Issue with boot-sel.d Hooks

[RLovelett]

Hi everyone,

I am attempting to setup ZFSBootMenu to boot between two environments, each with its own HOME dataset. Here’s the structure I’m aiming for:

• Boot Environments:

  • zroot/ROOT/debian dataset with its home directory in the zroot/HOME/debian dataset
  • zroot/ROOT/ubuntu dataset with its home directory in the zroot/HOME/ubuntu dataset

My goal is to use the boot-sel.d hooks to dynamically switch the canmount property of the HOME datasets based on the selected boot environment:

• When booting into Debian, zroot/HOME/debian is mounted at /home.
• When booting into Ubuntu, zroot/HOME/ubuntu is mounted at /home.

Current Setup

• ZFSBootMenu 2.3.0
• I am booting from the zroot/ROOT/ubuntu dataset using ZFSBootMenu.
• In that dataset is a boot-sel.d script located at /etc/zfsbootmenu/hooks/boot-sel.d/mount-home.sh.

Here’s the script I’ve written to test if hooks are being executed:

#!/bin/bash
# Determine the directory of the current script
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
LOG_FILE="${SCRIPT_DIR}/zfsbootmenu_boot_state.log"

# Create or clear the log file
echo "Running mount-home.sh" > /dev/tty
echo "Log file: ${LOG_FILE}" > /dev/tty
echo "=== ZFSBootMenu Boot-Selection Hook Log ===" > "${LOG_FILE}"
echo "Timestamp: $(date)" >> "${LOG_FILE}"

# Log ZFSBootMenu environment variables
echo -e "\n--- ZFSBootMenu Environment Variables ---" >> "${LOG_FILE}"
echo "ZBM_SELECTED_BE: ${ZBM_SELECTED_BE}" >> "${LOG_FILE}"
echo "ZBM_SELECTED_KERNEL: ${ZBM_SELECTED_KERNEL}" >> "${LOG_FILE}"
echo "ZBM_SELECTED_INITRAMFS: ${ZBM_SELECTED_INITRAMFS}" >> "${LOG_FILE}"
echo "ZBM_SELECTED_MOUNTPOINT: ${ZBM_SELECTED_MOUNTPOINT}" >> "${LOG_FILE}"

# Log all shell environment variables
echo -e "\n--- Shell Environment Variables ---" >> "${LOG_FILE}"
env >> "${LOG_FILE}"

# Check mounted filesystems
echo -e "\n--- Mounted Filesystems ---" >> "${LOG_FILE}"
mount >> "${LOG_FILE}"

# Check if pools are available and list them
echo -e "\n--- ZFS Pool Status ---" >> "${LOG_FILE}"
if command -v zfs >/dev/null 2>&1; then
    echo "ZFS command is available" >> "${LOG_FILE}"
    zpool status >> "${LOG_FILE}"
    zfs list >> "${LOG_FILE}"
else
    echo "ZFS command is NOT available" >> "${LOG_FILE}"
fi

# Log script completion
echo -e "\n--- Script Completed ---" >> "${LOG_FILE}"
echo "Log file written to ${LOG_FILE}"

Problems

1. After booting into Ubuntu, I don’t see anything that indicates if the hook ran (nothing in TTY and no log file). I expected a log file at /etc/zfsbootmenu/hooks/boot-sel.d/zfsbootmenu_boot_state.log.
2. I’m unsure how to debug the script in ZFSBootMenu. Is there a way to output directly to the ZFSBootMenu interface (e.g., using echo to /dev/tty) or log to a more reliable location?
3. I am trying to get a picture of the state/value of the variables ZBM_SELECTED_BE, ZBM_SELECTED_MOUNTPOINT so I can write my script.

What I’ve Checked

• The script is executable (chmod +x /etc/zfsbootmenu/hooks/boot-sel.d/mount-home.sh).
• The ZFSBootMenu version is latest. I assume so at least, I just downloaded it from https://get.zfsbootmenu.org/efi, I do not know how to get ZFSBootMenu to display the current version.
• The dataset containing the script (zroot/ROOT/ubuntu) is the one I am booting from.

Questions

1. Is there anything I need to do to enable boot-sel.d hooks, or should they run automatically if the script is correctly placed and executable?
2. Is my script in the correct location? Does ZFSBootMenu expect hooks to be elsewhere?
3. How can I debug or verify that hooks are being executed? For example, is there a way to output to the ZFSBootMenu interface during execution?

Any guidance or suggestions would be greatly appreciated. Thanks in advance!

[zdykstra]

User hooks stored at /etc/zfsbootmenu/hooks/... only work if you're building your own copy of ZFSBootMenu with generate-zbm. They're then copied to a specific location inside of ZFSBootMenu where they're executed during different loader stages. Your script isn't being executed by ZFSBootMenu because it's not available to it.

If you want to use the pre-built image, you'll need to put the hooks on your ESP, and pass in a parameter to the ZFSBootMenu KCL - specifically the zbm.hookroot option, detailed at https://docs.zfsbootmenu.org/en/v2.3.x/man/zfsbootmenu.7.html#command-line-parameters .

Alternatively, you can just add an entry to /etc/fstab on each BE to mount the appropriate home directory on boot.

[RLovelett]

Thank you! This clarified why my boot-sel.d hook wasn’t working; it hadn’t been included in the ZFSBootMenu environment. I didn’t realize hooks in /etc/zfsbootmenu/hooks/... only work when building ZFSBootMenu with generate-zbm, or that I could use the zbm.hookroot parameter for pre-built images.

I’ll follow the advice of using /etc/fstab and mountpoint=legacy though and abandon the whole boot-sel.d thing. So I’m marking this as the accepted answer. I really appreciate the help!

[ahesford]

You should absolutely use mountpoint=legacy and a proper /etc/fstab configuration for each boot environment. Changing ZFS properties in a ZFSBootMenu hook is going to be incredibly fragile and also requires you to import your pool read-write every boot. This increases boot times and also presents a negligible but nonzero risk of harm should your script do something wrong.

[RLovelett]

Thanks for this suggestion! Your explanation about the risks of requiring a writable pool convinced me this is the better approach. Using mountpoint=legacy with /etc/fstab is so much simpler and more robust than dynamically switching ZFS properties with hooks.

I’m marking the other answer as accepted since it addressed my question about boot-sel.d, but your advice is the solution I’ll be using.