fix: read the current location from the session, not from $PWD

A variable named PWD in the scope of the caller, such as a loop variable,
hid the automatic variable, and every cmdlet failed with a
NullReferenceException, also for an absolute path. The cmdlets now read
the current file system location from the session state, and only for a
relative path (#86).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: AI Assistant <ai@example.com>

CHANGELOG.md

@@ -172,5 +172,10 @@ The format is based on
 - Fix a `FormatException` in the cmdlets for a path with braces, such as
   `C:\Data\{Archive}`: their messages formatted the path a second time
   ([#3](https://github.com/raandree/NTFSSecurity/issues/3))
+- Fix a `NullReferenceException` in every cmdlet when a variable named
+  `PWD` in the scope of the caller, such as a loop variable, hid the
+  automatic variable; the cmdlets now read the current location from the
+  session, and only for a relative path
+  ([#86](https://github.com/raandree/NTFSSecurity/issues/86))
 
 [Unreleased]: https://github.com/raandree/NTFSSecurity/compare/4.2.6...HEAD

NTFSSecurity/BaseCmdlets.cs

@@ -26,32 +26,7 @@ namespace NTFSSecurity
         #region GetFileSystemInfo
         protected System.IO.FileSystemInfo GetFileSystemInfo(string path)
         {
-            string currentLocation = GetVariableValue("PWD").ToString();
+            path = GetRelativePath(path);
-
-            if (path == ".")
-            {
-                path = currentLocation;
-            }
-            if (path.StartsWith(".."))
-            {
-                path = System.IO.Path.Combine(
-                    string.Join("\\", currentLocation.Split('\\').Take(currentLocation.Split('\\').Count() - path.Split('\\').Count(s => s == "..")).ToArray()),
-                    string.Join("\\", path.Split('\\').Where(e => e != "..").ToArray()));
-            }
-            else if (path.StartsWith("."))
-            {
-                //combine . and .\path\subpath
-                path = System.IO.Path.Combine(currentLocation, path.Substring(2));
-            }
-            else if (path.StartsWith("\\"))
-            {
-                //do nothing
-            }
-            else
-            {
-                ////combine . and \path\subpath or path\subpath
-                path = System.IO.Path.Combine(currentLocation, path.Substring(0));
-            }
 
             if (System.IO.File.Exists(path))
             {
@@ -114,18 +89,17 @@ namespace NTFSSecurity
         #region GetRelativePath
         protected string GetRelativePath(string path)
         {
-            string currentLocation = GetVariableValue("PWD").ToString();
-
             if (string.IsNullOrEmpty(path))
             {
-                path = currentLocation;
+                path = GetCurrentLocation();
             }
             else if (path == ".")
             {
-                path = currentLocation;
+                path = GetCurrentLocation();
             }
             else if (path.StartsWith(".."))
             {
+                var currentLocation = GetCurrentLocation();
                 path = System.IO.Path.Combine(
                     string.Join("\\", currentLocation.Split('\\').Take(currentLocation.Split('\\').Count() - path.Split('\\').Count(s => s == "..")).ToArray()),
                     string.Join("\\", path.Split('\\').Where(e => e != "..").ToArray()));
@@ -133,20 +107,28 @@ namespace NTFSSecurity
             else if (path.StartsWith("."))
             {
                 //combine . and .\path\subpath
-                path = System.IO.Path.Combine(currentLocation, path.Substring(2));
+                path = System.IO.Path.Combine(GetCurrentLocation(), path.Substring(2));
             }
-            else if (path.StartsWith("\\"))
+            else if (path.StartsWith("\\") || System.IO.Path.IsPathRooted(path))
             {
-                //do nothing
+                //an absolute path needs no location
             }
             else
             {
-                ////combine . and \path\subpath or path\subpath
+                ////combine . and path\subpath
-                path = System.IO.Path.Combine(currentLocation, path);
+                path = System.IO.Path.Combine(GetCurrentLocation(), path);
             }
 
             return path;
         }
+
+        // Read from the session state, not from $PWD: a variable named PWD in the caller's scope shadows the
+        // automatic variable and can be $null (#86). In a location of another provider, such as the registry, this
+        // is the last file system location.
+        private string GetCurrentLocation()
+        {
+            return SessionState.Path.CurrentFileSystemLocation.ProviderPath;
+        }
         #endregion
 
         #region InvokeAsOwner

Tests/Owner.Tests.ps1

@@ -4,6 +4,9 @@
 [Diagnostics.CodeAnalysis.SuppressMessageAttribute(
     'PSUseDeclaredVarsMoreThanAssignments', '', Justification = 'Pester shares variables between blocks.'
 )]
+[Diagnostics.CodeAnalysis.SuppressMessageAttribute(
+    'PSAvoidAssignmentToAutomaticVariable', '', Justification = 'A test shadows $PWD on purpose (#86).'
+)]
 param ()
 
 BeforeDiscovery {
@@ -52,3 +55,33 @@ Describe 'Get-NTFSOwner' {
         }
     }
 }
+
+Describe 'Current location' {
+    BeforeAll {
+        function Invoke-WithShadowedPwd {
+            param ([scriptblock] $Command)
+
+            $PWD = $null
+            & $Command
+        }
+    }
+
+    # Before 5.0.0, a variable named PWD in the scope of the caller, such as a loop variable, made every cmdlet
+    # fail with a NullReferenceException, also for an absolute path (#86).
+    It 'Should ignore a variable named PWD for an absolute path' {
+        $file = New-TestSandboxItem -Sandbox $sandbox -Name 'Pwd'
+
+        $result = Invoke-WithShadowedPwd -Command { Get-NTFSOwner -Path $file -ErrorAction Stop }
+
+        $result.FullName | Should -Be $file
+    }
+
+    It 'Should resolve a relative path against the current location despite a variable named PWD' {
+        $file = New-TestSandboxItem -Sandbox $sandbox -Name 'PwdRelative'
+        $name = Split-Path -Path $file -Leaf
+
+        $result = Invoke-WithShadowedPwd -Command { Get-NTFSOwner -Path $name -ErrorAction Stop }
+
+        $result.FullName | Should -Be $file
+    }
+}