How do I deal with Paths when writing a PowerShell Cmdlet?

Question

What is the proper way to receive a file as a parameter when writing a C# cmdlet? So far I just have a property LiteralPath (aligning with their parameter naming convention) that is a string. This is a problem because you just get whatever is typed into the console; which could be the full path or could be a relative path.

Using Path.GetFullPath(string) doesn't work. It thinks I'm currently at ~, I'm not. Same problem occurs if I change the property from a string to a FileInfo.

EDIT: For anyone interested, this workaround is working for me:

    SessionState ss = new SessionState();
    Directory.SetCurrentDirectory(ss.Path.CurrentFileSystemLocation.Path);

    LiteralPath = Path.GetFullPath(LiteralPath);

LiteralPath is the string parameter. I'm still interested in learning what is the recommended way to handle file paths that are passed as parameters.

EDIT2: This is better, so that you don't mess with the users current directory, you should set it back.

            string current = Directory.GetCurrentDirectory();
            Directory.SetCurrentDirectory(ss.Path.CurrentFileSystemLocation.Path);
            LiteralPath = Path.GetFullPath(LiteralPath);
            Directory.SetCurrentDirectory(current);

Answer 1

This is a surprisingly complex area, but I have a ton of experience here. In short, there are some cmdlets that accept win32 paths straight from the System.IO APIs, and these typically use a -FilePath parameter. If you want to write a well behaved "powershelly" cmdlet, you need -Path and -LiteralPath, to accept pipeline input and work with relative and absolute provider paths. Here's an excerpt from a blog post I wrote a while ago:

Paths in PowerShell are tough to understand [at first.] PowerShell Paths - or PSPaths, not to be confused with Win32 paths - in their absolute forms, they come in two distinct flavours:

• Provider-qualified: FileSystem::c:\temp\foo.txt
• PSDrive-qualified: c:\temp\foo.txt

It's very easy to get confused over provider-internal (The ProviderPath property of a resolved System.Management.Automation.PathInfo – the portion to the right of :: of the provider-qualified path above) and drive-qualified paths since they look the same if you look at the default FileSystem provider drives. That is to say, the PSDrive has the same name (C) as the native backing store, the windows filesystem (C). So, to make it easier for yourself to understand the differences, create yourself a new PSDrive:

ps c:\> new-psdrive temp filesystem c:\temp\
ps c:\> cd temp:
ps temp:\>

Now, let's look at this again:

• Provider-qualified: FileSystem::c:\temp\foo.txt
• Drive-qualified: temp:\foo.txt

A bit easier this time to see what’s different this time. The bold text to the right of the provider name is the ProviderPath.

So, your goals for writing a generalized provider-friendly Cmdlet (or advanced function) that accepts paths are:

• Define a LiteralPath path parameter aliased to PSPath
• Define a Path parameter (which will resolve wildcards / glob)
• Always assume you are receiving PSPaths, NOT native provider-paths (e.g. Win32 paths)

Point number three is especially important. Also, obviously LiteralPath and Path should belong in mutually exclusive parameter sets.

Relative Paths

A good question is: how do we deal with relative paths being passed to a Cmdlet. As you should assume all paths being given to you are PSPaths, let’s look at what the Cmdlet below does:

ps temp:\> write-zip -literalpath foo.txt

The command should assume foo.txt is in the current drive, so this should be resolved immediately in the ProcessRecord or EndProcessing block like (using the scripting API here to demo):

$provider = $null;
$drive = $null
$pathHelper = $ExecutionContext.SessionState.Path
$providerPath = $pathHelper.GetUnresolvedProviderPathFromPSPath(
    "foo.txt", [ref]$provider, [ref]$drive)

Now you everything you need to recreate the two absolute forms of PSPaths, and you also have the native absolute ProviderPath. To create a provider-qualified PSPath for foo.txt, use $provider.Name + “::” + $providerPath. If $drive is not $null (your current location might be provider-qualified in which case $drive will be $null) then you should use $drive.name + ":\" + $drive.CurrentLocation + "\" + "foo.txt" to get a drive-qualified PSPath.

Quickstart C# Skeleton

Here's a skeleton of a C# provider-aware cmdlet to get you going. It has built in checks to ensure it has been handed a FileSystem provider path. I am in the process of packaging this up for NuGet to help others get writing well-behaved provider-aware Cmdlets:

using System;
using System.Collections.Generic;
using System.IO;
using System.Management.Automation;
using Microsoft.PowerShell.Commands;
namespace PSQuickStart
{
    [Cmdlet(VerbsCommon.Get, Noun,
        DefaultParameterSetName = ParamSetPath,
        SupportsShouldProcess = true)
    ]
    public class GetFileMetadataCommand : PSCmdlet
    {
        private const string Noun = "FileMetadata";
        private const string ParamSetLiteral = "Literal";
        private const string ParamSetPath = "Path";
        private string[] _paths;
        private bool _shouldExpandWildcards;
        [Parameter(
            Mandatory = true,
            ValueFromPipeline = false,
            ValueFromPipelineByPropertyName = true,
            ParameterSetName = ParamSetLiteral)
        ]
        [Alias("PSPath")]
        [ValidateNotNullOrEmpty]
        public string[] LiteralPath
        {
            get { return _paths; }
            set { _paths = value; }
        }
        [Parameter(
            Position = 0,
            Mandatory = true,
            ValueFromPipeline = true,
            ValueFromPipelineByPropertyName = true,
            ParameterSetName = ParamSetPath)
        ]
        [ValidateNotNullOrEmpty]
        public string[] Path
        {
            get { return _paths; }
            set
            {
                _shouldExpandWildcards = true;
                _paths = value;
            }
        }
        protected override void ProcessRecord()
        {
            foreach (string path in _paths)
            {
                // This will hold information about the provider containing
                // the items that this path string might resolve to.                
                ProviderInfo provider;
                // This will be used by the method that processes literal paths
                PSDriveInfo drive;
                // this contains the paths to process for this iteration of the
                // loop to resolve and optionally expand wildcards.
                List<string> filePaths = new List<string>();
                if (_shouldExpandWildcards)
                {
                    // Turn *.txt into foo.txt,foo2.txt etc.
                    // if path is just "foo.txt," it will return unchanged.
                    filePaths.AddRange(this.GetResolvedProviderPathFromPSPath(path, out provider));
                }
                else
                {
                    // no wildcards, so don't try to expand any * or ? symbols.                    
                    filePaths.Add(this.SessionState.Path.GetUnresolvedProviderPathFromPSPath(
                        path, out provider, out drive));
                }
                // ensure that this path (or set of paths after wildcard expansion)
                // is on the filesystem. A wildcard can never expand to span multiple
                // providers.
                if (IsFileSystemPath(provider, path) == false)
                {
                    // no, so skip to next path in _paths.
                    continue;
                }
                // at this point, we have a list of paths on the filesystem.
                foreach (string filePath in filePaths)
                {
                    PSObject custom;
                    // If -whatif was supplied, do not perform the actions
                    // inside this "if" statement; only show the message.
                    //
                    // This block also supports the -confirm switch, where
                    // you will be asked if you want to perform the action
                    // "get metadata" on target: foo.txt
                    if (ShouldProcess(filePath, "Get Metadata"))
                    {
                        if (Directory.Exists(filePath))
                        {
                            custom = GetDirectoryCustomObject(new DirectoryInfo(filePath));
                        }
                        else
                        {
                            custom = GetFileCustomObject(new FileInfo(filePath));
                        }
                        WriteObject(custom);
                    }
                }
            }
        }
        private PSObject GetFileCustomObject(FileInfo file)
        {
            // this message will be shown if the -verbose switch is given
            WriteVerbose("GetFileCustomObject " + file);
            // create a custom object with a few properties
            PSObject custom = new PSObject();
            custom.Properties.Add(new PSNoteProperty("Size", file.Length));
            custom.Properties.Add(new PSNoteProperty("Name", file.Name));
            custom.Properties.Add(new PSNoteProperty("Extension", file.Extension));
            return custom;
        }
        private PSObject GetDirectoryCustomObject(DirectoryInfo dir)
        {
            // this message will be shown if the -verbose switch is given
            WriteVerbose("GetDirectoryCustomObject " + dir);
            // create a custom object with a few properties
            PSObject custom = new PSObject();
            int files = dir.GetFiles().Length;
            int subdirs = dir.GetDirectories().Length;
            custom.Properties.Add(new PSNoteProperty("Files", files));
            custom.Properties.Add(new PSNoteProperty("Subdirectories", subdirs));
            custom.Properties.Add(new PSNoteProperty("Name", dir.Name));
            return custom;
        }
        private bool IsFileSystemPath(ProviderInfo provider, string path)
        {
            bool isFileSystem = true;
            // check that this provider is the filesystem
            if (provider.ImplementingType != typeof(FileSystemProvider))
            {
                // create a .NET exception wrapping our error text
                ArgumentException ex = new ArgumentException(path +
                    " does not resolve to a path on the FileSystem provider.");
                // wrap this in a powershell errorrecord
                ErrorRecord error = new ErrorRecord(ex, "InvalidProvider",
                    ErrorCategory.InvalidArgument, path);
                // write a non-terminating error to pipeline
                this.WriteError(error);
                // tell our caller that the item was not on the filesystem
                isFileSystem = false;
            }
            return isFileSystem;
        }
    }
}

Cmdlet Development Guidelines (Microsoft)

Here is some more generalized advice that should help you out in the long run: http://msdn.microsoft.com/en-us/library/ms714657%28VS.85%29.aspx