update-initramfs

-c
    create
Creates a new initramfs image for the specified kernel version. If no version is given with -k, it defaults to the currently running kernel.

-u
    update
Updates an existing initramfs image for the specified kernel version. This is the most common operation after system updates.

-d
    delete
Deletes an existing initramfs image for the specified kernel version. Useful for cleaning up old kernel images.

-k version
    kernel version
Specifies the kernel version(s) to operate on. Can be a specific version string (e.g., 5.15.0-78-generic), all (to operate on all installed kernels), or current (to operate on the currently running kernel).

-b builddir
    build directory
Specifies an alternative build directory to use instead of the default.

-t
    test mode
Runs in test mode; the actual image is not created or modified, but the process is simulated.

-v
    verbose output
Provides more detailed output during the operation, showing which files and modules are being processed.

The update-initramfs command is a crucial utility on Debian-based Linux systems, responsible for managing the initial RAM filesystem (initramfs) images. These images are small, self-contained filesystems loaded into memory by the bootloader (like GRUB) before the main root filesystem is mounted. Their primary purpose is to provide the necessary drivers and tools to mount the actual root filesystem. This includes support for complex setups like LVM (Logical Volume Management), software RAID, disk encryption (e.g., LUKS), or non-standard filesystem types.

When a new kernel is installed, updated, or when critical hardware configurations (such as disk controllers or storage devices) change, the initramfs image often needs to be rebuilt to include the correct modules and utilities. update-initramfs automates this process by scanning the system for required modules and configuration, then packaging them into an executable image. Without a correctly built initramfs, the system may fail to find and mount its root filesystem, rendering it unbootable. It relies heavily on configuration files in /etc/initramfs-tools/ and hooks/scripts to determine what to include in the image.

Root Privileges Required: This command must be run with superuser (root) privileges.
System Bootability: Incorrect or incomplete initramfs images can prevent your system from booting. Always exercise caution, especially when manually modifying configurations or using unknown options.
Distribution Specific: update-initramfs is part of the initramfs-tools package, primarily used on Debian, Ubuntu, Mint, and other Debian-derived distributions. Other Linux distributions (e.g., Fedora, CentOS, openSUSE) typically use different tools like Dracut for similar purposes.

The behavior of update-initramfs is heavily influenced by configuration files located in /etc/initramfs-tools/.
/etc/initramfs-tools/initramfs.conf: Defines general settings like the COMPRESS method, modules to include, and whether to create a debug initramfs.
/etc/initramfs-tools/modules: A list of additional kernel modules that should always be included in the initramfs image, beyond what is automatically detected.

update-initramfs uses a hook-based system to extend its functionality. Scripts placed in specific directories are executed during the image creation process:
/etc/initramfs/update-initramfs.d/: User-defined hooks that run at various stages of the initramfs build.
/usr/share/initramfs-tools/hooks/: System-provided hooks that add essential functionality (e.g., LVM support, encryption).
These scripts allow for custom drivers, utilities, or pre-boot configurations to be included in the initramfs.

The concept of an initial RAM disk has evolved from initrd (initial RAM disk), which was a gzipped filesystem image, to initramfs (initial RAM filesystem). initramfs is a cpio archive that is unpacked directly into a tmpfs (temporary filesystem) in memory, offering greater flexibility and efficiency. The update-initramfs command is part of the initramfs-tools package, which was developed to provide a robust and extensible framework for building initramfs images on Debian systems. Its development has focused on automating the complex process of identifying and including necessary modules and scripts, ensuring system bootability across various hardware and software configurations.

mkinitramfs(8), initramfs-tools(8), dracut(8), update-grub(8)