Systemd

Introduction

The main aim of systemd (a "system and service manager") is to unify service configuration and behavior across Linux distributions. It bootstraps the user space and manages user processes. It also provides replacements for various daemons and utilities, including device management, login management, network connection management, and event logging.

Since 2015, the majority of Linux distributions have adopted systemd, having replaced other init systems such as SysV init.

Preventing shutdown and reboot while VDR is running

By default, the power button on a remote control that is managed by a kernel LIRC driver will be mapped to systemd-logind. Unless you have overridden the default HandlePowerKey=poweroff in /etc/systemd/logind.conf, the system would be shut down immediately when you hit a power button on any input device (remote control, keyboard, or power button on the computer case).

Often, VDR is the main application on the system. You would not want the system to be accidentally shut down or rebooted while someone is watching TV, or a recording is in progress. System shutdown would be initiated by a script that would be invoked by VDR itself; see the -s parameter of ExecStart below.

To disable or enable the normal handling of shutdown and reboot, you can use the following script, say, /etc/systemd/system/vdr-keep-alive.sh. Executing it will require super user privileges.

#!/bin/sh

TARGETS="
/lib/systemd/system/poweroff.target.d
/lib/systemd/system/reboot.target.d
/lib/systemd/system/halt.target.d
"
CONF=vdr-keep-alive.conf

case "$1" in
start)
  for t in $TARGETS
  do
    if [ ! -f "$t/$CONF" ]
    then
      if [ ! -d "$t/" ]
      then
        mkdir "$t"
      fi
      echo "[Unit]\nRefuseManualStart=yes" > "$t/$CONF"
    fi
  done
  ;;
stop)
  for t in $TARGETS
  do
    rm -f "$t/$CONF"
  done
  ;;
esac

exec systemctl daemon-reload

You can execute this script as follows:

sudo /etc/systemd/system/vdr-keep-alive.sh start
sudo /etc/systemd/system/vdr-keep-alive.sh stop

You might want to make the TARGETS above to include suspend, hibernate, or others mentioned in man 7 systemd.special.

Making Systemd start up VDR

Before systemd, you might have edited /etc/gettydefs to prevent a virtual console from being associated with a normal login prompt, and then have init invoke a shell script that would invoke VDR, often named runvdr. With systemd, all you need is a single file, say, /etc/systemd/system/vdr.service, with contents like the following:

[Unit]
Description=Video Disk Recorder
After=systemd-user-sessions.service plymouth-quit-wait.service
After=rc-local.service
After=getty@tty1.service
Conflicts=getty@tty1.service
Conflicts=shutdown.target
ConditionPathExists=/video/video

[Service]
User=pi
ExecStartPre=+/etc/systemd/system/vdr-keep-alive.sh start
ExecStart=/usr/local/bin/vdr --no-kbd --lirc=/dev/lirc0 -Prpihddevice -v /video/video -s /var/lib/vdr/vdr-shutdown.sh
TimeoutStartSec=infinity
Type=idle
Restart=on-failure
RestartSec=1s
TTYVTDisallocate=yes

Adjustments

• User= refers to the user account that will run the VDR service.
• ExecStart= must refer to the full VDR invocation.
• ConditionPathExists= is for the directory that contains the recordings, matching the -v parameter in ExecStart. If the recordings cannot be mounted, the service would not start up.

The shutdown script (named /var/lib/vdr/vdr-shutdown.sh in the above example) could do at least one of the following:

• Invoke sudo /etc/systemd/system/vdr-keep-alive.sh stop to allow normal reboot and shutdown.
• Adjust Nvram_wakeup to have the system start up automatically on the next scheduled recording.
• Invoke sudo systemctl stop vdr or sudo systemctl halt to shut down VDR or the entire system. This will also terminate the shell that runs the script!

Installation

sudo systemctl enable vdr
sudo systemctl start vdr

Example: Auto-starting VDR with swappable video storage

On a small system like the Raspberry_Pi, the internal storage is just large enough for the basic installation and system configuration. For recordings, you might want to use a USB-powered HDD or SSD.

You might make a virtue out of necessity and implement swappable video directories. One USB SSD might not be large enough for your entire collection of recordings. So, why not implement swappable storage, with the following user interface?

1. When you plug in the USB cable (or it is attached on system startup), VDR will start up automatically.
2. When you press the power button, VDR will shut down and the USB storage will be powered off, so that it can be safely unplugged.
3. When you plug in the USB cable again, VDR will start up again. This could be a different drive.

Preparations

Create a file system and label it VDR. Create the subdirectory video inside it.

Below, we assume that the storage devices that you want to initialize is /dev/sdd. Substitute the correct name for sdd below, and ensure that you are accessing the right device!

sudo mkfs.ext4 /dev/sdd1
sudo tune2fs -L /dev/sdd1
sudo mount /dev/sdd1 /mnt
mkdir /mnt/video
sudo umount /mnt

On the VDR system, create a mount point for the storage devices that have been prepared as above.

sudo mkdir -m 000 /video

You might think of a simple /etc/fstab entry like this:

LABEL=VDR /video ext4 defaults,noatime,nofail 0 1

But, we want the file system to be mounted automatically when it is labeled VDR. To achieve that, we create a file /etc/systemd/system/video.mount (the name must match the mount point /video):

[Unit]
BindsTo=dev-disk-by\x2dlabel-VDR.device
After=dev-disk-by\x2dlabel-VDR.device
Requires=systemd-fsck@dev-disk-by\x2dlabel-VDR.service
After=systemd-fsck@dev-disk-by\x2dlabel-VDR.service

[Mount]
Where=/video
What=/dev/disk/by-label/VDR
Type=ext4
Options=defaults,noatime,nofail

[Install]
WantedBy=dev-disk-by\x2dlabel-VDR.device

The magic .device unit refers to the path /dev/disk/by-label/VDR, which will become available when a device containing a file system labeled VDR is attached. We use that name also in the What= directive.

Now we are ready to enable the unit:

sudo systemctl enable video.mount

To check the status, you may find the following commands useful:

systemctl status
journalctl -u video.mount
journalctl -xe

We also want VDR to start automatically once the file system has been mounted. To do that, the following lines have to be added to /etc/systemd/system/vdr.service:

[Install]
WantedBy=dev-disk-by\x2dlabel-VDR.device

To notify systemd of the update:

sudo systemctl daemon-reload

Last, we need a VDR shutdown script /var/lib/vdr/vdr-shutdown.sh that will power off the storage so that it can be safely detached:

#!/bin/sh
set -eu
if [ "$5" = 1 ]
then
  sudo systemd-mount -u /dev/disk/by-label/VDR
  sudo sh -x /etc/systemd/system/vdr-keep-alive.sh stop
  sudo udisksctl power-off -b /dev/disk/by-label/VDR
fi

If anything is using the file system (whether it is VDR playing back a recording, or another process), unmounting will fail, and thanks to set -e the entire script will be terminated.

If unmounting the file system succeeds, we will re-enable the shutdown and reboot actions, and finally power off the drive. Thanks to the declared dependency, the VDR service will be shut down as part of the udisksctl command.

If the power button is pressed while VDR is not running, the system will be shut down. The power LED indicator on the USB storage device will become an indicator on whether VDR is running.