Meeting Pearls 1

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Text File | 1993-05-29 | 9.3 KB | 211 lines

1 GZIP NAME gzip, gunzip, zcat - compress or expand files SYNOPSIS gzip [ -cdfhLrtvV19 ] [ name ... ] gunzip [ -cfhLrtvV ] [ name ... ] zcat [ -hLV ] [ name ... ] 2 DESCRIPTION Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension .gz, while keeping the same ownership modes, access and modification times. (The default exten- sion is -gz for VMS, z for MSDOS, OS/2 and Atari.) If no files are specified, the standard input is compressed to the standard output. If the new file name is too long, gzip truncates it and keeps the original file name in the compressed file. Gzip will only attempt to compress regular files. In particular, it will ignore symbolic links. Compressed files can be restored to their original form using gzip -d or gunzip or zcat. 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. gunzip can currently decompress files created by gzip, zip, compress 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 compress format was not designed to allow con- sistency 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 gen- erally means that the standard uncompress does not check its input, and happily generates garbage output. 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. Typi- cally, 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 expan- sion 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. 2 OPTIONS -c --stdout Write output on standard output; keep original files unchanged. If there are several input files, the out- put consists of a sequence of independently compressed members. To obtain better compression, concatenate all input files before compressing them. -d --decompress --uncompress Decompress. -f --force Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or writ- ten to a terminal. If -f is not given, and when not running in the background, gzip prompts to verify whether an existing file should be overwritten. -h --help Display a help screen and quit. -L --license Display the gzip license. -q --quiet Suppress all warnings. -r --recurse Travel the directory structure recursively. If any of the file names specified on the command line are directories, gzip will descend into the directory and compress all the files it finds there (or decompress them in the case of gunzip ). -S -z --suffix -z Use suffix -z instead of -gz. Any suffix can be given, but suffixes other than -z and -gz should be avoided to avoid confusion when files are transferred to other systems. Previous versions of gzip used the -z suffix. This was changed to avoid a conflict with pack. -t --test Test. Check the compressed file integrity. -v --verbose Verbose. Display the name and percentage reduction for each file compressed. -V --version Version. Display the version number and compilation options then quit. -# --fast --best Regulate the speed of compression using the specified digit #, where -1 or --fast indicates the fastest compression method (less compression) and -9 or --best indicates the slowest compression method (optimal compression). The default compression level is -5. 2 ENVIRONMENT The environment variable GZIP_OPT can hold a set of default options for gzip. These options are interpreted first and can be overwritten by explicit command line parameters. For example: GZIP_OPT == "-8 -v" 2 SEE ALSO compress, zip, unzip 2 DIAGNOSTICS Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2. Usage: gzip [-cdfhLrStvV19] [file ...] Invalid options were specified on the command line. file: not in gzip format The file specified to gunzip has not been compressed. file: Corrupt input. Use zcat to recover some data. The compressed file has been damaged. The data up to the point of failure can be recovered using define /user sys$output file.recover zcat file file: compressed with xx bits, can only handle yy bits File was compressed (using LZW) by a program that could deal with more bits than the decompress code on this machine. Recompress the file with gzip, which compresses better and uses less memory. file: already has gz suffix -- no change The file is assumed to be already compressed. Rename the file and try again. file already exists; do you wish to overwrite (y or n)? Respond "y" if you want the output file to be replaced; "n" if not. gunzip: corrupt input A SIGSEGV violation was detected which usually means that the input file has been corrupted. xx.x% Percentage of the input saved by compression. (Relevant only for -v.) -- not a regular file or directory: ignored When the input file is not a regular file or direc- tory, it is left unaltered. 2 CAVEATS On VMS: - upper case options need quotes: gzip "-V". - restoration of timestamps and version numbers is not supported - If a compressed file already exists, gzip -f overwrites it, it does not create a new version. - multi-part gzip files are not supported. - gunzip does not preserve the input file format. You can use a separate utility to restore the original format. - gunzip and zcat can be used only if you have created the links to gzip as documented in makegzip.com. Otherwise you must use explicit parameters ("gzip -c" or "gzip -dc"). When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boun- dary. When the data is read and the whole block is passed to gunzip for decompression, gunzip detects that there is extra trailing garbage after the compressed data and emits a warn- ing by default. You have to use the --quiet option to suppress the warning. This option can be set in the GZIP environment variable as in: GZIP == "-q" tar xfz /dev/rmt/datn In the above example, gzip is invoked implicitly by the -z option of GNU tar. Make sure that the same block size (-b option of tar) is used for reading and writing compressed data on tapes. 2 BUGS On VMS, files in VFC record format are not correctly handled by the C runtime library (the linefeed character is suppressed).