diff --git a/main-command/src/main/java/sbt/internal/NGUnixDomainSocket.java b/main-command/src/main/java/sbt/internal/NGUnixDomainSocket.java
index 1a9942ad9..b70bef611 100644
--- a/main-command/src/main/java/sbt/internal/NGUnixDomainSocket.java
+++ b/main-command/src/main/java/sbt/internal/NGUnixDomainSocket.java
@@ -29,6 +29,8 @@ import java.nio.ByteBuffer;
 
 import java.net.Socket;
 
+import java.util.concurrent.atomic.AtomicInteger;
+
 /**
  * Implements a {@link Socket} backed by a native Unix domain socket.
  *
@@ -41,6 +43,25 @@ public class NGUnixDomainSocket extends Socket {
   private final InputStream is;
   private final OutputStream os;
 
+  public NGUnixDomainSocket(String path) throws IOException {
+    try {
+      AtomicInteger fd = new AtomicInteger(
+          NGUnixDomainSocketLibrary.socket(
+              NGUnixDomainSocketLibrary.PF_LOCAL,
+              NGUnixDomainSocketLibrary.SOCK_STREAM,
+              0));
+      NGUnixDomainSocketLibrary.SockaddrUn address =
+        new NGUnixDomainSocketLibrary.SockaddrUn(path);
+      int socketFd = fd.get();
+      NGUnixDomainSocketLibrary.connect(socketFd, address, address.size());
+      this.fd = new ReferenceCountedFileDescriptor(socketFd);
+      this.is = new NGUnixDomainSocketInputStream();
+      this.os = new NGUnixDomainSocketOutputStream();
+    } catch (LastErrorException e) {
+      throw new IOException(e);
+    }
+  }
+
   /**
    * Creates a Unix domain socket backed by a native file descriptor.
    */
diff --git a/main-command/src/main/java/sbt/internal/NGUnixDomainSocketLibrary.java b/main-command/src/main/java/sbt/internal/NGUnixDomainSocketLibrary.java
index 7e760d37a..4d781b6b6 100644
--- a/main-command/src/main/java/sbt/internal/NGUnixDomainSocketLibrary.java
+++ b/main-command/src/main/java/sbt/internal/NGUnixDomainSocketLibrary.java
@@ -131,6 +131,8 @@ public class NGUnixDomainSocketLibrary {
   public static native int listen(int fd, int backlog) throws LastErrorException;
   public static native int accept(int fd, SockaddrUn address, IntByReference addressLen)
     throws LastErrorException;
+  public static native int connect(int fd, SockaddrUn address, int addressLen)
+    throws LastErrorException;
   public static native int read(int fd, ByteBuffer buffer, int count)
     throws LastErrorException;
   public static native int write(int fd, ByteBuffer buffer, int count)
diff --git a/main-command/src/main/scala/sbt/internal/server/Server.scala b/main-command/src/main/scala/sbt/internal/server/Server.scala
index c4d3b542c..0b8ee4b32 100644
--- a/main-command/src/main/scala/sbt/internal/server/Server.scala
+++ b/main-command/src/main/scala/sbt/internal/server/Server.scala
@@ -9,7 +9,7 @@ package sbt
 package internal
 package server
 
-import java.io.File
+import java.io.{ File, IOException }
 import java.net.{ SocketTimeoutException, InetAddress, ServerSocket, Socket }
 import java.util.concurrent.atomic.AtomicBoolean
 import java.nio.file.attribute.{ UserPrincipal, AclEntry, AclEntryPermission, AclEntryType }
@@ -54,15 +54,17 @@ private[sbt] object Server {
       val serverThread = new Thread("sbt-socket-server") {
         override def run(): Unit = {
           Try {
-            ErrorHandling.translate(s"server failed to start on ${connection.shortName}. ") {
-              connection.connectionType match {
-                case ConnectionType.Local if isWindows =>
-                  new NGWin32NamedPipeServerSocket(pipeName)
-                case ConnectionType.Local =>
-                  prepareSocketfile()
-                  new NGUnixDomainServerSocket(socketfile.getAbsolutePath)
-                case ConnectionType.Tcp => new ServerSocket(port, 50, InetAddress.getByName(host))
-              }
+            connection.connectionType match {
+              case ConnectionType.Local if isWindows =>
+                // Named pipe already has an exclusive lock.
+                addServerError(new NGWin32NamedPipeServerSocket(pipeName))
+              case ConnectionType.Local =>
+                tryClient(new NGUnixDomainSocket(socketfile.getAbsolutePath))
+                prepareSocketfile()
+                addServerError(new NGUnixDomainServerSocket(socketfile.getAbsolutePath))
+              case ConnectionType.Tcp =>
+                tryClient(new Socket(InetAddress.getByName(host), port))
+                addServerError(new ServerSocket(port, 50, InetAddress.getByName(host)))
             }
           } match {
             case Failure(e) => p.failure(e)
@@ -87,6 +89,24 @@ private[sbt] object Server {
       }
       serverThread.start()
 
+      // Try the socket as a client to make sure that the server is not already up.
+      // f tries to connect to the server, and flip the result.
+      def tryClient(f: => Socket): Unit = {
+        if (portfile.exists) {
+          Try { f } match {
+            case Failure(e) => ()
+            case Success(socket) =>
+              socket.close()
+              throw new IOException("sbt server is already running.")
+          }
+        } else ()
+      }
+
+      def addServerError(f: => ServerSocket): ServerSocket =
+        ErrorHandling.translate(s"server failed to start on ${connection.shortName}. ") {
+          f
+        }
+
       override def authenticate(challenge: String): Boolean = synchronized {
         if (token == challenge) {
           token = nextToken
diff --git a/main/src/main/scala/sbt/internal/CommandExchange.scala b/main/src/main/scala/sbt/internal/CommandExchange.scala
index 476ea5f76..32175d256 100644
--- a/main/src/main/scala/sbt/internal/CommandExchange.scala
+++ b/main/src/main/scala/sbt/internal/CommandExchange.scala
@@ -152,11 +152,13 @@ private[sbt] final class CommandExchange {
         Await.ready(x.ready, Duration("10s"))
         x.ready.value match {
           case Some(Success(_)) =>
+            // rememeber to shutdown only when the server comes up
+            server = Some(x)
           case Some(Failure(e)) =>
             s.log.error(e.toString)
+            server = None
           case None => // this won't happen because we awaited
         }
-        server = Some(x)
     }
     s
   }