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Full Discussion: tar and gzip problem
Top Forums Shell Programming and Scripting tar and gzip problem Post 302338482 by Scott on Tuesday 28th of July 2009 04:10:56 AM
Old 07-28-2009
Hi.

The - in tar means (with the c option) write to standard output (or read from standard input with the x option).

The -9 means maximum compression (9) (you can use -c also, which means read from standard input with gzip, and write to standard output with gunzip, but it seems most gzips are happy without it)

It's all explained in the man pages.

----
Sorry I didn't explain it very well (or at all!).

Code:
tar cvf - files_or_directories

Will tar up the files or directories you specify and write the tarfile to standard output.

Code:
 | gzip -9

will pipe the output (the tar file) into gzip and compress it (using maximum compression, -9) and write this to standard output (as it doesn't have either an input or output filename to work with)

Code:
 > myfile.tar.gz

will direct standard output (your compressed tar archive) to myfile.tar.gz.

You can do the reverse also:

Code:
gunzip -c myfile.tar.gz | tar xvf -

This is quicker and saves diskspace in the short term. You can do similar things with pipes, such as when exporting large database schemas directly to compressed files, etc, if you don't have enough space for the .dmp file.

In fact, you can set up just about anything to write directly to a compressed file.

i.e.
Code:
mknod myLogFile.txt p
cat myLogFile.txt | gzip -9 > myLogFile.txt.gz &

(run your program or script that writes to myLogFile.txt)

The background process "cat .... &" will end when your program or script does
Last edited by Scott; 07-28-2009 at 02:09 PM..
 

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LEARN ABOUT SUSE

bf_tar

BF_TAR(1)																 BF_TAR(1)

NAME

       bf_tar - shell script to write a tar file of a bogofilter directory to stdout

SYNOPSIS

       bf_tar [-r] [-R] bogofilter_directory

DESCRIPTION

       bf_tar bundles a bogofilter working directory in tar format and copies it to standard output (your console, or where you redirect it, see
       EXAMPLES below).

OPTIONS

       The -r option causes bf_tar to remove inactive log files after the archive has been written successfully. The default is to leave log
       files.

       The -R option causes bf_tar to remove inactive log files before the archive is written. This may reduce chances that the resulting archive
       is recoverable should it become damaged. The archive may be smaller though. The default is to leave log files.

EXIT STATUS

       The script exits with status code 0 if everything went well, and nonzero if it encountered trouble.

EXAMPLES

       o  bf_tar ~/.bogofilter > outfile.tar

	  Writes a standard .tar file containing the essential files from ~/.bogofilter to outfile.tar.

       o  bf_tar ~/.bogofilter | gzip -9 -c > outfile.tar.gz

	  Writes a gzipped .tar.gz file containing the essential files from ~/.bogofilter to outfile.tar.gz.

       o  bf_tar `pwd`/mydirectory > outfile.tar

	  Prepend $(pwd)/ or `pwd`/ if you want to specify an absolute path instead of a relative path.

NOTES

       This script is meant for use with Berkeley DB based bogofilter versions.

       This script requires a SUSv2 compliant pax utility.

       This script expects a SUSv2 compliant shell. Solaris systems should have the SUNWxcu4 package installed (when bogofilter is configured) so
       that /usr/xpg4/bin/sh can be used.

								    07/23/2007								 BF_TAR(1)
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