Tag Archives : PowerShell

An Architecture Aware VsVars.ps1

Like many in the Microsoft dev community, I’m a heavy user of Visual Studio and Powershell. And so, of course, I’ve been a heavy user Chris Tavares’ vsvars32.ps1 script. However, recently I needed the ability to specify my desired processor architecture when setting up a VS command line session. Unfortunately, Chris’s script wraps vsvars32.bat which only supports generating 32-bit apps. Luckily, VC++ includes a vcvarsall.bat script that let’s you specify processor architecture. So I updated my local copy of vsvars.ps1 to use vcvarsall.bat under the hood and added an -x64 switch to enable setting up a 64-bit command line environment. Vcvarsall.bat supports a variety of additional options, but 64-bit support is all I needed so that’s all I added. I didn’t change the name of the script because there’s WAY too much muscle memory associated with typing “vsvars” to bother changing that now.

If you want it, you can get my architecture aware version of vsvars.ps1 from my OneDrive here: http://1drv.ms/1kf8g9I.

Variadic Powershell Functions With Optional Named Params

I’ve been doing a little CPython coding lately. Even though I left the IronPython team a while ago (and IronPython is now under new management) I’m still still a big fan of the Python language and it’s great for prototyping.

However, one thing I don’t like about Python is how it uses the PYTHONPATH environment variable. I like to keep any non-standard library dependencies in my project folder, but then you have to set the PYTHONPATH environment variable in order for the Python interpreter to resolve those packages. Personally, I wish there was a command line parameter for specifying PYTHONPATH – I hate having to modify the environment in order to execute my prototype. Yes, I realize I don’t have to modify the machine-wide environment – but I would much prefer a stateless approach to an approach that requires modification of local shell state.

I decided to build a Powershell script that takes allows the caller to invoke Python while specifying the PYTHONPATH as a parameter. The script saves off the current PYTHONPATH, sets it to the passed in value, invokes the Python interpreter with the remaining script parameters, then sets PYTHONPATH back to its original value. While I was at it, I added the ability to let the user optionally specify which version of Python to use (defaulting to the most recent) as well as a switch to let the caller chose between invoking python.exe or pythonw.exe.

The details of the script are fairly mundane. However, building a Powershell script that supported optional named parameters and collected all the unnamed arguments together in a single parameter took a little un-obvious Powershell voodoo that I thought was worth blogging about.

I started with the following param declaration for my function

param (
    [string] $LibPath="", 
    [switch] $WinApp, 
    [string] $PyVersion=""

These three named parameters control the various features of my Python Powershell script. Powershell has an automatic variable named $args that holds the arguments that don’t get bound to a named argument. My plan was to pass the contents of the $args parameter to the Python interpreter. And that plan works fine…so long as none of the non-switch parameters are omitted.

I mistakenly (and in retrospect, stupidly) thought that since I had provided default values for the named parameters, they would only bind to passed-in arguments by name. However, Powershell binds non-switch parameters by position if the names aren’t specified . For example, this is the command line I use to execute tests from the root of my prototype project:

cpy -LibPath .Libsite-packages .Scriptsunit2.py discover -s .src

Obviously, the $LibPath parameter gets bound to the “.Libsite-package” argument. However, since $PyVersion isn’t specified by name, it gets bound by position and picks up the “.Scriptsunit2.py” argument. Clearly, that’s not what I intended – I want “.Scriptsunit2.py” along with the remaining arguments to be passed to the Python interpreter while the PyVersion parameter gets bound to its default value.

What I needed was more control over how incoming arguments are bound to parameters. Luckily, Powershell 2 introduced Advanced Function Parameters which gives script authors exactly that kind of control over parameters binding. In particular, there are two custom attributes for parameters that allowed me to get the behavior I wanted:

  • Position – allows the script author to specify what positional argument should be bound to the parameter. If this argument isn’t specified, parameters are bound in the order they appear in the param declaration
  • ValueFromRemainingArguments – allows the script author to specify that all remaining arguments that haven’t been bound should be bound to this parameter. This is kind of like the Powershell equivalent of params in C# or the ellipsis in C/C++.

A little experimentation with these attributes yielded the following solution:

param (
    [string] $LibPath="", 
    [switch] $WinApp, 
    [string] $PyVersion="",
    [parameter(Position=0, ValueFromRemainingArguments=$true)] $args

Note, the first three parameters are unchanged. However, I added an explicit $args parameter (I could have named it anything, but I had already written the rest of my script against $args) with the Position=0 and ValueFromRemainingArguments=$true parameter attribute values.The combination of these two attribute values means that the $args parameter is bound to an array of all the positional (aka unnamed) incoming arguments, starting with the first position. In other words – exactly the behavior I wanted.

Not sure how many people need a Powershell script that sets PYTHONPATH and auto-selects the latest version of Python, but maybe someone will find it useful. Also, I would think this approach to variadic functions with optional named parameters could be useful in other scenarios where you are wrapping an existing tool or utility in PowerShell, but need the ability to pass arbitrary parameters thru to the tool/utility being wrapped.

Fixing Powershell’s Busted Resolve-Path Cmdlet

Usually, my PowerShell posts are effusive in their praise. However, who thought up this “feature” gets no praise from me:
PS»Resolve-Path ~missing.file
Resolve-Path : Cannot find path 'C:Usershpiersonmissing.file' because it does not exist.
In my opinion, this is a bad design. Resolve-Path assumes that if the filename being resolved doesn’t exist, then it must be an error. But in the script I’m building, I’m resolving the path of a file that I’m going to create. In other words, I know a priori that the file doesn’t exist. Yet Resolve-Path insists on throwing an error. I would have expected there to be some switch you could pass to Resolve-Path telling it to skip path validation, but there’s not.
And the worst thing is, I can see that Resolve-Path came up with the “right” answer – it’s right there in the error message!
Searching around, I found a thread where someone else was having the same problem. Jeffrey Snover – aka Distinguished Engineer, inventor of Powershell and target of Erik Meijer’s Lang.NET coin throwing stunt – suggested using –ErrorAction and –ErrorVariable to ignore the error and retrieve the resolved path from the TargetObject property error variable. Like Maximilian from the thread, using this approach feels fragile and frankly kinda messy, but I needed a solution. So I wrote the following function that wraps up access to the error variable so at least I don’t have fragile messy code sprinkled through out my script.
function force-resolve-path($filename)
  $filename = Resolve-Path $filename -ErrorAction SilentlyContinue
                                     -ErrorVariable _frperror
  if (!$filename)
    return $_frperror[0].TargetObject
  return $filename
The script is pretty straightforward. –ErrorAction SilentlyContinue is PowerShell’s version of On Error Resume Next in Visual Basic. If the cmdlet encounters an error, it gets stashed away in the variable specified by ErrorVariable (it’s also added to $Error so you can still retrieve the error object if ErrorVariable isn’t specified) and continues processing. Then I manually check to see if resolve-path succeeded – i.e. did it return a value – and return the TargetObject of the Error object if it didn’t.
As I said, fragile and kinda messy. But it works.

Functions that Create Functions in Powershell

Since I started using Powershell, I’m very picky about what I let on my path. I feel it’s much cleaner to create aliases or functions rather than letting all kinds of crud creep into my path.

Recently, I installed the latest IronRuby release and discovered there’s a whole bunch of little batch file wrappers around common Ruby commands like gem and rake. While being able to simply type “igem” or “irake” is much easier than typing “ir “C:Program Filesironruby-0.6.0binigem””, I didn’t want to pollute my path – even with a product from my team. Instead, I wanted to create a Powershell function for each of those IronRuby-fied commands. Furthermore, I wanted to avoid manually creating a function for each Ruby command – these batchfiles are literally identical except for their name, so I figured it would be possible automate the function creation in Powershell. Here’s what I came up with:

$iralias = get-alias ir -EA SilentlyContinue
if ($iralias -eq $null) {return}

$irbindir = split-path $iralias.Definition

function make-rubyfunction($cmd)
  $cmdpath = join-path $irbindir $cmd
  set-item function:global:$cmd -Value {ir $cmdpath $args}.GetNewClosure()
  write-host "Added IronRuby $_ command"

("igem","iirb","irackup","irails","irake","irdoc","iri") |
  %{make-rubyfunction $_}

I start by getting the ir alias, which I’m setting in my traditional fashion. The Ruby command files are in the same directory as ir.exe, which is what ir is aliased to. If the ir alias isn’t set, I quit out of the script without setting anything.

The make-rubyfunction function is the primary workhorse of this script. You pass in a command name as a string, and it uses set-item on the function provider to create a new function. Note, I had to explicitly create this function in the global scope since I’m running the set-item cmdlet inside a script.

Getting the value for the function took a bit of head banging to figure out. I’m used to Python, which automatically closes over variables, so my first attempt was to set the function value to something like { ir $cmdpath $args }. But Powershell doesn’t close automatically, so that fails since $cmd isn’t defined inside the function. I asked around on the internal Powershell alias, and someone pointed me to the new GetNewClosure function in Powershell v2. In other words, Powershell only supports manual closures, which is kind of wonky, but works OK for this scenario. I create a new script block that references in-scope variable $cmdpath and GetNewClosure automatically creates a new script block where that value is captured and embedded. More info on GetNewClosure in the docs.

Now, I’m using Win7 exclusively at this point, so depending on a v2 feature didn’t bother me. However, if you’re using Powershell v1, you could still accomplish something similar using text substitution. Here’s my original (i.e. pre-GetNewClosure) version of make-rubyfunction

function make-rubyfunction($cmd)
  $cmdpath = join-path $irbindir $cmd
  $p = "ir `"$cmdpath`" `$args"
  set-item function:global:$cmd -Value $p
  write-host "Added IronRuby $_ command"

I’m using Powershell’s standard text substitution mechanism to create the function value as a string. Note that I’m escaping the dollar sign in $args, so that does not get substituted the way $cmdpath does. GetNewClosure feels cleaner, so that’s how I ended up doing it, but both ways seem to work fine.

Finally, I pass an array of IronRuby commands down the pipe to make-rubyfunction. I love the pipe command, though it feels strange to use parentheses instead of square brackets for list comprehensions like Python and F#!

Anyway, the script – as usual – is up on my SkyDrive. At some point, I want to do something similar for common IronPython scripts like pyc and ipydbg. Until then, hopefully someone out there will find it useful (like maybe the IronRuby team?).


I *LOVE* the new boot from VHD feature in Win7. I am primarily using it for doing some VS 2010 dogfooding without messing up my primary drive partition. But man, the process for setting up a VHD for booting is brutal. Scott Hanselman did a great job laying out the steps, but I wanted something a bit more productive.

First, I created a clean Win7 RC VHD and zipped it up for easy storage. The basic Win7 RC VHD is just under 5GB, but compresses down to about 1.5GB with 7-zip. I used the ImageX process Aviraj described though in the future I’ll use the Install-WindowsImage script. Install-WindowsImage is more convenient to use because it will list the indexes within a given .wim file instead of making you grovel thru an XML file like ImageX does. Also Install-WindowsImage is 27k download while ImageX is part of the 1.4 gigabyte Windows Automated Installation Kit. Look, I’m not hurting for bandwidth, but I don’t see the point of downloading 54442 times more data for a utility that isn’t as useful.

Once you’ve created the VHD, you need to update your Boot Configuration Data, or BCD for short, using the appropriately named BCDEdit utility. The process is fairly straight forward, if tedious. You have to run BCDEdit four times, copy the configuration GUID to the clipboard and type out the path to the VHD in a slightly funky syntax. Blech. So I built a PowerShell script to automate updating the BCD, called add-bcd-vhd. You can get it from my skydrive. Pass in the name of the BCD entry and the path to the VHD and add-bcd-vhd will do the rest.

I was whining on Twitter yesterday that there’s no PowerShell specific tools for managing the BCD data. Add-bcd-vhd just runs bcdedit behind the scenes and processes the text output with regular expressions. Ugly, but effective. I decided to spend some time trying accessing the BCD data from its WMI provider, but that turned out to be way too much of a hassle to be effective. If someone else out there knows how to use the BCD WMI provider from PowerShell, I’d appreciate some sample code.