Spawn getty over serial: systemd


The switch to systemd from init has been wonderful in the world of *nix. Here’s how you could quickly get a getty running on a serial ttyUSB device in the old system:

  • Create a ttyUSB0.conf file inside /etc/init/ and fill it with start on stopped and RUNLEVEL information, followed by an exec statement, that reads something like: /sbin/getty -L 9600 ttyUSB0 vt102. (see addendum)
  • Tell init to pick it up, by adding to /etc/rc.local the line sudo start ttyUSB0.

(Warning: this is the way I do it, which naturally means there’s something ugly about it!).

Here’s how you’d do this through systemd:

  • Modify the system-file in /lib/systemd/system/serial-getty@.service to change the baud-rate to what you need (technically you shouldn’t do this, but we need to get moving quick).
    The line: ExecStart:-/sbin/agetty 9600 %I $TERM instead of --keep-baud. This will tell it to use a specific baudrate as opposed to scanning through a list.
  • The systemctl daemon needs to know that you meddled with the file (which you shouldn’t have, you know.. but we did anyway, spilt milk). Do:
    sudo systemctl daemon-relaod
  • Fun part: just enable this setup once, and systemd will remember over reboots:
    sudo systemctl enable serial-getty@ttyUSB0

    Of course you can disable, start or stop this. Apparently it is not smart enough to ignore you when you start it twice. The way it enables is by creating a symlink from /etc/systemd/system/getty.target.wants/serial-getty@ttyUSB0.service to /lib/systemd/system/serial-getty@.service, in case the command doesn’t work (note that the latter is the file you edited previously). Feel free to explain this to me after reading more on this RHEL KB. :)


Addendum:

My old setup (using init) had this inside the ttyUSB0.conf file:
start on stopped rc RUNLEVEL=[2345]
stop on runlevel [!2345]
respawn
exec /sbin/getty -L 9600 ttyUSB0 vt102


Ooh, how do you know if you’re using systemd or init? If you’re system is up, then PID 1 should tell you :)
(ps -p1).

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