#+TITLE: 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 [[https://manpages.debian.org/bookworm/kbd/openvt.1.en.html][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 [[https://manpages.debian.org/bookworm/systemd/systemd.exec.5.en.html#:~:text=Sets%20the%20terminal%20device%20node%20to%20use][=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:

#+BEGIN_SRC
[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
#+END_SRC

It only occured to me to have a look at how =getty.service= is configured, and
indeed it [[https://github.com/systemd/systemd/blob/e127c66985b9e338fcb79900a88a24848d5d31fb/units/getty%40.service.in#L46][uses the same options]]:

#+BEGIN_SRC
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
#+END_SRC

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.