Merge pull request docker#622 from mstanleyjones/add_info_about_docker_log

Add info about using docker logs

Author: Misty Stanley-Jones

_data/toc.yaml

@@ -155,6 +155,8 @@ toc:
       title: Run a local registry mirror
     - sectiontitle: Logging
       section:
+      - path: /engine/admin/logging/view_container_logs/
+        title: View a container's logs
       - path: /engine/admin/logging/overview/
         title: Configuring Logging Drivers
       - path: /engine/admin/logging/log_tags/

engine/admin/logging/view_container_logs.md

@@ -0,0 +1,50 @@
+---
+description: How to write to and view a container's logs
+keywords: docker, logging
+title: View a container's logs
+---
+
+The `docker logs` command shows information logged by a running container. The
+information that is logged and the format of the log depends almost entirely on
+the container's endpoint command.
+
+By default, `docker logs` shows the command's output just as it would appear if
+you ran the command interactively in a terminal. UNIX and Linux commands
+typically open three I/O streams when they run, called `STDIN`, `STDOUT`, and
+`STDERR`. `STDIN` is the commmand's input stream, which may include input from
+the keyboard or input from another command. `STDOUT` is usually a command's
+normal output, and `STDERR` is typically used to output error messages. By
+default, `docker logs` shows the command's `STDOUT` and `STDERR`. To read more
+about I/O and Linux, see the
+[Linux Documentation Project article on I/O redirection](http://www.tldp.org/LDP/abs/html/io-redirection.html)
+
+In some cases, `docker logs` may not show useful information unless you take
+additional steps.
+
+- If you use a [logging driver](overview.md) which sends logs to a file, an
+  external host, a database, or another logging back-end, `docker logs` may not
+  show useful information.
+
+- If your image runs a non-interactive process such as a web server or a
+  database, that application may send its output to log files instead of `STDOUT`
+  and `STDERR`.
+
+In the first case, your logs are processed in other ways and you may choose not
+to use `docker logs`. In the second case, the official `nginx` image shows one
+workaround, and the official Apache `httpd` image shows another.
+
+The official `nginx` image creates a symbolic link from
+`/var/log/nginx/access.log` to `/dev/stdout`, and creates another symbolic link
+from `/var/log/nginx/error.log` to `/dev/stderr`, overwriting the previous
+devices in the process. See the
+[Dockerfile](https://github.com/nginxinc/docker-nginx/blob/8921999083def7ba43a06fabd5f80e4406651353/mainline/jessie/Dockerfile#L21-L23).
+
+The official `httpd` driver changes the `httpd` application's configuration to
+write its normal output directly to `/proc/self/fd/1` (which is `STDOUT`) and
+its errors to `/proc/self/fd/2` (which is `STDERR`). See the
+[Dockerfile](https://github.com/docker-library/httpd/blob/b13054c7de5c74bbaa6d595dbe38969e6d4f860c/2.2/Dockerfile#L72-L75).
+
+## Next steps
+
+- Learn about using custom [logging drivers](overview.md).
+- Learn about writing a [Dockerfile](../reference/builder.md).