xdelta

Apply a patch

$ xdelta -d -s [path/to/input_file] [path/to/delta_file.xdelta] [path/to/output_file]

$ xdelta -e -s [path/to/old_file] [path/to/new_file] [path/to/output_file.xdelta]

xdelta3 [OPTIONS] [COMMAND] [SOURCE] [TARGET] [DELTA]

Common Usage:
xdelta3 diff SOURCE TARGET [DELTA_OUTPUT_FILE]
xdelta3 patch SOURCE DELTA_FILE [TARGET_OUTPUT_FILE]

-s SOURCE, --source=SOURCE
    Specifies the source file, which serves as the base for creating or applying a delta. Required for delta operations.

-e, --encode
    Sets the command to create a delta. Alias for the 'diff' command. Requires a source and target file.

-d, --decode
    Sets the command to apply a delta. Alias for the 'patch' command. Requires a source file and a delta file.

-0 to -9
    Sets the compression level. 0 means no compression, 9 means best compression. Higher levels use more CPU and memory but produce smaller deltas.

-v, --verbose
    Increases the verbosity of output, showing more details about the operation.

-q, --quiet
    Decreases the verbosity of output, suppressing informational messages.

-W SIZE, --window-size=SIZE
    Sets the VCDIFF window size. Larger windows can improve compression but consume more memory during delta creation and application.

--test
    Tests a delta by applying it and verifying the resulting file's integrity against the original target, without writing the output file.

--help
    Displays a help message with available commands and options.

xdelta (most commonly referring to xdelta3) is a powerful command-line utility designed for computing and applying binary differences (deltas) between two files. It implements the VCDIFF (RFC 3284) delta compression standard, making it highly efficient for large files and interoperable with other VCDIFF-compliant tools. Its primary use case is distributing software updates or syncing large datasets, where only the changes between file versions need to be transferred. This significantly reduces bandwidth consumption and download times compared to transferring entire files. xdelta can generate a delta file from a source and target file, and then reconstruct the target file from the source and the delta, even on different systems. It's optimized for speed and compression, making it a critical tool in many large-scale distribution systems.

Memory Usage: xdelta can consume significant amounts of memory, especially with large files or when using large window sizes (-W option), which might lead to performance issues or out-of-memory errors on systems with limited RAM.
Source File Integrity: For successful patching, the source file used during the patch operation must be absolutely identical to the source file used when the delta was created. Any discrepancy will result in a failed patch.
Binary Data Only: While it technically works on any file, xdelta is designed for binary data. For text files, diff and patch are usually more appropriate and provide human-readable output.
No Built-in Checksum Verification: While VCDIFF includes some integrity checks, users often need to implement external checksum verification (e.g., SHA256) of the resulting patched file to ensure full data integrity.

xdelta performs two primary operations:
1. Diffing (Encoding): Taking a SOURCE file and a TARGET file, it computes the binary differences between them and writes these differences to a DELTA file. This DELTA file is typically much smaller than the TARGET file itself.
2. Patching (Decoding): Taking a SOURCE file and a DELTA file, it applies the changes described in the DELTA to the SOURCE to reconstruct the TARGET file. This is how incremental updates are delivered and applied.

xdelta excels in scenarios where a large number of users need to update software or data from one version to another. Instead of downloading the entire new version, users only download the relatively small delta file, significantly reducing bandwidth usage and accelerating the update process, especially over slow or metered connections.

The original xdelta was developed by Joshua MacDonald. The most widely used version today is xdelta3, which is a significant rewrite and implements RFC 3284 (VCDIFF Generic Differencing and Compression Data Format). This adherence to a standard format makes xdelta3 highly interoperable. Its development has been driven by the need for efficient binary diffing in large-scale software distribution systems, notably being utilized by Google for updating Chrome and Android operating system over-the-air (OTA) updates.

diff(1), patch(1), bsdiff(1), gzip(1)