Saturday, December 17, 2011

Setting environment variables for MSBuild Exec tasks

MSBuild has an <Exec> task for calling external programs, but (bafflingly) it doesn’t allow you to set the environment the program runs in. In my case, I need to run a Python script with certain directories in the PYTHONPATH.

The Short Way

On Unix machines, this is trivial:

PYTHONPATH=”~/foo” python

For Windows’ wonderful cmd.exe shell (which MSBuild uses to run Exec) it’s a little longer:

(set PYTHONPATH=C:\Foo) & python

If you want, you can chain multiple set commands together to set multiple variables:

(set PYTHONPATH=C:\Foo) & (set FOO=42) & python

To actually use this in the MSBuild file, you’ll need to escape it like so:

<Exec Command=”(set PYTHONPATH=C:\Foo) &amp; python” />

Getting the quoting right for <Exec> can be tricky; I use the <Message> task for debugging the command line. Remember to use &quot; instead of double quotes.

The Long Way

This method takes more typing but is a bit more clear, especially if you have multiple variables to set. Actually, it can be used to store whole batch files inside the MSBuild file, if necessary.

set FOO=42

<Exec Command="$(PythonExec)" />

A CDATA section is required because the newlines need to be preserved. When running an <Exec> task, all MSBuild does is write the contents of Command to a temporary batch file an execute. This just provides more than the usual single line command.