Quickstart #1 - A Cmdlet That Processes Files and Directories

It seems lots of people are catching the PowerShell bug these days. People who previously considered themselves as administrators have learned enough of the magic from using PowerShell from day to day over the last few months that they feel comfortable enough to tinker around with C#.

This is an example Cmdlet skeleton that is designed to work with Files and Directories. It looks after resolving wildcards, supports –WhatIf and –Confirm, accepting files and/or directories from the pipeline or accepting strings of paths from the pipeine. It also ensures that the only types of paths it can accept lie on the File System. Here’s an example of the invocation styles it supports:

cmdlet

Here’s the source code for this. It doesn’t really do a lot. It spits out two different types of custom objects: one for a file, and another for a directory. Feel free to do what you like with it; it’s yours. Have fun.

using System;
using System.Collections.Generic;
using System.IO;
using System.Management.Automation;
using Microsoft.PowerShell.Commands;
namespace PSQuickStart
{
    [Cmdlet(VerbsCommon.Get, Noun,
        DefaultParameterSetName = ParmamSetPath,
        SupportsShouldProcess = true)
    ]
    public class GetFileMetadataCommand : PSCmdlet
    {
        private const string Noun = "FileMetadata";
        private const string ParamSetLiteral = "Literal";
        private const string ParmamSetPath = "Path";
        private string[] _paths;
        private bool _shouldExpandWildcards;
        [Parameter(
            Position = 0,
            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 = ParmamSetPath)
        ]
        [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;
        }
    }
}
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About the author

Irish, PowerShell MVP, .NET/ASP.NET/SharePoint Developer, Budding Architect. Developer. Montrealer. Opinionated. Montreal, Quebec.

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