PostCreateHook: How To Populate A Newly Created Partition With Some Files?

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Introduction

When creating a new partition, it can be challenging to populate it with the necessary files. In this article, we will explore how to use the postCreateHook to create a file in the newly created partition. We will also discuss the context of using a password to decrypt a ZFS partition and how to achieve a no-password boot while maintaining encrypted ZFS incremental backups.

Context and Goal

The goal is to create a file with a password to decrypt a ZFS partition and store it in the /boot partition. This password will be used to create the ZFS partition, and the file will be copied to the newly created /boot partition using the postCreateHook. This approach may seem insecure, but it is a temporary solution to achieve a no-password boot while maintaining encrypted ZFS incremental backups.

Plan and Requirements

The plan involves the following steps:

  1. Use nixos-anywhere to create a disk with encryption keys from a file (/boot/mysecretfile.txt).
  2. Use disko's keylocation option to use the password from the file when creating the ZFS partition.
  3. Use postCreateHook to copy the file (/boot/mysecretfile.txt) to the newly created /boot partition.

Understanding postCreateHook

The postCreateHook is a hook that is executed after the creation of a partition. It allows you to perform actions on the newly created partition, such as creating files or directories. To use postCreateHook, you need to specify a script that will be executed after the partition is created.

Mounting the Partition

When creating a partition, it is not automatically mounted. However, you can specify a mount point using the mountpoint option in the configuration file. If you do not specify a mount point, the partition will not be mounted.

Environment Variables

The postCreateHook script has access to environment variables that provide information about the newly created partition. These variables include:

  • DISK_ID: The ID of the newly created disk.
  • PARTITION_ID: The ID of the newly created partition.
  • MOUNTPOINT: The mount point of the newly created partition.

Using postCreateHook to Populate the Partition

To use postCreateHook to populate the newly created partition, you need to create a script that will be executed after the partition is created. The script should create the necessary files or directories in the partition.

Here is an example script that copies the file (/boot/mysecretfile.txt) to the newly created /boot partition:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Copy the file to the newly created partition
cp /boot/mysecretfile.txt $MOUNTPOINT

Conclusion

In this article, we explored how to use the postCreateHook to populate a newly created partition with some files. We discussed the context of using a password to decrypt a ZFS partition and how to achieve a no-password boot while maintaining encrypted ZFS incremental backups. We also provided an example script that uses postCreateHook to copy a file to the newly created /boot partition.

Example Use Case

Here is an example use case that demonstrates how to use postCreateHook to populate a newly created partition:

{ pkgs, ... }:

{
  boot.loader.grub.device = "/dev/sda1";

  boot.postCreateHook = pkgs.writeShellScriptBin "populate-boot-partition" ''
    # Get the ID of the newly created partition
    PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

    # Get the mount point of the newly created partition
    MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

    # Copy the file to the newly created partition
    cp /boot/mysecretfile.txt $MOUNTPOINT
  '';

  fileSystems."/boot".options = [ "noatime" "nodiratime" ];

  fileSystems."/boot".fsType = "ext4";
}

Q: What is postCreateHook?

A: The postCreateHook is a hook that is executed after the creation of a partition. It allows you to perform actions on the newly created partition, such as creating files or directories.

Q: How do I use postCreateHook?

A: To use postCreateHook, you need to specify a script that will be executed after the partition is created. The script should create the necessary files or directories in the partition.

Q: What environment variables are available in postCreateHook?

A: The postCreateHook script has access to environment variables that provide information about the newly created partition. These variables include:

  • DISK_ID: The ID of the newly created disk.
  • PARTITION_ID: The ID of the newly created partition.
  • MOUNTPOINT: The mount point of the newly created partition.

Q: How do I get the ID of the newly created partition?

A: You can get the ID of the newly created partition using the lsblk command. For example:

lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}'

This command will print the ID of the newly created partition.

Q: How do I get the mount point of the newly created partition?

A: You can get the mount point of the newly created partition using the lsblk command. For example:

lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}'

This command will print the mount point of the newly created partition.

Q: Can I use postCreateHook to create a file in a specific directory?

A: Yes, you can use postCreateHook to create a file in a specific directory. For example:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Create a file in the specific directory
mkdir -p $MOUNTPOINT/mydir
touch $MOUNTPOINT/mydir/myfile.txt

This script creates a file (myfile.txt) in the directory (mydir) on the newly created partition.

Q: Can I use postCreateHook to create a directory in a specific location?

A: Yes, you can use postCreateHook to create a directory in a specific location. For example:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblko NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Create a directory in the specific location
mkdir -p $MOUNTPOINT/mydir/mydir2

This script creates a directory (mydir2) in the directory (mydir) on the newly created partition.

Q: Can I use postCreateHook to copy a file from one location to another?

A: Yes, you can use postCreateHook to copy a file from one location to another. For example:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Copy a file from one location to another
cp /boot/mysecretfile.txt $MOUNTPOINT/mydir/myfile.txt

This script copies the file (mysecretfile.txt) from the /boot directory to the directory (mydir) on the newly created partition.

Q: Can I use postCreateHook to delete a file or directory?

A: Yes, you can use postCreateHook to delete a file or directory. For example:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Delete a file or directory
rm -rf $MOUNTPOINT/mydir/myfile.txt

This script deletes the file (myfile.txt) in the directory (mydir) on the newly created partition.

Q: Can I use postCreateHook to create a symbolic link?

A: Yes, you can use postCreateHook to create a symbolic link. For example:

#!/bin/sh

# Get the ID of the newly created partition
PARTITION_ID=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $1}')

# Get the mount point of the newly created partition
MOUNTPOINT=$(lsblk -o NAME,FSTYPE,MOUNTPOINT -d -n | grep -E "nixos-boot" | awk '{print $3}')

# Create a symbolic link
ln -s $MOUNTPOINT/mydir/myfile.txt $MOUNTPOINT/mydir/mylink.txt

This script creates a symbolic link (mylink.txt) to the file (myfile.txt) in the directory (mydir) on the newly created partition.