How can I send a process a message from a shell script?

Would love to know as well! D-Bus and Websockets come to mind.

I can think of solving this a number of ways. Do you have distribution available? If not i would use a domain socket if you are on a Linux or OSx machine. (i think OSX supports unix domain sockets)
From an iex session:

{:ok, sock} = :gen_udp.open(0, [{:ifaddr, {:local, '/tmp/testsockin'}}])

and from a shell

echo 'Hello, World!' | socat - UNIX-SENDTO:/tmp/testsockin

I’m thinking of trying to solve this without distribution since I don’t want to trust the network. That said these two processes will be on the same machine (although there will be another machine running Nerves and Scenic, but that will be a different communication mechanism). The machine running pianobar will be Linux (although it would be nice to support OS X as well).

What advantages does a domain socket have over a named pipe if we want to operate within the same operating system?

I know how to setup a domain socket and not a named pipe in Elixir

Just in case, you can use something like config :kernel, inet_dist_use_interface: {127, 0, 0, 1} to bind only on localhost (loopback).

Well, that was unexpectedly tediuous …

• Elixir port using named pipe via UNDOCUMENTED open_port/2 functionality
• Elixir port using named pipe with forwarding script
• Elixir port using named pipe with releasing script

I found that useful to send messages from system cron. Note the use of a backslash in front of %. I have a GenServer called CronServer in charge of parsing the message and calling the appropriate client function in the interested GenServers. I also use sched_ex for cron jobs internal to the app.

5 7 * * * ID=xxx_cron echo "minute. datetime: $(date +'\%FT\%T\%:z')" | socat - UNIX-SENDTO:/tmp/elixir_xxx_cron