How to write to stdin of a service

Question

I have a service running which has a CLI connected to stdin. When I ssh into the machine, I'd like to be able to send commands to the stdin of that service.

systemd.exec says that StandardInput=file:/path/to/file is a thing and supports FIFOs. That sounds like the simplest method.

I've tried this in my ~/.config/systemd/user/foo.service

[Service]
ExecStartPre=mkfifo %t/foo.stdin
ExecStart=cat -
StandardInput=file:%t/foo.stdin
ExecStopPost=rm -f %t/foo.stdin

In this example, I expect that when I run the following, I'll see the output echo'd in the journal.

echo "hello" > /run/user/1000/foo.stdin

I have two problems with this:

1. foo.service: Failed to set up standard input: No such file or directory . It appears that StandardInput= must exist before ExecStartPre=. Will I need to create a permanent pipe in a static location during installation or is there a work-around? If I make the file manually, and remove ExecStartPre/ExecStopPost, then things work.
2. The first echo "command" is processed fine, but sends an EOF and stdin closes. I wanted the stdin to remain open. The answer (exec 3> stdin, ..., exec 3>&-) seems to be to use bash FD redirection, but that isn't available in systemd.

Answer 1

You can create a socket unit for your FIFO:

• foo.socket:

  [Socket]
  ListenFIFO=%t/foo.stdin
  Service=foo.service
  

• foo.service:

  [Service]
  ExecStart=/bin/cat -
  Sockets=foo.socket
  StandardInput=socket
  StandardOutput=journal
  StandardError=journal
  

Start it up as follows:

systemctl --user start foo.socket

The service itself will start as soon as the FIFO is written into. If you want it to start immediately, instead launch it like this:

systemctl --user start foo.service