I made a systemd service that launches a simple .sh to read data from a serial connection. If I run the service manually after booting it works fine, but when I try to run it automatically at boot it fails because the .sh can't yet read from ttyUSB0 (awk: fatal: cannot open file /dev/ttyUSB0' for reading (No such file or directory).

Is there any way to make the service wait for ttyUSB0 and then run the .sh? I tried something like after=ttyUSB0 but that doesn't work.

Description=Serial logger



Consider using an udev rule instead of a systemd service to start your script.

Something like this in /etc/udev/rules.d/99-serial-logger.rules:

SUBSYSTEM=="tty", ACTION=="add", KERNEL=="ttyUSB0", RUN+="/serial_script.sh"

Or if you want to keep the script as a systemd service (e.g. to allow it to be controlled using the systemctl command), remove the WantedBy=default.target line from your service and make your udev rule like this:

SUBSYSTEM=="tty", KERNEL=="ttyUSB0", TAG+="systemd", ENV{SYSTEMD_WANTS}+="your-serial-logger.service"

As a result, udev should tell systemd to start your service when the device appears, and to stop it if/when the device is removed.

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