If the compressed file name is too long for its file system, //gzip// truncates it. //Gzip// attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length.
By default, //gzip// keeps the original file name and timestamp in the compressed file. These are used when decompressing the file with the **-N** option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer.
Compressed files can be restored to their original form using //gzip -d// or //gunzip// or //zcat.// If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal.
//gunzip// takes a list of files on its command line and replaces each file whose name ends with .gz, -gz, .z, -z, _z or .Z and which begins with the correct magic number with an uncompressed file without the original extension. //gunzip// also recognizes the special extensions **.tgz** and **.taz** as shorthands for **.tar.gz** and **.tar.Z** respectively. When compressing, //gzip// uses the **.tgz** extension if necessary instead of truncating a file with a **.tar** extension.
//gunzip// can currently decompress files created by //gzip, zip, compress, compress -H// or //pack.// The detection of the input format is automatic. When using the first two formats, //gunzip// checks a 32 bit CRC. For //pack, gunzip// checks the uncompressed length. The standard //compress// format was not designed to allow consistency checks. However //gunzip// is sometimes able to detect a bad .Z file. If you get an error when uncompressing a .Z file, do not assume that the .Z file is correct simply because the standard //uncompress// does not complain. This generally means that the standard //uncompress// does not check its input, and happily generates garbage output. The SCO compress -H format (lzh compression method) does not include a CRC but also allows some consistency checks.
Files created by //zip// can be uncompressed by gzip only if they have a single member compressed with the 'deflation' method. This feature is only intended to help conversion of tar.zip files to the tar.gz format. To extract zip files with several members, use //unzip// instead of //gunzip.//
//zcat// is identical to //gunzip// **-c.** (On some systems, //zcat// may be installed as //gzcat// to preserve the original link to //compress.)// //zcat// uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. //zcat// will uncompress files that have the correct magic number whether they have a **.gz** suffix or not.
//Gzip// uses the Lempel-Ziv algorithm used in //zip// and PKZIP. The amount of compression obtained depends on the size of the input and the distribution of common substrings. Typically, text such as source code or English is reduced by 60-70%. Compression is generally much better than that achieved by LZW (as used in //compress//), Huffman coding (as used in //pack//), or adaptive Huffman coding (//compact//).
Compression is always performed, even if the compressed file is slightly larger than the original. The worst case expansion is a few bytes for the gzip file header, plus 5 bytes every 32K block, or an expansion ratio of 0.015% for large files. Note that the actual number of used disk blocks almost never increases. //gzip// preserves the mode, ownership and timestamps of files when compressing or decompressing.
compressed size: size of the compressed file uncompressed size: size of the uncompressed file ratio: compression ratio (0.0% if unknown) uncompressed_name: name of the uncompressed file
The uncompressed size is given as -1 for files not in gzip format, such as compressed .Z files. To get the uncompressed size for such a file, you can use:
zcat file.Z | wc -c
In combination with the --verbose option, the following fields are also displayed:
method: compression method crc: the 32-bit CRC of the uncompressed data date & time: time stamp for the uncompressed file
The compression methods currently supported are deflate, compress, lzh (SCO compress -H) and pack. The crc is given as ffffffff for a file not in gzip format.
With --name, the uncompressed name, date and time are those stored within the compress file if present.
With --verbose, the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With --quiet, the title and totals lines are not displayed.
gunzip -S ""** (**.** for MSDOS)
Previous versions of gzip used the .z suffix. This was changed to avoid a conflict with //pack//(1).
gzip -c file1 > foo.gz gzip -c file2 >> foo.gz
Then
gunzip -c foo
is equivalent to
cat file1 file2
In case of damage to one member of a .gz file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once:
cat file1 file2 | gzip > foo.gz
compresses better than
gzip -c file1 file2 > foo.gz
If you want to recompress concatenated files to get better compression, do:
gzip -cd old.gz | gzip > new.gz
If a compressed file consists of several members, the uncompressed size and CRC reported by the --list option applies to the last member only. If you need the uncompressed size for all members, you can use:
gzip -cd file.gz | wc -c
If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as tar or zip. GNU tar supports the -z option to invoke gzip transparently. gzip is designed as a complement to tar, not as a replacement.
-- Main.FredPettis - 2010-06-24