diary at Telent Netowrks

Hotplug scripts in NixOS#

Thu, 15 Dec 2016 08:46:21 +0000

Prompted partly by the new Met police online traffic incident reporting tool (but it's been something i've been thinking about for a while) I bought an action camera the other day: it's a "Savfy", which is a cheap clone of the SJ4000 (which is itself a cheap clone of a Gopro).

Since I need to plug it into a USB port every day or two to recharge (battery life is a claimed 1.5 hours, haven't tested this yet) I thought it would be good to automate downloading the data off it as well. This is almost my first foray into systemd and udev, and certainly the first time I've tried it in NixOS, so I have reproduced my findings below.

First, we need to recognise when the camera is connected. It's USB mass storage, which means it shows up as a SCSI disk device (e.g. sdb). In /etc/nixos/configuration.nix we add a stanza something like this:

  services.udev = {
    path = [ "/home/dan/udev/bin" ];
    extraRules = ''
    ACTION=="add", KERNEL=="sd*[0-9]", ENV{ID_SERIAL}=="NOVATEKN_vt-DSC*", RUN+="${pkgs.systemd}/bin/systemctl --no-block start copyCamFiles@%k.service"
    '' ;

There are a few things worth noting here.

Great. We've got the trigger, where's the service? Again in configuration.nix

  systemd.services."copyCamFiles@" = {
    bindsTo = [ "dev-%i.device"] ;
    environment = { 
        RSYNC = "${pkgs.rsync}/bin/rsync"; 
        MOUNT = "${pkgs.utillinux}/bin/mount"; 
        UMOUNT = "${pkgs.utillinux}/bin/umount"; 
    serviceConfig = {
        Type = "simple";
        ExecStart = "${pkgs.bash}/bin/bash /home/dan/udev/bin/cp-actioncam.sh %I";

The @-sign in the service name means this isn't actually a service, it's a template (i.e. we can pass parameters to it to instantiate services). When it's invoked by udev, the parameter passed will be the device name of the newly-added partition. We export the pathnames of some utilities that the script will need, because I haven't built a nixos derivation for the script itself. simple as a service type means (I hope) that it's not started unless asked for and that nothing is going to try to restart it when it terminates.

Finally, here's the cp-actioncam.sh script

#!/usr/bin/env bash
set -e
unmount() {
    $UMOUNT $MP;
    rmdir $MP;
trap unmount 0
mkdir -p $MP $OUT 

And there you have it. There are a bunch of refinements that could profitably be made: most obviously, notifying the user somehow when the copying is finished and the device may be unplugged, and not putting root-owned files into a non-root-users home directory. But this will do for now.

Some browser tabs I can now close: