Backups are important, regular ones doubly so. Unfortunately it's very easy to forget to make regular backups. Since Qubes stores the date and time when a VM was backed up, we will use this information to remind the user (probably you) when it has been more than N days since a backup.

Architecture

We use an "exclude file" to exclude some VMs which we don't backup, and therefore for which we should not check timestamps for.

Since we're probably running on a laptop, we run the script daily using anacron instead of cron, since anacron will deal with situtions when the machine is frequently in "suspend" mode. (Which laptops are)

We set up an anacrontab for our user (as opposed to using a system-wide one) and run anacron hourly using the user's crontab. This gives some minor security benefits (by not running things as root) and reduces the chance of accidentally causing problems in the system through misconfiguration.

Implementation

Excludes file

A plain text file containing the names of VMs which are not backed up. In this example, we save it in dom0, /home/${USER}/backup/exclude_vms.txt:

fedora-30
whonix-gw-15
whonix-ws-15

Backup script

#!/bin/bash

# Remind if this many days since backup
DAYS_THRESHOLD=3

# Setup variables
BACKUP_DIR="/home/${USER}/backup"
EXCLUDE_FILE=${BACKUP_DIR}/exclude_vms.txt


# Build backup VM list
ALL_VMS=(`/usr/bin/qvm-ls --raw-list`)
EXCLUDE_VMS=(`/usr/bin/cat $EXCLUDE_FILE`)
EXCLUDE_VMS+=("dom0")
BACKUP_VMS=()
for i in "${ALL_VMS[@]}"; do
        skip=
        for j in "${EXCLUDE_VMS[@]}"; do
                [[ $i == $j ]] && { skip=1; break; }
        done
        [[ -n $skip ]] || BACKUP_VMS+=("$i")
done

# Get oldest known backup TS
TS=`/usr/bin/date +%s`
echo "TS now: $TS"
for vm in "${BACKUP_VMS[@]}"; do
        vm_ts=`/usr/bin/qvm-prefs --get $vm backup_timestamp`

        if [ "$vm_ts" -lt "$TS" ]; then
                echo "New oldest TS: $vm_ts"
                TS=$vm_ts
        fi
done

# Get delta between current time and oldest backup
NOW=`/usr/bin/date +%s`
DELTA=`/usr/bin/expr $NOW - $TS`
DELTA_DAYS=`/usr/bin/expr $DELTA / 86400`
echo "delta in seconds: $DELTA / days: $DELTA_DAYS"
if [ "$DELTA_DAYS" -gt "$DAYS_THRESHOLD" ]; then
        /usr/bin/notify-send --expire-time 86400000 "It has been $DELTA_DAYS days since last backup"
fi

echo `/usr/bin/date` >> $BACKUP_DIR/reminders.log
remind.sh (END)

You can try running it right away to see how long it's been since your last backup.

Anacrontab

Anacron should be installed by default in dom0, but you can check it by running (in dom0): dnf info cronie-anacron and verifying the first line of the info is "Installed Packages".

Now open up the file ${HOME}/.config/anacrontab in your text editor of choice and add to it the following, replacing USERNAME_GOES_HERE with the dom0 user account name (to find it, echo ${USER}:

# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=USERNAME_GOES_HERE
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

#period in days   delay in minutes   job-identifier   command
@daily 0        backup-reminder /home/USERNAME_GOES_HERE/backup/remind.sh

Crontab

Finally, add the following to your crontab file, accessed by running crontab -e: (crontab -e is the only way you should access your crontab)