Visit The New, Modern Unix Linux Community

Linux and UNIX Man Pages

Test Your Knowledge in Computers #576
Difficulty: Medium
All programming languages have automatic garbage collection that monitors the dynamically allocated pieces of memory and determine if any variable in the program still references it. If the memory is no longer referenced, it is 'garbage' and becomes eligible to be 'collected'.
True or False?
Linux & Unix Commands - Search Man Pages

compress(1) [hpux man page]

compress(1)						      General Commands Manual						       compress(1)

NAME
compress, uncompress, zcat, compressdir, uncompressdir - compress and expand data SYNOPSIS
Compress Files maxbits] [file ...] [file ...] [file ...] Compress Entire Directory Subtrees [options] [directory ...] [options] [directory ...] DESCRIPTION
The following commands compress and uncompress files and directory subtrees as indicated: Reduce the size of the named files using adaptive Lempel-Ziv coding. If reduction is possible, each file is replaced by a new file of the same name with the suffix added to indicate that it is a compressed file. Original ownership, modes, access, and modification times are preserved. If no file is specified, or if is specified, standard input is compressed to the standard output. Restore the compressed files to original form. Resulting files have the original filename, ownership, and permissions, and the file- name suffix is removed. If no file is specified, or if is specified, standard input is uncompressed to the standard output. Restore the compressed files to original form and send the result to standard output. If no file is specified, or if is specified, standard input is uncompressed to the standard output. Front-end processor. Recursively descend each specified directory subtree and use to compress each file in directory. Existing files are replaced by a compressed file having the same name plus the suffix provided the resulting file is smaller than the original. If no directories are specified, compression is applied to all files starting with the cur- rent directory. options may include any valid command options (they are passed through to To force compression of all files, even when the result is larger than the original file, use the option. Opposite of Restore compressed files to their original form. options may include any valid command options (they are passed through to The amount of compression obtained depends on the size of the input, the maximum number of bits (maxbits) per code, and the distribution of common substrings. Typically, text such as source code or English is reduced by 50-60 percent. Compression is generally much better than that achieved by Huffman coding (as used in or adaptive Huffman coding and takes less time to compute. Options These commands recognize the following options in the combinations shown above in SYNOPSIS: Decompress file. is equivalent to Force compression of file. This is useful for compressing an entire directory, even if some of the files do not actually shrink. If is not given and is run in the foreground, the user is prompted as to whether an existing file should be over- written. This is the same as the option except that it does not force compression when there is null compression. Print a message describing the percentage of reduction for each file compressed. Force and to write to the standard output; no files are changed. The nondestructive behavior of is identical to that of Print the current version and compile options onto the standard error. Specify the maximum number of bits the algorithm will use. The default is 16 and the range can be any integer between 9 and 16. uses the modified Lempel-Ziv algorithm popularized in A Technique for High Performance Data Compression , Terry A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pages 8-19. Common substrings in the file are first replaced by 9-bit codes 257 and up. When code 512 is reached, the algorithm switches to 10-bit codes and continues to use more bits until the limit specified by the flag is reached (default 16). After the maxbits limit is attained, periodically checks the compression ratio. If it is increasing, continues to use the existing code dictionary. However, if the compression ratio is decreasing, discards the table of substrings and rebuilds it from scratch. This allows the algorithm to adapt to the next "block" of the file. Note that the flag is omitted for since the maxbits parameter specified during compression is encoded within the output, along with a magic number to ensure that neither decompression of random data nor recompression of compressed data is attempted. Access Control Lists retains a file's access control list when compressing and expanding data. EXTERNAL INFLUENCES
Environment Variables determines the language in which messages are displayed. If is not specified in the environment or is set to the empty string, the value of is used as a default for each unspecified or empty vari- able. If is not specified or is set to the empty string, a default of "C" (see lang(5)) is used instead of If any internationalization variable contains an invalid setting, and behave as if all internationalization variables are set to "C". See environ(5). International Code Set Support Single- and multi-byte character code sets are supported. RETURN VALUE
These commands return the following values upon completion: Completed successfully. Last file is larger after (attempted) compression. An error occurred. DIAGNOSTICS
Invalid options were specified on the command line. maxbits must follow The file specified to has not been compressed. file was compressed by a program that could deal with a higher value of maxbits than the compress code on this machine. Recompress the file with a lower value of maxbits. The file is assumed to be already compressed. Rename the file and try again. The output file name, which is the source file name with a extension, is too long for the file system on which the source file resides. Make the source file name shorter and try again. Respond if you want the output to replace the existing file; otherwise, respond A violation was detected which usually means that the input file has been corrupted. Percentage of the input saved by compression. (Relevant only for When the input file is not a regular file (a directory for example), it is left unaltered. The input file has links which are not symbolic links and has been left unchanged. See ln(1) for more information. The input file has symbolic links and has been left unchanged. See ln(1) for more information. No savings is achieved by compression. The input remains unaltered. EXAMPLES
Compress the file named and print compression information to the terminal: The terminal display shows either a line resembling indicating that the compressed file is 23.55% smaller than the original, or a line resembling indicating that an additional 12.04% space must be used to compress the file. Undo the compression by typing either of the following commands: This restores file to its original uncompressed form and name. will perform on standard input if no files are specified. For example, to list a compressed tar file: WARNINGS
Although compressed files are compatible between machines with large memory, should be used for file transfer to architectures with a small process data space (64K bytes or less). NFS Access control lists of networked files are summarized (as returned in by but not copied to the new file (see stat(2)). AUTHOR
was developed by Joseph M. Orost, Kenneth E. Turkowski, Spencer W. Thomas, and James A. Woods. FILES
Compressed file created by and removed by SEE ALSO
compact(1), pack(1), acl(5). STANDARDS CONFORMANCE
compress(1)

Featured Tech Videos