patch 7.4.1514
Problem:    Channel output to file not implemented yet.
Solution:   Implement it for Unix.
diff --git a/src/testdir/test_channel_pipe.py b/src/testdir/test_channel_pipe.py
index 2097d3e..d5da687 100644
--- a/src/testdir/test_channel_pipe.py
+++ b/src/testdir/test_channel_pipe.py
@@ -21,10 +21,13 @@
         if typed.startswith("echo "):
             print(typed[5:-1])
             sys.stdout.flush()
-        if typed.startswith("echoerr"):
-            print(typed[8:-1], file=sys.stderr)
-            sys.stderr.flush()
-        if typed.startswith("double"):
+        if typed.startswith("double "):
             print(typed[7:-1] + "\nAND " + typed[7:-1])
             sys.stdout.flush()
+        if typed.startswith("echoerr "):
+            print(typed[8:-1], file=sys.stderr)
+            sys.stderr.flush()
+        if typed.startswith("doubleerr "):
+            print(typed[10:-1] + "\nAND " + typed[10:-1], file=sys.stderr)
+            sys.stderr.flush()