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<a href="0249-0250.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0253-0254.html">Next</A></CENTER></P>
<A NAME="PAGENUM-251"><P>Page 251</P></A>
<P><B>
ADVANCED USAGE
</B></P>
<P>Multiple compressed FILES can be concatenated. In this case,
gunzip will extract all members at once. For example,
</P>
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<PRE>
gzip -c file1 >foo.gz gzip -c file2>>> foo.gz
</PRE>
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<P>Then
</P>
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<PRE>
gunzip -c foo
</PRE>
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<P>is equivalent to
</P>
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<PRE>
cat file1 file2
</PRE>
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<P>In case of damage to one member of a .gz file, other members can still be recovered (if the damaged member is
removed). However, you can get better compression by compressing all members at once.
</P>
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<PRE>
cat file1 file2 | gzip > foo.gz
</PRE>
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<P>compresses better than
</P>
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<PRE>
gzip -c file1 file2 >foo.gz
</PRE>
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<P>If you want to recompress concatenated FILES to get better compression, use
</P>
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<PRE>
gzip -cd old.gz | gzip > new.gz
</PRE>
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<P>If a compressed file consists of several members, the uncompressed size and CRC reported by the
_list option applies to the last member only. If you need the uncompressed size for all members, you can use
</P>
<!-- CODE SNIP //-->
<PRE>
gzip -cd file.gz | wc -c
</PRE>
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<P>If you wish to create a single archive file with multiple members so that members can later be extracted independently, use
an archiver such as tar or zip. GNU tar supports the
-z option to invoke gzip transparently. gzip is designed as a
complement to tar, not as a replacement.
</P>
<P><B>
ENVIRONMENT
</B></P>
<P>The ENVIRONMENT variable GZIP can hold a set of default OPTIONS for
gzip. These OPTIONS are interpreted first and can
be overwritten by explicit command-line parameters. For example,
</P>
<P>For sh: GZIP="-8v _name"
</P>
<P>Export GZIP for csh: setenv GZIP "-8v _name"
</P>
<P>For MS-DOS: set GZIP=-8v _name
</P>
<P>On Vax/VMS, the name of the ENVIRONMENT variable is
GZIP_OPT, to avoid a conflict with the symbol set for invocation of
the program.
</P>
<P><B>
SEE ALSO
</B></P>
<P>znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1),
zip(1), unzip(1), compress(1), pack(1), compact(1)
</P>
<P><B>
DIAGNOSTICS
</B></P>
<P>Exit status is normally 0; if an error occurs, exit status is 1. If a warning occurs, exit status is 2.
</P>
<P>Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
</P>
<P>Invalid OPTIONS were specified on the command line.
</P>
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<PRE>
file: not in gzip format
</PRE>
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<P>The file specified to gunzip has not been compressed.
</P>
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<PRE>
file: Corrupt input. Use zcat to recover some data.
</PRE>
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<P>The compressed file has been damaged. The data up to the point of failure can be recovered using
</P>
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<PRE>
zcat file > recover
</PRE>
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<A NAME="PAGENUM-252"><P>Page 252</P></A>
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<PRE>
file: compressed with xx bits, can only handle yy bits
</PRE>
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<P>file was compressed (using LZW) by a program that could deal with more bits than the decompress code on this
machine. Recompress the file with gzip, which compresses better and uses less memory.
</P>
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<PRE>
file: already has .gz suffix--no change
</PRE>
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<P>The file is assumed to be already compressed. Rename the file and try again.
</P>
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<PRE>
file already exists; do you wish to overwrite (y or n)?
</PRE>
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<P>Respond y if you want the output file to be replaced;
n if not.
</P>
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<PRE>
gunzip: corrupt input
</PRE>
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<P>A SIGSEGV violation was detected, which usually means that the input file has been corrupted.
</P>
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<PRE>
xx.x%
</PRE>
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<P>Percentage of the input saved by compression. (Relevant only for
_v and _l.)
</P>
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<PRE>
_ not a regular file or directory: ignored
</PRE>
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<P>When the input file is not a regular file or directory, (such as a symbolic link, socket, FIFO, device file), it is left unaltered.
</P>
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<PRE>
_ has xx other links: unchanged
</PRE>
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<P>The input file has links; it is left unchanged. See
ln(1) for more information.
</P>
<P>Use the _f flag to force compression of FILES that are multiply linked.
</P>
<P><B>
CAVEATS
</B></P>
<P>When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block
boundary. When the data is read and the whole block is passed to
gunzip for decompression, gunzip detects that there is extra
trailing garbage after the compressed data and emits a warning by default. You have to use the
_quiet option to suppress the warning. This option can be set in the
GZIP ENVIRONMENT variable as in the following:
</P>
<!-- CODE SNIP //-->
<PRE>
for sh: GZIP="-q" tar -xfz _block-compress /dev/rst0 for csh:
(setenv GZIP -q; tar -xfz _block-compr /dev/rst0
</PRE>
<!-- END CODE SNIP //-->
<P>In the preceding example, gzip is invoked implicitly by the
-z option of GNU tar. Make sure that the same block size
(-b option of tar) is used for reading and writing compressed data on tapes. (This example assumes you are using the
GNU version of tar.)
</P>
<P><B>
BUGS
</B></P>
<P>The _list option reports incorrect sizes if they exceed two gigabytes. The
_list option reports sizes as -1 and crc as
ffffffff if the compressed file is on a nonseekable media.
</P>
<P>In some rare cases, the _best option gives worse compression than the default compression level (-6). On some
highly redundant FILES, compress compresses better than
gzip.
</P>
<P>
Local
</P>
<H3><A NAME="ch01_ 108">
gzexe
</A></H3>
<P>gzexe—Compress executable FILES in place
</P>
<P><B>
SYNOPSIS
</B></P>
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<PRE>
gzexe [ name ... ]
</PRE>
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