STraTOS 1997 April & May

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GZIP(1) UNIX Programmer's Manual GZIP(1) NAME gzip, gunzip, zcat - compress or expand files SYNOPSIS gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ _n_a_m_e ... ] gunzip [ -acfhlLnNrtvV ] [-S suffix] [ _n_a_m_e ... ] zcat [ -fhLV ] [ _n_a_m_e ... ] DESCRIPTION _G_z_i_p 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 FAT, Windows NT FAT and Atari.) If no files are specified, or if a file name is "-", the standard input is compressed to the standard out- put. _G_z_i_p will only attempt to compress regular files. In particular, it will ignore symbolic links. If the compressed file name is too long for its file system, _g_z_i_p truncates it. _G_z_i_p attempts to truncate only the parts of the file name longer than 3 characters. (A part is del- imited 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, _g_z_i_p 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 _g_z_i_p -_d or _g_u_n_z_i_p or _z_c_a_t. 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. _g_u_n_z_i_p 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. _g_u_n_z_i_p also recognizes the special extensions .tgz and .taz as shorthands for .tar.gz and .tar.Z respectively. When compressing, _g_z_i_p uses the .tgz extension if necessary instead of truncating a file with a .tar extension. _g_u_n_z_i_p can currently decompress files created by _g_z_i_p, _z_i_p, _c_o_m_p_r_e_s_s, _c_o_m_p_r_e_s_s -_H or _p_a_c_k. The detection of the input format is automatic. When using the first two formats, Printed 8/24/93 local 1 GZIP(1) UNIX Programmer's Manual GZIP(1) _g_u_n_z_i_p checks a 32 bit CRC. For _p_a_c_k, _g_u_n_z_i_p checks the uncompressed length. The standard _c_o_m_p_r_e_s_s format was not designed to allow consistency checks. However _g_u_n_z_i_p 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 _u_n_c_o_m_p_r_e_s_s does not complain. This generally means that the standard _u_n_c_o_m_p_r_e_s_s does not check its input, and happily generates garbage out- put. The SCO compress -H format (lzh compression method) does not include a CRC but also allows some consistency checks. Files created by _z_i_p 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 _u_n_z_i_p instead of _g_u_n_z_i_p. _z_c_a_t is identical to _g_u_n_z_i_p -c. (On some systems, _z_c_a_t may be installed as _g_z_c_a_t to preserve the original link to _c_o_m_p_r_e_s_s.) _z_c_a_t uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. _z_c_a_t will uncompress files that have the correct magic number whether they have a .gz suffix or not. _G_z_i_p uses the Lempel-Ziv algorithm used in _z_i_p 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 _c_o_m_p_r_e_s_s), Huffman coding (as used in _p_a_c_k), or adaptive Huffman coding (_c_o_m_p_a_c_t). 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. _g_z_i_p preserves the mode, ownership and timestamps of files when compressing or decompressing. OPTIONS -a --ascii Ascii text mode: convert end-of-lines using local con- ventions. This option is supported only on some non- Unix systems. For MSDOS, CR LF is converted to LF when compressing, and LF is converted to CR LF when decompressing. -c --stdout --to-stdout Printed 8/24/93 local 2 GZIP(1) UNIX Programmer's Manual GZIP(1) 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 the input data is not in a format recognized by _g_z_i_p, and if the option --stdout is also given, copy the input data without change to the stan- dard ouput: let _z_c_a_t behave as _c_a_t. If -f is not given, and when not running in the background, _g_z_i_p prompts to verify whether an existing file should be overwritten. -h --help Display a help screen and quit. -l --list For each compressed file, list the following fields: 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 Printed 8/24/93 local 3 GZIP(1) UNIX Programmer's Manual GZIP(1) for all files is also displayed, unless some sizes are unknown. With --quiet, the title and totals lines are not displayed. -L --license Display the _g_z_i_p license and quit. -n --no-name When compressing, do not save the original file name and time stamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original file name if present (remove only the _g_z_i_p suffix from the compressed file name) and do not restore the original time stamp if present (copy it from the compressed file). This option is the default when decompressing. -N --name When compressing, always save the original file name and time stamp; this is the default. When decompress- ing, restore the original file name and time stamp if present. This option is useful on systems which have a limit on file name length or when the time stamp has been lost after a file transfer. -q --quiet Suppress all warnings. -r --recursive Travel the directory structure recursively. If any of the file names specified on the command line are direc- tories, _g_z_i_p will descend into the directory and compress all the files it finds there (or decompress them in the case of _g_u_n_z_i_p ). -S .suf --suffix .suf Use suffix .suf 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. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in: gunzip -S "" * (*.* for MSDOS) Previous versions of gzip used the .z suffix. This was changed to avoid a conflict with _p_a_c_k(1). -t --test Test. Check the compressed file integrity. -v --verbose Printed 8/24/93 local 4 GZIP(1) UNIX Programmer's Manual GZIP(1) Verbose. Display the name and percentage reduction for each file compressed or decompressed. -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 (best compres- sion). The default compression level is -6 (that is, biased towards high compression at expense of speed). ADVANCED USAGE Multiple compressed files can be concatenated. In this case, _g_u_n_z_i_p will extract all members at once. For example: 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 Printed 8/24/93 local 5 GZIP(1) UNIX Programmer's Manual GZIP(1) If you wish to create a single archive file with multiple members so that members can later be extracted indepen- dently, 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. ENVIRONMENT The environment variable GZIP can hold a set of default options for _g_z_i_p. These options are interpreted first and can be overwritten by explicit command line parameters. For example: for sh: GZIP="-8v --name"; export GZIP for csh: setenv GZIP "-8v --name" for MSDOS: set GZIP=-8v --name On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid a conflict with the symbol set for invo- cation of the program. SEE ALSO znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1), compress(1), pack(1), compact(1) DIAGNOSTICS Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2. Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...] Invalid options were specified on the command line. _f_i_l_e: not in gzip format The file specified to _g_u_n_z_i_p has not been compressed. _f_i_l_e: 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 zcat file > recover _f_i_l_e: compressed with _x_x bits, can only handle _y_y bits _F_i_l_e was compressed (using LZW) by a program that could deal with more _b_i_t_s than the decompress code on this machine. Recompress the file with gzip, which compresses better and uses less memory. _f_i_l_e: already has .gz suffix -- no change The file is assumed to be already compressed. Rename the file and try again. _f_i_l_e 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. _x_x._x% Percentage of the input saved by compression. Printed 8/24/93 local 6 GZIP(1) UNIX Programmer's Manual GZIP(1) (Relevant only for -v and -l.) -- not a regular file or directory: ignored When the input file is not a regular file or direc- tory, (e.g. a symbolic link, socket, FIFO, device file), it is left unaltered. -- has _x_x other links: unchanged The input file has links; it is left unchanged. See _l_n(1) for more information. Use the -f flag to force compression of multiply-linked files. CAVEATS 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 _g_u_n_z_i_p for decompression, _g_u_n_z_i_p 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: for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0 for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0 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. (This example assumes you are using the GNU version of tar.) BUGS The --list option reports incorrect sizes if they exceed 2 gigabytes. The --list option reports sizes as -1 and crc as ffffffff if the compressed file is on a non seekable media. In some rare cases, the --best option gives worse compres- sion than the default compression level (-6). On some highly redundant files, _c_o_m_p_r_e_s_s compresses better than _g_z_i_p. Printed 8/24/93 local 7