Docker – Systemd Service Start With Dynamic Port Value From Docker

Using CoreOS, Docker and systemd to manage my services I want to properly perform service discovery. Since CoreOS utilizes etcd (distributed key-value) there is a very convenient way to do this. On systemd's ExecStartPost I can just insert the started service into etcd without problems. My usecase needs something like this:

ExecStartPost=/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": 5555 }'

which works like a charm.

But this is where my idea popped up. Docker has the power to randomly assign a port if I just run docker run -p 5555 which is awesome since I don't have to set it statically in the *.service file and I could possibly run multiple instances on the same host. What if I could get the randomly assigned port and insert instead of the static 5555?

Turns out I can use the docker port command to get the port and with some formatting we can get just the port with

$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)

which works if I set it (using bash) like this:

/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": '$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)' }'

but using systemd I just can't get it to work. This is the code I'm using:

ExecStartPost=/usr/bin/etcdctl set /services/myServiceName '{ \"host\": \"%H\", \"port\": '$(echo $(/usr/bin/docker port my-container-name 5555) | cut -d':' -f2)'}'

Somehow I got something wrong but it's hard to debug since it works when typed in the terminal.