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bashsocketssystemd

Communicating with Systemd service through socket mapped to stdin

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)?

Solution

  • 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 [email protected] rather than myservice.service.

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