TIL: The TTYPath= setting to starting an interactive program on boot

For $WORK I had to setup a Linux machine (a Raspberry Pi) so that right after boot, an interactive CLI tool would start, ready to receive input and show information.

Of course if you start a script at boot, its stdin is closed and stdout goes to the Journal. There exists openvt(1) to launch that script in a new VT (or to take over tty1). (But then you have to mind that openvt needs to run as root to open a new VT, so it should wrap sudo to drop privileges, and that get a bit fiddly.)

Then I happened upon the TTYPath setting, which does exactly what I need; and of course it combines perfectly fine with settings for User= or WorkingDirectory. I ended up with the following:

[Unit]
Description=Interactive script started on boot

[Install]
WantedBy=multi-user.target

[Service]
# The magic to have prompts displayed to the user
TTYPath=/dev/tty1
StandardInput=tty
StandardOutput=tty

User=someuser
WorkingDirectory=/opt/software/
ExecStart=/opt/software/bin/start_soft

# Always restart on exit, disable restart counter, delay a little bit
# the restart to avoid spam in case of repeating error
Restart=always
StartLimitIntervalSec=0
RestartSec=5s

It only occured to me to have a look at how getty.service is configured, and indeed it uses the same options:

StandardInput=tty
StandardOutput=tty
TTYPath=/dev/%I
TTYReset=yes          # reset $TTYPath before and after execution
TTYVHangup=yes        # disconnect all clients which have open $TTYPath
TTYVTDisallocate=yes  # deallocate before and after execution; this ensures screen and scrollback buffer is clear

A inconvenience is that if you start on tty1, then messages from systemd or the kernel get mixed up with your program output. If you start on tty2, I tried to add ExecStartPre=chvt 2, but the VT would switch back to tty1 depending on exact boot order.