run_null.sh: use file descriptors instead of temporary files to get the exit status of the first part of a pipeline

run_null.sh

@@ -17,14 +17,24 @@ while [ "$#" -gt 0 ]; do
 	shift
 done
 
-# subshell so that we can cd without effecting the rest
+# - Run command with fds 3 and 4 closed so that whatever test.sh does it
-(
+#   cannot interfere with these.
-	set +e
+# - Both stdin and stderr of test.sh are written to stdout
-	cd ./shared;
+# - Write exit status of test.sh to fd 3
-	$SUDO sh -x ./test.sh;
+# - Write stdout to shared/output.txt as well as to fd 4
-	echo $?;
+# - Redirect fd 3 to stdout
-) 2>&1 | tee shared/output.txt
+# - Read fd 3 and let the group exit with that value
-if [ "$(cat shared/exitstatus.txt)" -ne 0 ]; then
+# - Redirect fd 4 to stdout
+ret=0
+{ { { {
+        ret=0;
+        ( exec 3>&- 4>&-; env --chdir=./shared $SUDO sh -x ./test.sh 2>&1) || ret=$?;
+        echo $ret >&3;
+      } | tee shared/output.txt >&4;
+    } 3>&1;
+  } | { read -r xs; exit "$xs"; }
+} 4>&1 || ret=$?
+if [ "$ret" -ne 0 ]; then
 	echo "test.sh failed"
 	exit 1
 fi