Adding docker exec support in CLI.

Fixed a bug in daemon that resulted in accessing of a closed pipe.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)

docs/sources/reference/commandline/cli.md

@@ -1295,6 +1295,36 @@ It is even useful to cherry-pick particular tags of an image repository
 
    $ sudo docker save -o ubuntu.tar ubuntu:lucid ubuntu:saucy
 
+## exec
+
+    Usage: docker exec CONTAINER COMMAND [ARG...]
+
+    Run a command in an existing container
+
+      -d, --detach=false         Detached mode: run the process in the background and exit
+      -i, --interactive=false    Keep STDIN open even if not attached
+      -t, --tty=false            Allocate a pseudo-TTY
+
+The `docker exec` command runs a user specified command as a new process in an existing
+user specified container. The container needs to be active.
+
+The `docker exec` command will typically be used after `docker run`.
+
+### Examples:
+
+    $ sudo docker run --name ubuntu_bash --rm -i -t ubuntu bash
+
+This will create a container named 'ubuntu_bash' and start a bash session.
+
+    $ sudo docker exec -d ubuntu_bash touch /tmp/execWorks
+
+This will create a new file '/tmp/execWorks' inside the existing and active container
+'ubuntu_bash', in the background.
+
+    $ sudo docker exec ubuntu_bash -it bash
+
+This will create a new bash session in the container 'ubuntu_bash'.
+
 ## search
 
 Search [Docker Hub](https://hub.docker.com) for images