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Is it possible to configure systemd service that will delay the boot process (by about 10-20 minutes) to run a local backup script (something like dd if=/dev/sdb of=/dev/sdc) while filesystem is still read-only? Or maybe it's possible to execute such task just before shutdown?

I have tried several configurations, but still cannot achieve this behavior in my Ubuntu 22 box.

This variant causes timeout and boot process failed - system goes into maintenance mode:

[Unit]
Description=Backup on demand
DefaultDependencies=no
After=systemd-fsck-root.service
Before=systemd-remount-fs.service -.mount local-fs-pre.target local-fs.target

[Service]
Type=oneshot
TimeoutSec=0
ExecStart=/root/backup-on-boot.sh

[Install]
WantedBy=local-fs.target

This variant doesn't block boot process and run in background (which is undesired behavior)

[Unit]
Description=Backup on demand
After=local-fs.target
Wants=local-fs.target

[Service]
Type=oneshot
TimeoutSec=0
ExecStart=/root/backup-on-boot.sh

[Install]
WantedBy=multi-user.target
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  • Have you considered using snapshots rather than a full copy? (requires LVM, or BTRFS, or ZFS, or a SAN)
    – symcbean
    Commented Jun 8 at 11:05
  • This system is intended to run without kb/screen and boot from USB stick. The purpose of this backup is to have a bootable copy of the system on another flash drive, which can easily replace the system drive in case of a failure. (Of course dd is overkill, especially for flash drives, so I already switched to blocksync).
    – Andrey Chesnakov
    Commented Jun 8 at 12:02
  • Yes - so you create a snapshot, continue with boot, while writing the snapshot to the backup media in the background.
    – symcbean
    Commented Jun 8 at 12:05
  • Ah, I see. Yes, this could be a solution. Unfortunately I have used F2FS without LVM for system drive.
    – Andrey Chesnakov
    Commented Jun 8 at 12:16
  • Just a thought but you might want to make a custom target and make that the default; this target could have a unit that backs up the system and then switches to the normal target state (possibly via systemctl isolate). No idea if this would work, but it's something to think about!
    – Stephen Harris
    Commented Jun 8 at 12:43

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