How does this variable escaping work in a systemd unit file?

I have a fairly simple unit file for a discovery sidekick service for a server instance I'm running on CoreOS. The unit file looks like this:

[Unit]
Description=Discovery for frontend server (instance %i)
BindsTo=frontend@%i.service
After=frontend@%i.service

[Service]
EnvironmentFile=/etc/environment
ExecStart=/usr/bin/bash -c ' \
    while true; do \
        export PORT=$(docker port frontend%i 80 | sed s/.*://); \
        etcdctl set /services/frontend/%i "${COREOS_PRIVATE_IPV4}:$PORT" --ttl 60; \
        sleep 45; \
    done'
ExecStop=/usr/bin/etcdctl rm /services/frontend/%i

[X-Fleet]
MachineOf=frontend@%i.service

This works fine, but it took me ages to get to this stage, because if I change the etcdctl line to this:

etcdctl set /services/frontend/%i "${COREOS_PRIVATE_IPV4}:${PORT}" --ttl 60; \

Then it doesn't work - it ends up setting a value like 100.45.218.3:, with no port. Along the way I spent a lot of time playing with different uses of the $PORT variable, and I have no idea why the configuration I settled on works. At one point I had this in the script:

echo hi $PORT; \
echo "hi $PORT"; \
echo hi ${PORT}; \
echo "hi ${PORT}"; \

And got journal logs like this:

Aug 17 01:05:07 core-01 bash[53694]: hi 32769
Aug 17 01:05:07 core-01 bash[53694]: hi 32769
Aug 17 01:05:07 core-01 bash[53694]: hi
Aug 17 01:05:07 core-01 bash[53694]: hi

Essentially my question is: what's going on here? This flies in the face of how I understand {} to work in bash scripts. And why can I use curlies on the COREOS_PRIVATE_IPV4 variable (which is exported from /etc/environment, but not for PORT?

Solution 1:

This is documented in systemd.service(1). ${PORT} is expanded by systemd. To pass the $ to the shell you need to write $$, so $${PORT}. The important line is this:

To pass a literal dollar sign, use "$$". Variables whose value is not known at expansion time are treated as empty strings.

Solution 2:

1. if the content of PORT comes from some other bash variable you would be dealing with an indirect reference then please try:

   ${!PORT}
   

2. I assume you are sure your shell is Bash