How can I execute programs in my %PATH% with MSBui

Note: I'm using Mercurial as an example here, because that's what I'm trying to get to work with MSBuild right now.
But the problem is not limited to Mercurial, it happens with every external program that is somewhere in my %PATH% variable (I tried the same with PowerShell, for example).
So I didn't put the Mercurial tag on this question on purpose, because this is not about Mercurial!

What I actually want to do:
I want my build script to get the current revision number from my Mercurial repository and store it in a file.
The simplest way to do this from the command line is:

hg id -i >rev.txt

Mercurial is installed on my machine and the installation folder is in my %PATH% variable.
So I can run this line from anywhere on my machine (directly from the command line, or from a batch file), and it just works.

The problem occurs when I try to run this line from my build script.
I change the BeforeBuild (or AfterBuild) section of my .csproj file as follows:

<Target Name="AfterBuild">
     <Exec Command="hg id -i >rev.txt"/>
</Target>

When I compile my solution with Visual Studio, it works and the rev.txt file is created in the folder where my .csproj is.

But when I compile the exact same solution from the command line with MSBuild, the build fails with the following error message:

The command "hg id -i >rev.txt" exited with code 9009.

I googled "msbuild code 9009" and found some solutions, but all of them propose to provide the full path to the executable.
When I do this, the build succeeds with MSBuild as well.
But this is not an acceptable solution for me, because I can't be sure that everyone using my project (including the build server) has installed Mercurial in the exact same folder.
That's exactly what %PATH% is for...

The same happens when I put the <Exec Command="... line directly into the build script.
If I specify the path to the executable, it works.
If I don't specify the path, it doesn't.

Is there any trick to make MSBuild execute programs in my %PATH% variable without specifying the complete folder?

EDIT:

@leppie:

Output redirection:
You mean the fact that I save the output of my command in a text file inside the command , instead of just running hg id -i as a command and using an output parameter or something like that to get the output?
Doesn't make any difference...the error is the same when I omit >rev.txt.

Command line args:
No, it throws the same error, even if I shorten the command to just hg (without any parameters).

Don't forget: if I run the exact same Exec command in the exact same .csproj file from Visual Studio, or if I just provide the path to the .exe file in the command, everything works.
So IMO output redirection and command line args can't be the problem.

回答1:

Have you tried this extension pack for mercurial/msbuild? http://msbuildhg.codeplex.com/documentation

Seems to have a task for returning revision id, which is what your trying to achieve no?

<HgVersion LocalPath="$(MSBuildProjectDirectory)" Timeout="5000">
     <Output TaskParameter="Revision" PropertyName="AssemblyRevision" />
</HgVersion>

回答2:

Okay, I found the solution.
I have to admit, it was a classic case of PEBKAC :-)

I'll explain it anyway, maybe it will help someone who made the same mistake:

Basically everything I have tried (plus what James Woolfenden suggested in his answer) would have been worked...if only the batch file that I use to run the build script wouldn't have looked like this:

path="%windir%\Microsoft.net\Framework\v4.0.30319"

msbuild build.proj

Yes, exactly.
I'm editing the %PATH% variable for the duration of this batch file, and I'm overwriting it with the path to MSBuild instead of just appending it.

So when my build script tries to call Mercurial, it can't find it anymore because its location is not in the %PATH% variable anymore.
No idea why I didn't see this before.

The correct way would be to append the MSBuild path, leaving the other paths intact:

path=%path%;%windir%\Microsoft.net\Framework\v4.0.30319