diff --git a/.github/workflows/ci_cd.yml b/.github/workflows/ci_cd.yml
index 6361b5e..3de7608 100644
--- a/.github/workflows/ci_cd.yml
+++ b/.github/workflows/ci_cd.yml
@@ -197,7 +197,7 @@ jobs:
           expected: failure
           actual: ${{ steps.sad_path_timeout.outcome }}
 
-      - name: sad-path (wrong shell)
+      - name: sad-path (wrong shell for OS)
         id: wrong_shell
         uses: ./
         continue-on-error: true
@@ -215,6 +215,40 @@ jobs:
           expected: failure
           actual: ${{ steps.wrong_shell.outcome }}
 
+  ci_windows:
+    name: Run Windows Tests
+    if: startsWith(github.ref, 'refs/heads')
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12
+      - name: Install dependencies
+        run: npm ci
+      - name: Powershell test
+        uses: ./
+        with:
+          timeout_minutes: 1
+          max_attempts: 2
+          shell: powershell
+          command: Get-ComputerInfo
+      - name: CMD.exe test
+        uses: ./
+        with:
+          timeout_minutes: 1
+          max_attempts: 2
+          shell: cmd
+          command: echo %PATH%
+      - name: Python test
+        uses: ./
+        with:
+          timeout_minutes: 1
+          max_attempts: 2
+          shell: python
+          command: print 1, 2, 3
 
   # runs on push to master only
   cd:
diff --git a/README.md b/README.md
index 270c27d..793817e 100644
--- a/README.md
+++ b/README.md
@@ -20,10 +20,6 @@ Retries an Action step on failure or timeout. This is currently intended to repl
 
 **Required** The command to run
 
-### `OS`
-
-**Required** The OS passed from the runner (runner.OS)
-
 ### `retry_wait_seconds`
 
 **Optional** Number of seconds to wait before attempting the next retry. Defaults to `10`
@@ -69,7 +65,6 @@ uses: nick-invision/retry@v2
 with:
   timeout_minutes: 10
   max_attempts: 3
-  os: ${{ runner.os }}
   shell: pwsh
   command: dir
 ```
diff --git a/action.yml b/action.yml
index 2fa4197..d9978b3 100644
--- a/action.yml
+++ b/action.yml
@@ -18,13 +18,9 @@ inputs:
     description: Number of seconds to wait before attempting the next retry
     required: false
     default: 10
-  OS:
-    description: OS Passed from Runner (runner.OS)
-    required: true
   shell:
     description: Shell to Use to retry (Default is pwsh)
     required: false
-    default: pwsh 
   polling_interval_seconds:
     description: Number of seconds to wait for each check that command has completed running
     required: false
diff --git a/dist/index.js b/dist/index.js
index 5493451..88822fb 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -247,13 +247,13 @@ var util_1 = __webpack_require__(322);
 var TIMEOUT_MINUTES = getInputNumber('timeout_minutes', false);
 var TIMEOUT_SECONDS = getInputNumber('timeout_seconds', false);
 var MAX_ATTEMPTS = getInputNumber('max_attempts', true) || 3;
-var OS = core_1.getInput('os', { required: true });
 var COMMAND = core_1.getInput('command', { required: true });
 var RETRY_WAIT_SECONDS = getInputNumber('retry_wait_seconds', false) || 10;
-var SHELL = core_1.getInput('shell') || 'pwsh';
+var SHELL = core_1.getInput('shell');
 var POLLING_INTERVAL_SECONDS = getInputNumber('polling_interval_seconds', false) || 1;
 var RETRY_ON = core_1.getInput('retry_on') || 'any';
 var WARNING_ON_RETRY = core_1.getInput('warning_on_retry').toLowerCase() === 'true';
+var OS = process.platform;
 var OUTPUT_TOTAL_ATTEMPTS_KEY = 'total_attempts';
 var OUTPUT_EXIT_CODE_KEY = 'exit_code';
 var OUTPUT_EXIT_ERROR_KEY = 'exit_error';
@@ -307,6 +307,39 @@ function getTimeout() {
     }
     throw new Error('Must specify either timeout_minutes or timeout_seconds inputs');
 }
+function getExecutable() {
+    if (!SHELL) {
+        return OS === 'win32' ? 'powershell' : 'bash';
+    }
+    var executable;
+    switch (SHELL) {
+        case "bash":
+        case "python":
+        case "pwsh": {
+            executable = SHELL;
+            break;
+        }
+        case "sh": {
+            if (OS === 'win32') {
+                throw new Error("Shell " + SHELL + " not allowed on OS " + OS);
+            }
+            executable = SHELL;
+            break;
+        }
+        case "cmd":
+        case "powershell": {
+            if (OS !== 'win32') {
+                throw new Error("Shell " + SHELL + " not allowed on OS " + OS);
+            }
+            executable = SHELL + ".exe";
+            break;
+        }
+        default: {
+            throw new Error("Shell " + SHELL + " required");
+        }
+    }
+    return executable;
+}
 function runCmd() {
     var _a, _b;
     return __awaiter(this, void 0, void 0, function () {
@@ -315,48 +348,10 @@ function runCmd() {
             switch (_c.label) {
                 case 0:
                     end_time = Date.now() + getTimeout();
+                    executable = getExecutable();
                     exit = 0;
                     done = false;
-                    executable = SHELL + ".exe";
-                    switch (SHELL) {
-                        case "pwsh": {
-                            executable = SHELL;
-                            break;
-                        }
-                        case "bash": {
-                            executable = SHELL;
-                            break;
-                        }
-                        case "python": {
-                            executable = SHELL;
-                            break;
-                        }
-                        case "sh": {
-                            if (OS === 'Windows') {
-                                throw new Error("Shell " + SHELL + " not allowed on OS " + OS);
-                            }
-                            executable = SHELL;
-                            break;
-                        }
-                        case "cmd": {
-                            if (OS !== 'Windows') {
-                                throw new Error("Shell " + SHELL + " not allowed on OS " + OS);
-                            }
-                            executable = SHELL + ".exe";
-                            break;
-                        }
-                        case "powershell": {
-                            if (OS !== 'Windows') {
-                                throw new Error("Shell " + SHELL + " not allowed on OS " + OS);
-                            }
-                            executable = SHELL + ".exe";
-                            break;
-                        }
-                        default: {
-                            throw new Error("Shell " + SHELL + " required");
-                            break;
-                        }
-                    }
+                    core_1.debug("Running command " + COMMAND + " on " + OS + " using shell " + executable);
                     child = child_process_1.exec(COMMAND, { 'shell': executable });
                     (_a = child.stdout) === null || _a === void 0 ? void 0 : _a.on('data', function (data) {
                         process.stdout.write(data);
diff --git a/sample.env b/sample.env
index 5409962..f69d5fd 100644
--- a/sample.env
+++ b/sample.env
@@ -3,6 +3,5 @@ INPUT_MAX_ATTEMPTS=3
 INPUT_COMMAND="node -e 'process.exit(99)'"
 INPUT_RETRY_WAIT_SECONDS=10
 SHELL=pwsh
-OS=Linux
 INPUT_POLLING_INTERVAL_SECONDS=1
 INPUT_RETRY_ON=any
diff --git a/src/index.ts b/src/index.ts
index e6c7524..d55da15 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -9,14 +9,14 @@ import { wait } from './util';
 const TIMEOUT_MINUTES = getInputNumber('timeout_minutes', false);
 const TIMEOUT_SECONDS = getInputNumber('timeout_seconds', false);
 const MAX_ATTEMPTS = getInputNumber('max_attempts', true) || 3;
-const OS = getInput('os', { required: true });
 const COMMAND = getInput('command', { required: true });
 const RETRY_WAIT_SECONDS = getInputNumber('retry_wait_seconds', false) || 10;
-const SHELL = getInput('shell') || 'pwsh';
+const SHELL = getInput('shell');
 const POLLING_INTERVAL_SECONDS = getInputNumber('polling_interval_seconds', false) || 1;
 const RETRY_ON = getInput('retry_on') || 'any';
 const WARNING_ON_RETRY = getInput('warning_on_retry').toLowerCase() === 'true';
 
+const OS = process.platform;
 const OUTPUT_TOTAL_ATTEMPTS_KEY = 'total_attempts';
 const OUTPUT_EXIT_CODE_KEY = 'exit_code';
 const OUTPUT_EXIT_ERROR_KEY = 'exit_error';
@@ -63,42 +63,29 @@ function getTimeout(): number {
   throw new Error('Must specify either timeout_minutes or timeout_seconds inputs');
 }
 
-async function runCmd() {
-  const end_time = Date.now() + getTimeout();
-
-  exit = 0;
-  done = false;
+function getExecutable(): string {
+  if (!SHELL) {
+    return OS === 'win32' ? 'powershell' : 'bash';
+  }
 
-  let executable: string = SHELL + ".exe";
+  let executable: string;
   switch (SHELL) {
+    case "bash":
+    case "python":
     case "pwsh": {
       executable = SHELL;
       break;
     }
-    case "bash": {
-      executable = SHELL;
-      break;
-    }
-    case "python": {
-      executable = SHELL;
-      break;
-    }
     case "sh": {
-      if (OS === 'Windows') {
+      if (OS === 'win32') {
         throw new Error(`Shell ${SHELL} not allowed on OS ${OS}`);
       }
       executable = SHELL;
       break;
     }
-    case "cmd": {
-      if (OS !== 'Windows') {
-        throw new Error(`Shell ${SHELL} not allowed on OS ${OS}`);
-      }
-      executable = SHELL + ".exe";
-      break;
-    }
+    case "cmd":
     case "powershell": {
-      if (OS !== 'Windows') {
+      if (OS !== 'win32') {
         throw new Error(`Shell ${SHELL} not allowed on OS ${OS}`);
       }
       executable = SHELL + ".exe";
@@ -106,11 +93,19 @@ async function runCmd() {
     }
     default: {
       throw new Error(`Shell ${SHELL} required`);
-      break;
     }
   }
+  return executable
+}
+
+async function runCmd() {
+  const end_time = Date.now() + getTimeout();
+  const executable = getExecutable();
 
+  exit = 0;
+  done = false;
 
+  debug(`Running command ${COMMAND} on ${OS} using shell ${executable}`)
   var child = exec(COMMAND, { 'shell': executable });
 
   child.stdout?.on('data', (data) => {