stdin check with usage

TODO.norg

@@ -13,6 +13,7 @@ This is a list of tasks that would be cool to add.
 			- (x) Install script to system
 						Using update-alternatives is the most handy
 			- (x) Accept input from stdin when no file is given
+			- (x) Print error when no file and stdin is given
 			- ( ) Process only first N or last N Bytes
 			- ( ) Write a complete task for this stuff
 *** Networking

src/hexdumper.py

@@ -4,6 +4,7 @@ import os
 import sys
 import io
 import fcntl    # only for non blocking IO, if you dont know what it is, you can ignore it or study up
+import select   # to check if the file has data
 
 # 2**30 bytes aka 1 Mebibyte
 MAX_SIZE: int = 0x100000
@@ -69,6 +70,10 @@ def main():
         # i read somewhere that file.readline() does not work like this, but as
         # we are reading binary data anyways, I don't care.
         # check if the file is readable
+        if not select.select([file, ], [], [], 0.0)[0]:
+            print("no data from stdin, are you trying to dump a file?")
+            parser.print_usage()
+            sys.exit(1)
         if not file.peek(1):
             print("File is blocked, trying to wait...")
             file = open(args.file, "rb")