Monday, 10 January 2011

Intriguing SCHTASKS Bug

To get our system up and running quickly we chose to use the built-in Windows task scheduler to invoke our nightly batch. During initial development we added a couple of tasks by hand using the Scheduled Tasks UI, but as we got closer to our release date we created a batch file to script creation of the scheduled tasks so that we had a formal (and version controlled) deployment process, e.g.


The script was written and tested on a Windows XP desktop machine, but when we came to test it on our Windows server the script failed with the following error:-

ERROR: The creation of the scheduled task failed.
Reason: The Task Name may not contain the characters: <, >, :, /, \,|.

This was most curious as the server was running Windows Server 2003 (which is the same family as XP) and the task names were all pretty simple using just a number, dash and short name, e.g. “01-EstablishValueDate” – there were no out-of-the-ordinary characters at all. Naturally I took the dash[*] out first – no luck. Then the numbers – nope. Finally I resorted to a binary search to try and pin down exactly which character(s) it was…

It turned out to be the letter ‘D’ at the start of the word “Date”! The first three tasks I tried all had the word “Date” in them somewhere and they all worked after just removing the letter ‘D’. To be more specific it was a capital letter D as a lower case ‘d’ worked! This became our temporarily workaround whilst I did a little Googling in the background as I just couldn’t entertain the idea that this was the real problem; I’ll always assume I’m doing something wrong until Occam's Razor tells me it’s not me :-). Fortunately I quickly discovered the following forum post “Error In Schtasks Command Line” where there author was having no joy in creating a scheduled task called “Defrag”. Sadly it doesn’t say they resorted to just calling their task “defrag” as a workaround. Anyway a short time later I managed to discover an MSDN article that appeared to confirm the behaviour. In the KB article it says the following:-

The Schtasks command verifies the task name by comparing the task name with an invalid character set. If the task name contains one or more characters from the invalid character set, the Schtasks command denies the task creation request.

However, in race conditions, the invalid character set may contain a valid character. Therefore, the Schtasks command cannot create some tasks even if the task names contain only valid characters.

My mind boggles at what the SCHTASKS code is doing to create such a race condition? And why just the letter D?

[*] A common source of error is pasting command lines out of Word documents or web pages into console windows; this often results in curious error messages from the application. The problem is the “clever” document editor converting a dash (en dash, minus sign, etc) into a proper hyphen (em dash) or equivalent Unicode character at a different code point from the ASCII one. It may look like a dash to you and me but the “computer says no”.


  1. A repeatable error doesn't sound like a race condition to me, sounds like one of your actual bugs.

    Last time I was doing something like this, all we had was 'at'. If you really couldn't change the name, would that do instead?

  2. It's not much of a race is it if the same person always wins :-)