COMPRESS(1) UNIX Reference Manual COMPRESS(1)
c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs, u
\bun
\bnc
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs, z
\bzc
\bca
\bat
\bt - compress and expand data
S
\bSY
\bYN
\bNO
\bOP
\bPS
\bSI
\bIS
\bS
c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs [-
\b-f
\bf] [-
\b-v
\bv] [-
\b-c
\bc] [-
\b-b
\bb _
\bb_
\bi_
\bt_
\bs] [_
\bf_
\bi_
\bl_
\be ...]
u
\bun
\bnc
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs [-
\b-f
\bf] [-
\b-v
\bv] [-
\b-c
\bc] [_
\bf_
\bi_
\bl_
\be ...]
z
\bzc
\bca
\bat
\bt [_
\bf_
\bi_
\bl_
\be ...]
D
\bDE
\bES
\bSC
\bCR
\bRI
\bIP
\bPT
\bTI
\bIO
\bON
\bN
C
\bCo
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs reduces the size of the named files using adaptive Lempel-Ziv
coding. Whenever possible, each _
\bf_
\bi_
\bl_
\be is replaced by one with the exten-
sion ._
\bZ, while keeping the same ownership modes, access and modification
times. If no files are specified, the standard input is compressed to
the standard output. Compressed files can be restored to their original
form using u
\bun
\bnc
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs or z
\bzc
\bca
\bat
\bt
-
\b-f
\bf Force compression of _
\bf_
\bi_
\bl_
\be, even if it does not actually shrink or
the corresponding _
\bf_
\bi_
\bl_
\be._
\bZ file already exists. Except when run in
the background under /_
\bb_
\bi_
\bn/_
\bs_
\bh, if -
\b-f
\bf is not given the user is
prompted as to whether an existing _
\bf_
\bi_
\bl_
\be._
\bZ file should be overwrit-
-
\b-c
\bc (``cat'') makes c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs/
\b/u
\bun
\bnc
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs write to the standard output;
no files are changed. The nondestructive behavior of z
\bzc
\bca
\bat
\bt is
identical to that of u
\bun
\bnc
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs -
\b-c
\bc.
\b.
-
\b-b
\bb Specify _
\bb_
\bi_
\bt_
\bs code limit (see below).
-
\b-v
\bv Print the percentage reduction of each file.
C
\bCo
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs uses the modified Lempel-Ziv algorithm popularized in "A Tech-
nique for High Performance Data Compression", Terry A. Welch, _
\bI_
\bE_
\bE_
\bE
_
\bC_
\bo_
\bm_
\bp_
\bu_
\bt_
\be_
\br, vol. 17, no. 6 (June 1984), pp. 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 -
\b-b
\bb flag is reached (default 16).
_
\bB_
\bi_
\bt_
\bs must be between 9 and 16. The default can be changed in the source
to allow c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs to be run on a smaller machine.
After the _
\bb_
\bi_
\bt_
\bs limit is attained, c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs periodically checks the
compression ratio. If it is increasing, c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs continues to use the
existing code dictionary. However, if the compression ratio decreases,
c
\bco
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs 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 -
\b-b
\bb flag is omitted for _
\bu_
\bn_
\bc_
\bo_
\bm_
\bp_
\br_
\be_
\bs_
\bs since the _
\bb_
\bi_
\bt_
\bs 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 _
\bb_
\bi_
\bt_
\bs per code, and the distribution of common substrings. Typ-
ically, 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), or adaptive Huffman coding (as used in compact), and
takes less time to compute.
If an error occurs, exit status is 1, else if the last file was not
compressed because it became larger, the status is 2; else the status is
D
\bDI
\bIA
\bAG
\bGN
\bNO
\bOS
\bST
\bTI
\bIC
\bCS
\bS
Usage: compress [-
\b-f
\bfv
\bvc
\bc] [-
\b-b
\bb _
\bm_
\ba_
\bx_
\bb_
\bi_
\bt_
\bs] [_
\bf_
\bi_
\bl_
\be ...]
Invalid options were specified on the command line.
Maxbits must follow -
\b-b
\bb.
_
\bf_
\bi_
\bl_
\be: not in compressed format
The file specified to _
\bu_
\bn_
\bc_
\bo_
\bm_
\bp_
\br_
\be_
\bs_
\bs has not been compressed.
_
\bf_
\bi_
\bl_
\be: compressed with _
\bx_
\bx bits, can only handle _
\by_
\by bits
_
\bF_
\bi_
\bl_
\be was compressed by a program that could deal with more _
\bb_
\bi_
\bt_
\bs
than the compress code on this machine. Recompress the file with
smaller _
\bb_
\bi_
\bt_
\bs.
_
\bf_
\bi_
\bl_
\be: already has .Z suffix -- no change
The file is assumed to be already compressed. Rename the file and
_
\bf_
\bi_
\bl_
\be: filename too long to tack on .Z
The file cannot be compressed because its name is longer than 12
characters. Rename and try again. This message does not occur on
_
\bf_
\bi_
\bl_
\be already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n" if not.
uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the input
Compression: _
\bx_
\bx._
\bx_
\bx%
Percentage of the input saved by compression. (Relevant only for
-- not a regular file: unchanged
When the input file is not a regular file, (e.g. a directory), it
-- has _
\bx_
\bx other links: unchanged
The input file has links; it is left unchanged. See ln(1) for more
No savings is achieved by compression. The input remains virgin.
_
\bf_
\bi_
\bl_
\be._
\bZ compressed file is file.Z
H
\bHI
\bIS
\bST
\bTO
\bOR
\bRY
\bY
Although compressed files are compatible between machines with large
memory, -
\b-b
\bb_
\b1_
\b2 should be used for file transfer to architectures with a
small process data space (64KB or less, as exhibited by the DEC PDP
series, the Intel 80286, etc.)
C
\bCo
\bom
\bmp
\bpr
\bre
\bes
\bss
\bs should be more flexible about the existence of the `.Z' suffix.
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