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docs(README): Add more daemon info

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Gives the "Adding Additional Daemons" section of the README some extra
information, including another warning not to use `-it <cmd>` when
starting a given container.
This commit is contained in:
Cody Welsh
2018-09-03 21:31:55 -07:00
parent 3e62736510
commit 10c7bbf3b4
1 changed files with 33 additions and 14 deletions
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README.md
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@@ -156,26 +156,45 @@ The image is called `phusion/baseimage`, and is available on the Docker registry
<a name="adding_additional_daemons"></a>
### Adding additional daemons
You can add additional daemons (e.g. your own app) to the image by creating runit entries. You only have to write a small shell script which runs your daemon, and runit will keep it up and running for you, restarting it when it crashes, etc.
A daemon is a program which runs in the background of its system, such
as a web server.
The shell script must be called `run`, must be executable, and is to be placed in the directory `/etc/service/<NAME>`.
You can add additional daemons (for example, your own app) to the image
by creating runit service directories. You only have to write a small
shell script which runs your daemon;
[`runsv`](http://smarden.org/runit/runsv.8.html) will start your script,
and - by default - restart it upon its exit, after waiting one second.
Here's an example showing you how a memcached server runit entry can be made.
The shell script must be called `run`, must be executable, and is to be
placed in the directory `/etc/service/<NAME>`. `runsv` will switch to
the directory and invoke `./run` after your container starts.
In `memcached.sh` (make sure this file is chmod +x):
**Be certain that you do not start your container using interactive mode
(`-it`) with another command, as `runit` must be the first process to run. If you do this, your runit service directories won't be started. For instance, `docker run -it <name> bash` will bring you to bash in your container, but you'll lose all your daemons.**
Here's an example showing you how a `runit` service directory can be
made for a `memcached` server.
In `memcached.sh`, or whatever you choose to name your file (make sure
this file is chmod +x):
```bash
#!/bin/sh
# `/sbin/setuser memcache` runs the given command as the user `memcache`.
# If you omit that part, the command will be run as root.
exec /sbin/setuser memcache /usr/bin/memcached >>/var/log/memcached.log 2>&1
```
In an accompanying `Dockerfile`:
In `Dockerfile`:
```Dockerfile
RUN mkdir /etc/service/memcached
COPY memcached.sh /etc/service/memcached/run
RUN chmod +x /etc/service/memcached/run
Note that the shell script must run the daemon **without letting it daemonize/fork it**. Usually, daemons provide a command line flag or a config file option for that.
```
A given shell script must run **without daemonizing or forking itself**;
this is because `runit` will start and restart your script on its own.
Usually, daemons provide a command line flag or a config file option for
preventing such behavior - essentially, you just want your script to run
in the foreground, not the background.
<a name="running_startup_scripts"></a>
### Running scripts during container startup