I’ve been working on continuous integration and automated builds/deployment lately.  I put together a MSBuild script that does a whole bunch of typical stuff including getting code from my repository, compiling, and running tests.  I also want to create a Tag in my Subversion repository so I know which code was deployed.  That wasn’t too hard to do either.  The tricky part was that I want my build to fail if I try to make a Tag that already exists!

I fumbled around with this for a while trying a wide variety of ideas.  In the end, it turned out to be pretty simple.  There may be better ways to accomplish this, but here is how I am doing it…

I’m using the SvnClient task (it’s part of the MSBuild.Community.Tasks download) and executing a command:  “ls $(SvnRepository)/tags/$(SvnTagName)”.  Of course, my MSBuild script already has the variables $(SvnRepository) which is the path to my repository and $(SvnTagName) which is the name of the Tag I hope to create.  You may have to tweak this stuff depending on the layout of your SVN repository.  The command “ls” will actually list the contents of my Tag.  Since I know my tag just has a few folders in it (it isn’t a recursive task), it isn’t too harmful if it succeeds.  If the Tag does not exist in my repository, the command actually fails.  Oddly, I actually want this task to fail!  Yes, in my case, I am hoping the task does not find the Tag with the specified name.  When the task is executed, it returns an ExitCode which is 0 for success or 1 for failure.  Next I just check the returned code and if it is successful I generate an error.  Sure this feels a bit odd.  I don’t love using this kind of logic, hoping for an error to occur. I’d much rather have some subversion command I could execute to see if the Tag exists but I couldn’t find anything like that.  Here is the actual xml from my Build file to check for the Tag:

<Target Name="CheckSvn">
  <Message Text="*** Checking to see if the tag already exists"/>
  <!-- this task will look up the tag supplied.  
    If it exists it will return a code of 0 
    Technically this is a success but for this instance, 
    it is a failure because we don't want to find it! -->
  <SvnClient Command="ls $(SvnRepository)/tags/$(SvnTagName)" 
             Username="xxx" password="xxx" ContinueOnError="true">
    <Output TaskParameter="ExitCode" PropertyName="CommandResult"/>
  </SvnClient>
  <Message Text="*** ExitCode = $(CommandResult)"/>
  <Error Condition="$(CommandResult)=='0'" Text="This Tag already exists!"/>
  <Message Text="*** Finished Checking to see if the tag already exists"/>
</Target>

Working with MSBuild is becoming a love/hate relationship for me.  I really love the power of what you can do to automate tasks.  But it can be a real pain to write, test, and debug these scripts.  I hope my sharing this code will make your life a little easier.

3 thoughts on “Using MSBuild to Check if a Subversion Tag Exists

  1. "Working with MSBuild is becoming a love/hate relationship for me. I really love the power of what you can do to automate tasks. But it can be a real pain to write, test, and debug these scripts."

    I couldn’t agree more!!!

    Thanks very much for the post – it has helped.

    Reply
  2. Thanks Travis! Yes, I guess I could have done a custom MSBuild Task. Not long after I wrote the MSBuild script shown above, I did start working with a custom MSBuild Task for something else. I didn’t realize that they are actually pretty easy to write! Maybe I’ll revisit this later as a custom build task. For now, it is getting the job done, even if it isn’t perfect.

    -Andy

    Reply
  3. Andy, you could pretty easily do the same sort of thing in a custom MSBuild task – that way you wouldn’t be "hoping for errors." Your MSBuild task could "shell out" to run any svn command and parse the output, and respond accordingly. There are examples in the download from a previous Code Camp talk I gave on the topic. http://www.datadeluxe.com/FreeStuff.aspx

    Reply

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