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 .z or .Z and which begins with the correct magic number with an uncompressed file without the original extension.
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 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.
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.z format. To extract zip files with several members, use unzip instead of gunzip.
zcat is identical to gunzip -c. 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 .z 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. gzip preserves the mode, ownership and timestamps of files when compressing or decompressing.
gzip -c file1 > foo.z
gzip -c file2 >> foo.z
Then
gunzip -c foo
is equivalent to
cat file1 file2
In case of damage to one member of a .z 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.z
compresses better than
gzip -c file1 file2 > foo.z
If you want to recompress concatenated files to get better compression, do:
Usage: gzip [-cdfhLrtvV19] [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 zcat file > recover 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 .z 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 directory, (e.g. a symbolic link, socket, FIFO, device file), it is left unaltered. -- has xx other links: unchanged The input file has links; it is left unchanged. See ln(1) for more information. Use the -f flag to force compression of multiply-linked files.