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- compress, uncompress files or display expanded files
compress [-fv] [-b bits] [file]...
compress [-cfv] [-b bits] [file]
uncompress [-cfv] [file]...
The compress utility will attempt to reduce the size of the named files by using adaptive Lempel-Ziv coding. Except when the output is to the standard output, each file will be replaced by one with the extension .Z, while keeping the same ownership modes, change times and modification times. If appending the .Z to the file pathname would make the pathname exceed 1023 bytes, the command will fail. If no files are specified, the standard input will be compressed to the standard output.
The amount of compression obtained depends on the size of the input, the number of bits per code, and the distribution of common substrings. Typically, text such as source code or English is reduced by 50-60%. Compression is generally much better than that achieved by Huffman coding (as used in pack(1)) and it takes less time to compute. The bits parameter specified during compression is encoded within the compressed file, along with a magic number to ensure that neither decompression of random data nor recompression of compressed data is subsequently allowed.
The uncompress utility will restore files to their original state after they have been compressed using the compress utility. If no files are specified, the standard input will be uncompressed to the standard output.
This utility supports the uncompressing of any files produced by compress. For files produced by compress on other systems, uncompress supports 9- to 16-bit compression (see -b).
The zcat utility will write to standard output the uncompressed form of files that have been compressed using compress. It is the equivalent of uncompress -c. Input files are not affected.
The following options are supported:
Writes to the standard output; no files are changed and no .Z files are created. The behavior of zcat is identical to that of `uncompress -c'.
When compressing, forces compression of file, even if it does not actually reduce the size of the file, or if the corresponding file.Z file already exists. If the -f option is not given, and the process is not running in the background, prompts to verify whether an existing file.Z file should be overwritten. When uncompressing, does not prompt for overwriting files. If the -f option is not given, and the process is not running in the background, prompts to verify whether an existing file should be overwritten. If the standard input is not a terminal and -f is not given, writes a diagnostic message to standard error and exits with a status greater than 0.
Verbose. Writes to standard error messages concerning the percentage reduction or expansion of each file.
Sets the upper limit (in bits) for common substring codes. bits must be between 9 and 16 (16 is the default). Lowering the number of bits will result in larger, less compressed files.
The following operand is supported:
A path name of a file to be compressed by compress, uncompressed by uncompress, or whose uncompressed form is written to standard out by zcat. If file is -, or if no file is specified, the standard input will be used.
See largefile(5) for the description of the behavior of compress, uncompress, and zcat when encountering files greater than or equal to 2 Gbyte ( 231 bytes).
See environ(5) for descriptions of the following environment variables that affect the execution of compress, uncompress, and zcat: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH.
The following error values are returned:
An error occurred.
One or more files were not compressed because they would have increased in size (and the -f option was not specified).
An error occurred.
See attributes(5) for descriptions of the following attributes:
Invalid options were specified on the command line.
Maxbits must follow -b, or invalid maxbits, not a numeric value.
The file specified to uncompress has not been compressed.
file was compressed by a program that could deal with more bits than the compress code on this machine. Recompress the file with smaller bits.
The file is assumed to be already compressed. Rename the file and try again.
Respond y if you want the output file to be replaced; n if not.
A SIGSEGV violation was detected, which usually means that the input file is corrupted.
Percentage of the input saved by compression. (Relevant only for -v.)
When the input file is not a regular file, (such as a directory), it is left unaltered.
The input file has links; it is left unchanged. See ln(1) for more information.
No savings are achieved by compression. The input remains uncompressed.
The path name is too long to append the .Z suffix.
Although compressed files are compatible between machines with large memory, -b 12 should be used for file transfer to architectures with a small process data space (64KB or less).
compress should be more flexible about the existence of the . Z suffix.