Introduction

To compute a delta between externally-compressed inputs (e.g., xdelta3 -e -s source.gz target.gz), xdelta3 recognizes several common compression programs and uncompresses the data before reading it, including gzip, bzip2, and compress. When the decoder finishes decompression, it re-applies the external compression.

External compression is not supported on Windows. Windows users should manually decompress the inputs prior to running xdelta3 (sorry).

Details

Xdelta decompresses the inputs using pipes to the external compression program.

Recognition of externally-compressed inputs can be disabled by -D.

External compression has a well known problem: xdelta3 does not know the original compression settings, so it when re-applying the external compression it uses defaul settings, which may break checksum verification of the compressed data. External recompression (of the output) can be disabled by -R. If you know the compression settings needed for the external compression to produce the exact output (e.g., gzip -9), consider setting an environment variable to control the external compression command (e.g., export GZIP=-9).

Example:

gzip release-1.tar
gzip release-2.tar
xdelta3 -e -s release-1.tar.gz release-2.tar.gz delta-1-2.xd3
xdelta3 -d -s release-1.tar.gz delta-1-2.xd3 release-2.tar.gz

Because of the above-mentioned pitfalls, xdelta3 prints the following warning:

xdelta3: WARNING: the encoder is automatically decompressing the input file;
xdelta3: WARNING: the decoder will automatically recompress the output file;
xdelta3: WARNING: this may result in different compressed data and checksums
xdelta3: WARNING: despite being identical data; if this is an issue, use -D
xdelta3: WARNING: to avoid decompression and/or use -R to avoid recompression
xdelta3: WARNING: and/or manually decompress the input file; if you know the
xdelta3: WARNING: compression settings that will produce identical output
xdelta3: WARNING: you may set those flags using the environment (e.g., GZIP=-9)