_etc_systemd_system_dynbg.service

[Unit]
Description=Create dynamic bg.svg in /tmp.

[Service]
User=nobody
Group=nobody
ExecStart=/usr/local/bin/dynbg.py /usr/local/etc/dynbg.svg /tmp/bg.svg
Restart=no
Type=oneshot

_etc_systemd_system_dynbg.timer

[Unit]
Description=Update /tmp/bg.svg

[Timer]
OnCalendar=*-*-* *:*:00
Persistent=true
Unit=dynbg.service

[Install]
WantedBy=multi-user.target

_usr_local_bin_dynbg.py

#!/bin/python
# Author: Milan Oberkirch <milan.py@oberkirch.org>

import subprocess
from sys import argv, exit
from string import printable
from re import sub


def dynbg(template, output):
    def bash(match):
        cmd = match.group(0)[2:-1]
        process = subprocess.Popen(
            ['bash', "-c",  cmd],
            stdout=subprocess.PIPE,
            stderr=subprocess.PIPE
        )
        return ''.join(
            filter(lambda x: x in printable, process.communicate()[0].decode())
        ).replace('\n', '</tspan><tspan dy="1em">')

    with open(template, 'r') as file:
        file_content = file.read()

    with open(output, 'w') as file:
        file.write(sub(r'\$\(.+\)', bash, file_content))


if __name__ == '__main__':
    if len(argv) == 3:
        this, template, output = argv
    else:
        print('Usage: %s template.svg output.svg' % argv[0])
        exit(1)

    dynbg(template, output)

_usr_local_etc_dynbg.svg