added exec and logs

README

@@ -239,3 +239,43 @@ Here are some useful options you might want to use:
 | -h     |   <hostname>    | set the container's hostname              |
 | -p     | <hport>:<cport> | map host's <hport> to container's <cport> |
 | -v     |  <hdir>:<cdir>  | mount host's <hdir> on <cdir>             |
+
+
+Executing Commands in a Running Container
+=========================================
+
+Sometimes you need to examine what's going on inside a container. That's
+where the *exec* command can come in handy. It's a lot like *run* but for
+a container, rather than an image. Here's a common thing you might want to
+do:
+
+    docker run --name=svc_instance my_service:latest
+    docker exec -ti svc_instance /bin/bash
+
+What this does is to first start a container using the latest version of
+the image my_service, and name the container svc_instance, and then to
+execute an interactive bash shell on that container. You don't have to exec an
+interactive command, though. There may be times when you want to run something
+like:
+
+    docker exec svc_instance touch /var/cache/magic_file
+
+in order to change the behavior of a running process. As with the other commands
+we've looked at, "docker exec" is now an alias for "docker container exec".
+
+
+Getting Process Output
+======================
+
+Many processes send their output to STDOUT or STDERR. Since there's no TTY
+available to the process in a container, this output would generally be lost.
+Docker saves this output for you, however, and you can retrieve these by
+running
+
+    docker logs <container>
+    docker container logs <container>
+
+The first command is now an alias for the second command. There are a number
+of options, such as "--since" to limit the timeframe of the logs returned,
+"-f" to continue to follow the logs rather than just dumping their current
+contents and exiting, and "-t" to show timestamps at the beginnings of lines.