Add 'checkup' command.

lib/diagnostic.ps1

@@ -0,0 +1,29 @@
+<#
+Diagnostic tests.
+Return $true if the test passed, otherwise $false.
+Use 'warn' to highlight the issue, and follow up with the recommended actions to rectify.
+#>
+
+
+function check_windows_defender($global) {
+    $defender = get-service -name WinDefend -errorAction SilentlyContinue
+    if($defender -and $defender.status) {
+        if($defender.status -eq [system.serviceprocess.servicecontrollerstatus]::running) {
+            $hasGetMpPreference = gcm get-mppreference -errorAction SilentlyContinue
+            if($hasGetMpPreference) {
+                $installPath = $scoopdir;
+                if($global) { $installPath = $globaldir; }
+
+                $exclusionPath = (Get-MpPreference).exclusionPath
+                if(!($exclusionPath -contains $installPath)) {
+                    warn "Windows Defender may slow down or disrupt installs with realtime scanning."
+                    write-host "  Consider running:"
+                    write-host "    sudo Add-MpPreference -ExclusionPath '$installPath'"
+                    write-host "  (Requires 'sudo' command. Run 'scoop install sudo' if you don't have it.)"
+                    return $false
+                }
+            }
+        }
+    }
+    return $true
+}

libexec/scoop-checkup.ps1

@@ -0,0 +1,19 @@
+# Usage: scoop checkup
+# Summary: Check for potential problems
+# Help: Performs a series of diagnostic tests to try to identify things that may
+# cause problems with Scoop.
+
+. "$psscriptroot\..\lib\core.ps1"
+. "$psscriptroot\..\lib\diagnostic.ps1"
+
+$issues = 0
+
+$issues += !(check_windows_defender $false)
+$issues += !(check_windows_defender $true)
+
+if($issues) {
+    warn "`nFound $issues potential $(pluralize $issues problem problems)."
+} else {
+    success "No problems identified!"
+}
+