COMPRESS(1) General Commands Manual COMPRESS(1)
compress, uncompress – compress and expand data
compress [-fv] [-b bits] [file ...]
compress -c [-b bits] [file ...]
uncompress [-fv] [file ...]
uncompress -c [file ...]
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 will be ignored except it will cause an
error exit after other arguments are processed. If compression would not
reduce the size of a file, the file is ignored.
The uncompress utility restores compressed files to their original form,
renaming the files by deleting the .Z extensions. A file specification
need not include the file's .Z extension. If a file's name in its file
system does not have a .Z extension, it will not be uncompressed and it
will cause an error exit after other arguments are processed.
If renaming the files would cause files to be overwritten and the standard
input device is a terminal, the user is prompted (on the standard error
output) for confirmation. If prompting is not possible or confirmation is
not received, the files are not overwritten.
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 or uncompressed 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
The options are as follows:
-b bits The code size (see below) is limited to bits, which must be
in the range 9..16. The default is 16.
-c Compressed or uncompressed output is written to the standard
output. No files are modified. The -v option is ignored.
Compression is attempted even if the results will be larger
than the original.
-f Files are overwritten without prompting for confirmation.
Also, for compress, files are compressed even if they are
not actually reduced in size.
-v Print the percentage reduction of each file. Ignored by
uncompress or if the -c option is also used.
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
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. This allows the
algorithm to adapt to the next "block" of the file.
The -b option is unavailable for uncompress since the bits 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.
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 the historical command pack), or adaptive Huffman coding (as
used in the historical command compact), and takes less time to compute.
The compress and uncompress utilities exit 0 on success, and >0 if an error
The compress utility exits 2 if attempting to compress a file would not
reduce its size and the -f option was not specified and if no other error
gunzip(1), gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)
Welch, Terry A., “A Technique for High Performance Data Compression”, IEEE
Computer, 17:6, pp. 8-19, June, 1984.
The compress and uncompress utilities conform to IEEE Std 1003.1-2001
The compress command appeared in 4.3BSD.
Some of these might be considered otherwise-undocumented features.
compress: If the utility does not compress a file because doing so would
not reduce its size, and a file of the same name except with an .Z
extension exists, the named file is not really ignored as stated above; it
causes a prompt to confirm the overwriting of the file with the extension.
If the operation is confirmed, that file is deleted.
uncompress: If an empty file is compressed (using -f), the resulting .Z
file is also empty. That seems right, but if uncompress is then used on
that file, an error will occur.
Both utilities: If a ‘-’ argument is used and the utility prompts the user,
the standard input is taken as the user's reply to the prompt.
Both utilities: If the specified file does not exist, but a similarly-named
one with (for compress) or without (for uncompress) a .Z extension does
exist, the utility will waste the user's time by not immediately emitting
an error message about the missing file and continuing. Instead, it first
asks for confirmation to overwrite the existing file and then does not
macOS 12.1 May 17, 2002 macOS 12.1