Splitting out the cli command into parts.

Signed-off-by: Mary Anthony <mary@docker.com>

docs/reference/commandline/exec.md

@@ -0,0 +1,55 @@
+<!--[metadata]>
++++
+title = "exec"
+description = "The exec command description and usage"
+keywords = ["command, container, run, execute"]
+[menu.main]
+parent = "smn_cli"
+weight=1
++++
+<![end-metadata]-->
+
+# exec
+
+    Usage: docker exec [OPTIONS] CONTAINER COMMAND [ARG...]
+
+    Run a command in a running container
+
+      -d, --detach=false         Detached mode: run command in the background
+      -i, --interactive=false    Keep STDIN open even if not attached
+      -t, --tty=false            Allocate a pseudo-TTY
+      -u, --user=                Username or UID (format: <name|uid>[:<group|gid>])
+
+The `docker exec` command runs a new command in a running container.
+
+The command started using `docker exec` only runs while the container's primary
+process (`PID 1`) is running, and it is not restarted if the container is
+restarted.
+
+If the container is paused, then the `docker exec` command will fail with an error:
+
+    $ docker pause test
+    test
+    $ docker ps
+    CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                   PORTS               NAMES
+    1ae3b36715d2        ubuntu:latest       "bash"              17 seconds ago      Up 16 seconds (Paused)                       test
+    $ docker exec test ls
+    FATA[0000] Error response from daemon: Container test is paused, unpause the container before exec
+    $ echo $?
+    1
+
+## Examples
+
+    $ docker run --name ubuntu_bash --rm -i -t ubuntu bash
+
+This will create a container named `ubuntu_bash` and start a Bash session.
+
+    $ docker exec -d ubuntu_bash touch /tmp/execWorks
+
+This will create a new file `/tmp/execWorks` inside the running container
+`ubuntu_bash`, in the background.
+
+    $ docker exec -it ubuntu_bash bash
+
+This will create a new Bash session in the container `ubuntu_bash`.
+