b/tests/test-hup.t
--- a/tests/test-hup.t
+++ b/tests/test-hup.t
@@ -11,7 +11,7 @@ Do test while holding fifo open
   $ (
   > echo lock
   > echo addchangegroup
-  > while [ ! -s .hg/store/journal ]; do true; done
+  > while [ ! -s .hg/store/journal ]; do sleep 0; done
   > kill -HUP $P
   > ) > p
 
diff --git a/tests/test-serve.t b/tests/test-serve.t
--- a/tests/test-serve.t
+++ b/tests/test-serve.t
@@ -14,7 +14,7 @@
   >    else
   >        kill `cat hg.pid`
   >    fi
-  >    while kill -0 `cat hg.pid` 2>/dev/null; do true; done
+  >    while kill -0 `cat hg.pid` 2>/dev/null; do sleep 0; done
   > }
 
   $ hg init test