Environment (GNU Gzip)

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5 Environment

The obsolescent environment variable GZIP can hold a set of default options for gzip. These options are interpreted first and can be overwritten by explicit command line parameters. As this can cause problems when using scripts, this feature is supported only for options that are reasonably likely to not cause too much harm, and gzip warns if it is used. This feature will be removed in a future release of gzip.

You can use an alias or script instead. For example, if gzip is in the directory ‘/usr/bin’ you can prepend $HOME/bin to your PATH and create an executable script $HOME/bin/gzip containing the following:

#! /bin/sh
export PATH=/usr/bin
exec gzip -9 "$@"

The following environment variables are applicable only when using gzip on IBM Z mainframes supporting DEFLATE COMPRESSION CALL instruction:

DFLTCC: Whether DEFLATE COMPRESSION CALL should be used. Default value is ‘1’. Set this to ‘0’ to disable DEFLATE COMPRESSION CALL altogether.
DFLTCC_LEVEL_MASK: Compression levels on which DEFLATE COMPRESSION CALL should be used. Represented as a bit mask in decimal or hexadecimal form, where each bit corresponds to a compression level. Default value is ‘2’, which means level 1 only. In order to make use of DEFLATE COMPRESSION CALL by default, that is, on levels 1-6, set this to ‘0x7e’.
DFLTCC_BLOCK_SIZE: Size of deflate blocks produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘1048576’ (1 megabyte). When using DEFLATE COMPRESSION CALL to compress a file containing heterogeneous data (e.g. a ‘.tar’ archive containing text and binary files), setting this to a smaller value may improve compression ratio.
DFLTCC_FIRST_FHT_BLOCK_SIZE: Size of the first fixed deflate block produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘4096’ (4 kilobytes). When using DEFLATE COMPRESSION CALL to compress a small file, setting this to a larger value may improve compression ratio.
DFLTCC_RIBM: Value of "Reserved for IBM" field of DEFLATE COMPRESSION CALL parameter block. Default value is ‘0’.
SOURCE_DATE_EPOCH: If set to any value, disables compression with DEFLATE COMPRESSION CALL. This variable is normally set during reproducible builds, where DEFLATE COMPRESSION CALL must be disabled, because its output may not be reproducible.