bootc-install-to-existing-root - Man Page

If you need to inject new configuration files (such as custom /etc/fstab entries, systemd mount units, or other configuration) into the newly installed system before rebooting, you can find the deployment directory in the ostree repository structure. The new deployment is located at:

/ostree/deploy/<stateroot>/deploy/<checksum>.<serial>/

Where <stateroot> defaults to default unless you specified a different value with --stateroot.

To find the path to the newly installed deployment:

# Get the full deployment path directly
DEPLOY_PATH=$(ostree admin --sysroot=/target --print-current-dir)

This will return the full path, for example: /target/ostree/deploy/default/deploy/807f233831a03d315289a4ba29c1670d8bd326d4569eabee7a84f25327997307.0

You can then modify files in that deployment. For example, to add systemd mount units:

# Get deployment path
DEPLOY_PATH=$(ostree admin --sysroot=/target --print-current-dir)
# Add a systemd mount unit
vi ${DEPLOY_PATH}/etc/systemd/system/data.mount

A better approach for machine-local configuration like filesystem mounts is to inject kernel arguments during installation. Kernel arguments are ideal for local/machine-specific state in a bootc system.

For filesystem mounts, use systemd.mount-extra instead of /etc/fstab:

# Add a mount via kernel argument (preferred over /etc/fstab)
bootc install to-existing-root \
  --karg="systemd.mount-extra=UUID=<uuid>:/data:xfs:defaults"

The systemd.mount-extra syntax is: source:path:type:options

You can also inject other local kernel arguments for machine-specific configuration:

# Add console settings for serial access
bootc install to-existing-root --karg="console=ttyS0,115200"

# Add storage-specific options
bootc install to-existing-root --karg="rootflags=subvol=root"

This approach is cleaner than editing configuration files because kernel arguments are explicitly designed for local/machine-specific state in a bootc system.

Note: In the future, this functionality will be provided via a dedicated bootc API to make finding and modifying the deployment more straightforward.

After rebooting into the new bootc system, the previous root filesystem data is accessible at /sysroot (the "physical root"). This allows you to migrate data from the old system to the new one.

Important: Any configuration data from /etc that you want to use in the new system must be manually copied from /sysroot/etc to /etc after rebooting. There is currently no automated mechanism for migrating this data.

For example, to migrate configuration after rebooting:

# After rebooting into the new system
# Copy network configuration from the old system
cp /sysroot/etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/

# Copy application configuration
cp -r /sysroot/etc/myapp /etc/

# Selectively merge configuration files
vi /etc/resolv.conf  # Add nameservers from /sysroot/etc/resolv.conf

# For user accounts, use proper tools
vipw  # Carefully review and merge users from /sysroot/etc/passwd

This applies to network configurations, user accounts, application settings, and other system configuration stored in /etc. Review files in /sysroot/etc and manually copy or merge what you need into /etc.

Note: For filesystem mounts from /etc/fstab in the old system, consider using kernel arguments (via systemd.mount-extra) injected before reboot instead of migrating the fstab entries. See the "Injecting kernel arguments" section above.