On Unix-like operating systems, the compress command compresses a file so that it becomes smaller. The compressed file’s name is given the extension .Z.

This page covers the GNU/Linux version of compress.

Description

The compress utility reduces the size of files using adaptive Lempel-Ziv coding. Each file is renamed to the same name plus the extension .Z. A file argument with a .Z extension is ignored except it causes an error exit after other arguments are processed. If compression would not reduce the size of a file, the file is ignored.

  • Description
  • Syntax
  • Options
  • Examples
  • Related commands
  • Linux commands help

As many of the modification time, access time, file flags, file mode, user ID, and group ID as allowed by permissions are retained in the new file.

If no files are specified or a file argument is a single dash (’-’), the standard input is compressed to the standard output. If either the input and output files are not regular files, the checks for reduction in size and file overwriting are not performed, the input file is not removed, and the attributes of the input file are not retained in the output file.

The compress utility uses a modified Lempel-Ziv algorithm. 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 -b option or its default is reached.

After the limit is reached, compress periodically checks the compression ratio. If it is increasing, compress continues to use the existing code dictionary. However, if the compression ratio decreases, compress discards the table of substrings and rebuilds it from scratch, which allows the algorithm to adapt to the next “block” of the file.

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 better than that achieved by Huffman coding (as used in the historical command pack), or adaptive Huffman coding (as used in the historical command “compact”), and takes less time to compute.

Syntax

compress [-fv] [-b bits] [file …]

compress -c [-b bits] [file]

Options

Examples

compress -v bigfile.exe

Compress bigfile.exe and rename that file to bigfile.exe.Z.

ln — Create a link, or a symbolic link, to a file or directory.uncompress — Extract files from compressed archives.zcat — Print the uncompressed contents of compressed files.zip — A compression and archiving utility.