cppo detects invalid SHK rather than creating empty folder

scripts/tools/cppo.txt

@@ -1169,15 +1169,21 @@ if g.SHK:
         targetDir = (args[3] if g.extractFile else args[2])
     unshkdir = ("/tmp/cppo-" + str(uuid.uuid4()))
     makedirs(unshkdir)
-    if not os.system("cd " + unshkdir + "; " +
-                     "nulib2 -xse " +
-                     os.path.abspath(g.imageFile) + " " +
-                     (args[2].replace('/', ':') if g.extractFile else "") +
-                     " | grep -q 'no records match' > /dev/null"):
+    result = os.system("/bin/bash -c 'cd " + unshkdir + "; " +
+                       "result=$(nulib2 -xse " + os.path.abspath(g.imageFile) +
+                       ((" " + args[2].replace('/', ':'))
+                       if g.extractFile else "") + " 2> /dev/null); " +
+                       "if [[ $result == \"Failed.\" ]]; then exit 3; " +
+                       "else if grep -q \"no records match\" <<< \"$result\"" +
+                       " > /dev/null; then exit 2; else exit 0; fi; fi'")
+    if (result == 512):
         print(
           "File not found in ShrinkIt archive. Try cppo -cat to get the path,")
         print("  and omit any leading slash or colon.")
         sys.exit(1)
+    elif (result != 0):
+        print("ShrinkIt archive is invalid, or some other problem happened.")
+        sys.exit(1)
     if g.extractFile:
         g.extractFile = g.extractFile.replace(':', '/')
         extractPath = (unshkdir + "/" + g.extractFile)