genfatimage

genfatimage is a simple tool to create a minimized FAT or exFAT filesystem populated from a set of files on a host system, optionally with a specified amount of extra space.

genfatimage is written in Perl and requires the following packages from CPAN; these may very well be included with your Perl interpreter distribution:

Crypt::Random::Source
Date::Parse
Digest::CRC
Digest::SHA
Encode
PathTools
indirect

As it is written in Perl, it should work on just about any platform.

Usage

By default genfatimage creates an image with a partition table. Use the --flat or --floppy options to create only a filesystem image. The partition table is MBR unless either the --gpt or --efi options are specified, or the image exceeds 2 TiB in size.

The --efi option creates a GPT-partitioned image (unless --mbr is also specified) and marks it as an EFI system partition. Use this to create a bootable EFI/UEFI image.

The image is by default heavily optimized for size, and is primarily intended to be used for virtual filesystem images or other applications where data persistence is not a requirement. To generate an image intended to be written to physical media, especially writable media, the --media option selects safer defaults and is strongly recommended.

The --strict option disables all optimizations that may affect compatibility with non-compliant FAT implementations; see genfatimage --strict=help for additional information.

The output of genfatimage can be piped to another command if desired, by setting the output filename to -.

Usage documentation is provided by running genfatimage --help:

genfatimage 0.10
Usage: genfatimage [options] [-o] outfile [[[destination]:]:]input_path...

Create and populate a FAT filesystem image. By default it tries to
minimize the image as much as possible, leaving no free space and
minimizing all data structures to the smallest possible.

Inputs can be files or directories. By default, if the input is a
directory, its input_path will be discarded entirely and the contents
of that directory will be added to the root directory; if the input is
a file, the input filename will be used as its name and the file is
added to the root directory.

Pathnames that are invalid in FAT are converted unless
--strict=filename is in use.

The prefix "destination::" specifies that an input should be located
at a certain destination path in the generated image. If the
destination ends in "/", the input file name or last level directory
name will be appended to the destination path.

A prefix of ":" is equivalent to no prefix at all. This is
recommended if input_path may contain "::".

A prefix of "::" alone as that the entire input_path should be used
as the destination name.

Most options can be negated by prefixing their long forms with --no-.

Options:
-o, --output             Specify the output file (- for standard output)
    --flat, --floppy     Do not create a partition table
    --part               Create a partion table, either MBR or GPT [default]
    --mbr                Create an MBR partition table
    --gpt[=<partitions>] Create a GPT partition table [default with --efi]
-e ,--efi, --uefi        Mark the partition as an EFI system partition
-b, --boot, --active     Mark the partition as legacy BIOS bootable
-L, --label=<name>       Set the filesystem volume name
-H, --heads=#            Define heads per cylinder for legacy geometry [255]
-S, --secpertrack=#      Define sectors per track legacy geometry [63]
-v, --verbose[=#]        Set message verbosity
-r, --readonly           Set the readonly flag on every file
    --archive            Set the archive flag on every file
-R, --reserve=<files>,<bytes>,<rootfiles>,<namelen>
                         Specify additional space to reserve in the filesystem
             files:      Specify the minimum number of (small) new files [0]
             bytes:      Specify additional bytes of free space [0]
             rootfiles:  Indicate how may of <files> may be in / [0]
             namelen:    Assumed maximum filename length for new files [64]
-m, --media              Settings suitable for writing to physical media
-s, --strict             Strictly adhere to recommended filesystem parameters
    --strict=...         Fine grained control of strictness flags
    --strict=help        Show detailed help about the --strict option
-x, --exfat              Generate an exFAT filesystem if advantageous
-X, --exfat=force        Unconditionally generate an exFAT filesystem
-a, --align              Align data structures to multiples of the cluster size
    --no-pad             Do not write out empty sectors at the end of the image
    --no-backup-gpt      Omit the end-of-image backup GPT partition table
    --random             Use random IDs by default instead of a repeatable hash
    --volid=<id>         Set the volume ID to <id> (exFAT: GUID, FAT: 8 hex)
    --partid=<id>        Set the partition ID to <id> (GPT only: GUID)
    --diskid=<id>        Set the disk ID to <id> (GPT: GUID, MBR: 8 hex)
-U, --utc                Create time stamps in UTC, not local time
-D, --date=<date>        Force the time stamp of all files to <date>
-C, --creator=<name>     Set the filesystem creator name to <name> [GenFatIm]
    --print-offset       Print filesystem offset in the image after completion
-V, --version            Display the version information
-h, --help               Display this help text

Authorship and License

genfatimage was developed by H. Peter Anvin of Intel Corporation, and is released under the 2-clause BSD license:

Copyright 2025 Intel Corporation - All Rights Reserved

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.