diff --git a/Cesium.Sdk.Tests/Cesium.Sdk.Tests.csproj b/Cesium.Sdk.Tests/Cesium.Sdk.Tests.csproj
index 6a912da0..67b1e517 100644
--- a/Cesium.Sdk.Tests/Cesium.Sdk.Tests.csproj
+++ b/Cesium.Sdk.Tests/Cesium.Sdk.Tests.csproj
@@ -18,6 +18,7 @@ SPDX-License-Identifier: MIT
     <ItemGroup>
         <PackageReference Include="MedallionShell" />
         <PackageReference Include="Microsoft.NET.Test.Sdk" />
+        <PackageReference Include="TruePath.SystemIo" />
         <PackageReference Include="xunit" />
         <PackageReference Include="xunit.runner.visualstudio" >
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
diff --git a/Cesium.Sdk.Tests/FileUtilTests.cs b/Cesium.Sdk.Tests/FileUtilTests.cs
new file mode 100644
index 00000000..ef6a5c98
--- /dev/null
+++ b/Cesium.Sdk.Tests/FileUtilTests.cs
@@ -0,0 +1,85 @@
+// SPDX-FileCopyrightText: 2025 Cesium contributors <https://github.com/ForNeVeR/Cesium>
+//
+// SPDX-License-Identifier: MIT
+
+using System.Runtime.InteropServices;
+using TruePath;
+using TruePath.SystemIo;
+
+namespace Cesium.Sdk.Tests;
+
+public class FileUtilTests
+{
+    private static void CreateUnixFile(AbsolutePath file, AbsolutePath? link = null, UnixFileMode? mode = null)
+    {
+        file.WriteAllText("empty");
+
+        if (!RuntimeInformation.IsOSPlatform(OSPlatform.Linux) &&
+            !RuntimeInformation.IsOSPlatform(OSPlatform.OSX)) return;
+        if (mode is { } m)
+            File.SetUnixFileMode(file.Value, m);
+        if (link is { } linkFile)
+        {
+            linkFile.Delete();
+            File.CreateSymbolicLink(linkFile.Value, file.Value);
+        }
+    }
+
+    private AbsolutePath TestFile = Temporary.CreateTempFile();
+    private AbsolutePath TestLink = Temporary.CreateTempFile();
+
+    [Fact]
+    public void ExecutablePermissionsCheckOnUnix()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            Assert.True(true);
+        else
+        {
+            var file = TestFile;
+            CreateUnixFile(file, mode: UnixFileMode.UserExecute);
+
+            Assert.True(FileSystemUtil.IsUnixFileExecutable(file.Value));
+        }
+    }
+
+    [Fact]
+    public void ExecutableLinkPermissionsCheckOnUnix()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            Assert.True(true);
+        else
+        {
+            var file = TestFile;
+            var link = TestLink;
+            CreateUnixFile(file, link: link, mode: UnixFileMode.UserExecute);
+
+            Assert.True(FileSystemUtil.IsUnixFileExecutable(link.Value));
+        }
+    }
+
+    [Fact]
+    public void NoExecutablePermissionsCheckOnUnix()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            Assert.True(true);
+        else
+        {
+            var file = TestFile;
+            CreateUnixFile(file, mode: UnixFileMode.UserRead | UnixFileMode.UserWrite);
+
+            Assert.False(FileSystemUtil.IsUnixFileExecutable(file.Value));
+        }
+    }
+
+    [Fact]
+    public void InvalidForDirOnUnix()
+    {
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            Assert.True(true);
+        else
+        {
+            var dir = "/etc";
+            Assert.False(FileSystemUtil.IsUnixFileExecutable(dir));
+        }
+    }
+}
diff --git a/Cesium.Sdk/Cesium.Sdk.csproj b/Cesium.Sdk/Cesium.Sdk.csproj
index 667f0885..073bb49a 100644
--- a/Cesium.Sdk/Cesium.Sdk.csproj
+++ b/Cesium.Sdk/Cesium.Sdk.csproj
@@ -18,6 +18,7 @@ SPDX-License-Identifier: MIT
         <Content Include="Sdk\Sdk.props" PackagePath="Sdk\Sdk.props" />
         <Content Include="Sdk\Sdk.targets" PackagePath="Sdk\Sdk.targets" />
         <None Include="$(OutputPath)\$(AssemblyName).dll" Pack="true" PackagePath="tools" Visible="false"/>
+        <InternalsVisibleTo Include="Cesium.Sdk.Tests" />
     </ItemGroup>
 
     <ItemGroup>
diff --git a/Cesium.Sdk/CesiumCompile.cs b/Cesium.Sdk/CesiumCompile.cs
index 222f96fa..92d4f45e 100644
--- a/Cesium.Sdk/CesiumCompile.cs
+++ b/Cesium.Sdk/CesiumCompile.cs
@@ -285,11 +285,11 @@ private static bool ExecutableFileExists(string path)
         var pathExtWithDot = new Lazy<string[]>(() =>
             Environment.GetEnvironmentVariable("PATHEXT")?.Split(Path.PathSeparator) ?? []);
 
-        if (IsExecutable(path)) return true;
+        if (File.Exists(path) && IsExecutable(path)) return true;
 
         foreach (var pathEntry in Environment.GetEnvironmentVariable("PATH")?.Split(Path.PathSeparator) ?? [])
         {
-            var fullPath = Path.Combine(pathEntry, pathEntry);
+            var fullPath = Path.Combine(pathEntry, path);
             if (IsExecutable(fullPath)) return true;
 
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
@@ -305,14 +305,12 @@ private static bool ExecutableFileExists(string path)
 
         bool IsExecutable(string exePath)
         {
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
-            {
-                var extension = Path.GetExtension(exePath);
-                return pathExtWithDot.Value.Contains(extension);
-            }
-
-            return true; // TODO[#840]: Proper executable check for Unix
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux)
+                || RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                return FileSystemUtil.IsUnixFileExecutable(exePath);
 
+            var extension = Path.GetExtension(exePath);
+            return pathExtWithDot.Value.Contains(extension);
         }
     }
 
diff --git a/Cesium.Sdk/FileSystemUtil.cs b/Cesium.Sdk/FileSystemUtil.cs
new file mode 100644
index 00000000..5e5948f9
--- /dev/null
+++ b/Cesium.Sdk/FileSystemUtil.cs
@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: 2025 Cesium contributors <https://github.com/ForNeVeR/Cesium>
+//
+// SPDX-License-Identifier: MIT
+
+using System.IO;
+using System.Runtime.InteropServices;
+
+namespace Cesium.Sdk;
+
+internal static class FileSystemUtil
+{
+    [DllImport("libc", EntryPoint = "access", SetLastError = true)]
+    private static extern int Access(string path, int mode);
+
+    private const int X_OK = 1;
+
+    public static bool IsUnixFileExecutable(string path)
+    {
+        if (Directory.Exists(path)) return false;
+        return Access(Path.GetFullPath(path), X_OK) == 0;
+    }
+}
diff --git a/Directory.Packages.props b/Directory.Packages.props
index 4821b01b..71f78430 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -19,6 +19,7 @@ SPDX-License-Identifier: MIT
     <PackageVersion Include="QuikGraph" Version="2.5.0" />
     <PackageVersion Include="QuikGraph.Graphviz" Version="2.5.0" />
     <PackageVersion Include="TruePath" Version="1.10.0" />
+    <PackageVersion Include="TruePath.SystemIo" Version="1.10.0" />
     <PackageVersion Include="Verify.Xunit" Version="31.0.0" />
     <PackageVersion Include="xunit" Version="2.9.3" />
     <PackageVersion Include="xunit.assert" Version="2.9.3" />