Communicating with Systemd service through socket mapped to stdin

user_1977415 0
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I'm creating my first background service and I want to communicate with it through a socket.

I have the following script /tmp/myservice.sh:

#! /usr/bin/env bash

while read received_cmd
do
    echo "Received command ${received_cmd}"
done

And the following socket /etc/systemd/user/myservice.socket

[Unit]
Description=Socket to communicate with myservice

[Socket]
ListenSequentialPacket=/tmp/myservice.socket

And the following service:

[Unit]
Description=A simple service example

[Service]
ExecStart=/bin/bash /tmp/myservice.sh
StandardError=journal
StandardInput=socket
StandardOutput=socket
Type=simple

The idea is to understand how to communicate with a background service, here using an unix file socket. The script works well when launched from the shell and reading stdin and I thought that by setting StandardInput = "socket" it would read from the socket the same way.

Nevertheless, when I run nc -U /tmp/myservice.socket the command returns right away and I have the following output:

$ journalctl --user -u myservice
-- Logs begin at Sat 2020-10-24 17:26:25 BST, end at Thu 2020-10-29 14:00:53 GMT. --
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21941]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21942]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21943]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21944]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21945]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Start request repeated too quickly.
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Failed with result 'start-limit-hit'.
Oct 29 08:40:16 shiny systemd[1689]: Failed to start A simple service example.

Did I misunderstand how sockets work? Why read fails to read from the socket? Should I use another mechanism to communicate with my background service (as I said, it's my first background service so I may do unconventional things here)?

systemd sockets bash
1 Answers

The only thing I have seen working with a shell script is ListenStream= rather than ListenSequentialPacket=. (Obviously, this means you lose packet boundaries, but the shell read is usually oriented to read lines ending \n from streams, so it is not usually a problem).

But the most important thing that is missing, is the extra Accept line:

[Socket]
ListenStream=...
Accept=true

As I understand it, without this the service will be passed a socket on which it must first do a socket accept() call, to get the actual connection socket (hence the read error). The service must also then handle all further connections.

By using Accept=true, a new service will be started for each new connection, and will be passed the immediately usable socket. Note, however, that this means the service must now be templated, i.e. called myservice@.service rather than myservice.service.

(For datagram sockets, Accept must be left defaulted to false). See man systemd.socket.

user_50082840
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