| 1 | 1 GZIP
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| 2 | NAME
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| 3 | gzip, gunzip, zcat - compress or expand files
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| 4 |
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| 5 | SYNOPSIS
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| 6 | gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
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| 7 | gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
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| 8 | zcat [ -fhLV ] [ name ... ]
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| 9 |
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| 10 | 2 DESCRIPTION
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| 11 | Gzip reduces the size of the named files using Lempel-Ziv
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| 12 | coding (LZ77). Whenever possible, each file is replaced by
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| 13 | one with the extension .gz, while keeping the same ownership
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| 14 | modes, access and modification times. (The default exten-
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| 15 | sion is -gz for VMS, z for MSDOS, OS/2 FAT, Windows NT FAT
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| 16 | and Atari.) If no files are specified, or if a file name is
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| 17 | "-", the standard input is compressed to the standard out-
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| 18 | put. Gzip will only attempt to compress regular files.
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| 19 |
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| 20 | If the compressed file name is too long for its file system,
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| 21 | gzip truncates it. Gzip attempts to truncate only the parts
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| 22 | of the file name longer than 3 characters. (A part is del-
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| 23 | imited by dots.) If the name consists of small parts only,
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| 24 | the longest parts are truncated. For example, if file names
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| 25 | are limited to 14 characters, gzip.msdos.exe is compressed
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| 26 | to gzi.msd.exe.gz. Names are not truncated on systems which
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| 27 | do not have a limit on file name length.
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| 28 |
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| 29 | By default, gzip keeps the original file name and timestamp
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| 30 | in the compressed file. These are used when decompressing
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| 31 | the file with the -N option. This is useful when the
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| 32 | compressed file name was truncated or when the time stamp
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| 33 | was not preserved after a file transfer.
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| 34 |
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| 35 | Compressed files can be restored to their original form
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| 36 | using gzip -d or gunzip or zcat. If the original name saved
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| 37 | in the compressed file is not suitable for its file system,
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| 38 | a new name is constructed from the original one to make it
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| 39 | legal.
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| 40 |
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| 41 | gunzip takes a list of files on its command line and
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| 42 | replaces each file whose name ends with .gz, -gz, .z, -z, _z
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| 43 | or .Z and which begins with the correct magic number with an
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| 44 | uncompressed file without the original extension. gunzip
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| 45 | also recognizes the special extensions .tgz and .taz as
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| 46 | shorthands for .tar.gz and .tar.Z respectively. When
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| 47 | compressing, gzip uses the .tgz extension if necessary
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| 48 | instead of truncating a file with a .tar extension.
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| 49 |
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| 50 | gunzip can currently decompress files created by gzip, zip,
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| 51 | compress, compress -H or pack. The detection of the input
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| 52 | format is automatic. When using the first two formats,
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| 53 | gunzip checks a 32 bit CRC. For pack, gunzip checks the
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| 54 | uncompressed length. The standard compress format was not
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| 55 | designed to allow consistency checks. However gunzip is
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| 56 | sometimes able to detect a bad .Z file. If you get an error
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| 57 | when uncompressing a .Z file, do not assume that the .Z file
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| 58 | is correct simply because the standard uncompress does not
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| 59 | complain. This generally means that the standard uncompress
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| 60 | does not check its input, and happily generates garbage out-
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| 61 | put. The SCO compress -H format (lzh compression method)
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| 62 | does not include a CRC but also allows some consistency
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| 63 | checks.
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| 64 |
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| 65 | Files created by zip can be uncompressed by gzip only if
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| 66 | they have a single member compressed with the 'deflation'
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| 67 | method. This feature is only intended to help conversion of
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| 68 | tar.zip files to the tar.gz format. To extract zip files
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| 69 | with several members, use unzip instead of gunzip.
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| 70 |
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| 71 | zcat is identical to gunzip -c. (On some systems, zcat may
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| 72 | be installed as gzcat to preserve the original link to
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| 73 | compress.) zcat uncompresses either a list of files on the
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| 74 | command line or its standard input and writes the
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| 75 | uncompressed data on standard output. zcat will uncompress
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| 76 | files that have the correct magic number whether they have a
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| 77 | .gz suffix or not.
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| 78 |
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| 79 | Gzip uses the Lempel-Ziv algorithm used in zip and PKZIP.
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| 80 | The amount of compression obtained depends on the size of
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| 81 | the input and the distribution of common substrings. Typi-
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| 82 | cally, text such as source code or English is reduced by
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| 83 | 60-70%. Compression is generally much better than that
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| 84 | achieved by LZW (as used in compress), Huffman coding (as
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| 85 | used in pack), or adaptive Huffman coding (compact).
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| 86 |
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| 87 | Compression is always performed, even if the compressed file
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| 88 | is slightly larger than the original. The worst case expan-
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| 89 | sion is a few bytes for the gzip file header, plus 5 bytes
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| 90 | every 32K block, or an expansion ratio of 0.015% for large
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| 91 | files. Note that the actual number of used disk blocks
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| 92 | almost never increases. gzip preserves the mode, ownership
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| 93 | and timestamps of files when compressing or decompressing.
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| 94 |
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| 95 | 2 OPTIONS
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| 96 | -a --ascii
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| 97 | Ascii text mode: convert end-of-lines using local con-
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| 98 | ventions. This option is supported only on some non-
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| 99 | Unix systems. For MSDOS, CR LF is converted to LF when
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| 100 | compressing, and LF is converted to CR LF when
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| 101 | decompressing.
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| 102 |
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| 103 | -c --stdout --to-stdout
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| 104 | Write output on standard output; keep original files
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| 105 | unchanged. If there are several input files, the out-
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| 106 | put consists of a sequence of independently compressed
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| 107 | members. To obtain better compression, concatenate all
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| 108 | input files before compressing them.
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| 109 |
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| 110 | -d --decompress --uncompress
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| 111 | Decompress.
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| 112 |
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| 113 | -f --force
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| 114 | Force compression or decompression even if the file has
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| 115 | multiple links or the corresponding file already
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| 116 | exists, or if the compressed data is read from or writ-
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| 117 | ten to a terminal. If the input data is not in a format
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| 118 | recognized by gzip, and if the option --stdout is also
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| 119 | given, copy the input data without change to the stan-
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| 120 | dard output: let zcat behave as cat. If -f is not given,
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| 121 | and when not running in the background, gzip prompts to
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| 122 | verify whether an existing file should be overwritten.
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| 123 |
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| 124 | -h --help
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| 125 | Display a help screen and quit.
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| 126 |
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| 127 | -l --list
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| 128 | For each compressed file, list the following fields:
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| 129 |
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| 130 | compressed size: size of the compressed file
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| 131 | uncompressed size: size of the uncompressed file
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| 132 | ratio: compression ratio (0.0% if unknown)
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| 133 | uncompressed_name: name of the uncompressed file
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| 134 |
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| 135 | The uncompressed size is given as -1 on VMS because it
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| 136 | it is not possible to seek reliably to the end of the
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| 137 | compressed file, where this size is stored.
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| 138 |
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| 139 | In combination with the --verbose option, the following
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| 140 | fields are also displayed:
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| 141 |
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| 142 | method: compression method
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| 143 | crc: the 32-bit CRC of the uncompressed data
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| 144 | date & time: time stamp for the uncompressed file
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| 145 |
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| 146 | The compression methods currently supported are
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| 147 | deflate, compress, lzh (SCO compress -H) and pack. The
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| 148 | crc is given as ffffffff on VMS for the reason given
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| 149 | above about the uncompressed size.
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| 150 |
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| 151 | With --name, the uncompressed name, date and time are
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| 152 | those stored within the compress file if present.
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| 153 |
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| 154 | With --verbose, the size totals and compression ratio
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| 155 | for all files is also displayed, unless some sizes are
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| 156 | unknown. With --quiet, the title and totals lines are
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| 157 | not displayed.
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| 158 |
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| 159 | -L --license
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| 160 | Display the gzip license and quit.
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| 161 |
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| 162 | -n --no-name
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| 163 | When compressing, do not save the original file name
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| 164 | and time stamp by default. (The original name is always
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| 165 | saved if the name had to be truncated.) When
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| 166 | decompressing, do not restore the original file name if
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| 167 | present (remove only the gzip suffix from the
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| 168 | compressed file name) and do not restore the original
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| 169 | time stamp if present (copy it from the compressed
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| 170 | file). This option is the default when decompressing.
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| 171 |
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| 172 | -N --name
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| 173 | When compressing, always save the original file name
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| 174 | and time stamp; this is the default. When decompress-
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| 175 | ing, restore the original file name and time stamp if
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| 176 | present. This option is useful on systems which have a
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| 177 | limit on file name length or when the time stamp has
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| 178 | been lost after a file transfer.
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| 179 |
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| 180 | -q --quiet
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| 181 | Suppress all warnings.
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| 182 |
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| 183 | -r --recursive
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| 184 | Travel the directory structure recursively. If any of
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| 185 | the file names specified on the command line are direc-
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| 186 | tories, gzip will descend into the directory and
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| 187 | compress all the files it finds there (or decompress
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| 188 | them in the case of gunzip ).
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| 189 |
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| 190 | -S suf --suffix suf
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| 191 | Use suffix suf instead of -gz. Any suffix can be
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| 192 | given, but suffixes other than -z and -gz should be
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| 193 | avoided to avoid confusion when files are transferred
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| 194 | to other. A null suffix forces gunzip to try
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| 195 | decompression on all given files regardless of suffix,
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| 196 | as in:
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| 197 |
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| 198 | gunzip --suffix "" *.*
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| 199 |
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| 200 | Previous versions of gzip used the -z suffix. This was
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| 201 | changed to avoid a conflict with pack on Unix.
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| 202 |
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| 203 | -t --test
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| 204 | Test. Check the compressed file integrity.
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| 205 |
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| 206 | -v --verbose
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| 207 | Verbose. Display the name and percentage reduction for
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| 208 | each file compressed or decompressed.
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| 209 |
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| 210 | -V --version
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| 211 | Version. Display the version number and compilation
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| 212 | options then quit.
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| 213 |
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| 214 | -# --fast --best
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| 215 | Regulate the speed of compression using the specified
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| 216 | digit #, where -1 or --fast indicates the fastest
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| 217 | compression method (less compression) and -9 or --best
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| 218 | indicates the slowest compression method (best compres-
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| 219 | sion). The default compression level is -6 (that is,
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| 220 | biased towards high compression at expense of speed).
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| 221 |
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| 222 | 2 ENVIRONMENT
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| 223 | The environment variable GZIP_OPT can hold a set of default
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| 224 | options for gzip. These options are interpreted first and
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| 225 | can be overwritten by explicit command line parameters. For
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| 226 | example:
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| 227 | define GZIP_OPT "-8 -v"
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| 228 |
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| 229 | 2 SEE ALSO
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| 230 | compress, zip, unzip
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| 231 |
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| 232 | 2 DIAGNOSTICS
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| 233 | Exit status is normally 0; if an error occurs, exit status
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| 234 | is 1. If a warning occurs, exit status is 2.
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| 235 |
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| 236 | Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
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| 237 | Invalid options were specified on the command line.
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| 238 | file: not in gzip format
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| 239 | The file specified to gunzip has not been
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| 240 | compressed.
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| 241 | file: Corrupt input. Use zcat to recover some data.
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| 242 | The compressed file has been damaged. The data up to
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| 243 | the point of failure can be recovered using
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| 244 | define /user sys$output file.recover
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| 245 | zcat file
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| 246 | file: compressed with xx bits, can only handle yy bits
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| 247 | File was compressed (using LZW) by a program that
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| 248 | could deal with more bits than the decompress code
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| 249 | on this machine. Recompress the file with gzip,
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| 250 | which compresses better and uses less memory.
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| 251 | file: already has -gz suffix -- no change
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| 252 | The file is assumed to be already compressed.
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| 253 | Rename the file and try again.
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| 254 | file already exists; do you wish to overwrite (y or n)?
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| 255 | Respond "y" if you want the output file to be
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| 256 | replaced; "n" if not.
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| 257 | gunzip: corrupt input
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| 258 | A SIGSEGV violation was detected which usually means
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| 259 | that the input file has been corrupted.
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| 260 | xx.x%
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| 261 | Percentage of the input saved by compression.
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| 262 | (Relevant only for -v and -l.)
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| 263 | -- not a regular file or directory: ignored
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| 264 | When the input file is not a regular file or direc-
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| 265 | tory, it is left unaltered.
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| 266 |
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| 267 | 2 CAVEATS
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| 268 | On VMS:
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| 269 | - upper case options need quotes: gzip "-V".
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| 270 | - restoration of timestamps and version numbers is not supported
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| 271 | - If a compressed file already exists, gzip -f overwrites it, it
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| 272 | does not create a new version.
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| 273 | - multi-part gzip files are not supported.
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| 274 | - gunzip does not preserve the input file format. You can use a
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| 275 | separate utility to restore the original format.
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| 276 | - gunzip and zcat can be used only if you have created the
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| 277 | links to gzip as documented in makegzip.com. Otherwise
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| 278 | you must use explicit parameters ("gzip -c" or "gzip -dc").
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| 279 | - gzip --list cannot give the uncompressed size and crc.
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| 280 |
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| 281 | When writing compressed data to a tape, it is generally
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| 282 | necessary to pad the output with zeroes up to a block boun-
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| 283 | dary. When the data is read and the whole block is passed to
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| 284 | gunzip for decompression, gunzip detects that there is extra
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| 285 | trailing garbage after the compressed data and emits a warn-
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| 286 | ing by default. You have to use the --quiet option to
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| 287 | suppress the warning. This option can be set in the GZIP_OPT
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| 288 | environment variable as in:
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| 289 | define GZIP_OPT "-q"
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| 290 |
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| 291 | 2 BUGS
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| 292 | On VMS, files in VFC record format are not correctly handled by
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| 293 | the C runtime library (the linefeed character is suppressed).
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| 294 |
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| 295 | In some rare cases, the --best option gives worse compres-
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| 296 | sion than the default compression level (-6). On some highly
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| 297 | redundant files, compress compresses better than gzip.
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