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GNU tar has several interesting additional features like:
-d, --diff, --compare find differences between archive and file system
To list the contents of a tarball without extracting files, use the t option as shown below. You do need option -f to specify the tarball. Omitting it is a very frequent mistake; in this case tar just wait forever input from stdin. Including the v option as well results in a long listing.
tar tf myfiles.tar tar tvf myfiles.tar
To extract to standard output a file from archive you can use -O or --to-stdout, for exampletar -xOf myfiles.tar hosts | moreHowever, ‘--to-command’ may be more convenient for use with multiple files.
Archives created with tar include the file ownership, file permissions, and access and creation dates of the files. The p (preserve) option restores file permissions to the original state. This is usually good since you'll ordinarily want to preserve permissions as well as dates so that executables will execute and you can determine how old they are. In some situations, you might not like that original owners are retrieved, since the original owners may be people at some other organization altogether. The tar command will set up ownership according to the numeric UID of the original owner. If someone in your local passwd file or network information service has the same UID, that person will become the owner; otherwise the owner will display numerically. Obviously, ownership can be altered later.tar xvpf myachive.tar
Extract each file from a shell prompt by typing tar xvzf file.tar.gz from the directory you saved the file.
You can extract to standard output (via option -O) and redirect it, for exampletar -xvOf etc_baseline110628_0900.tar hosts > /etc/hosts110628
Splitting big files into pieces is a common task. Another common task is to create a tar archive, and split it into smaller chunks that can be burned onto CD/DVD. The straightforward approach is to create the archive and then use 'split.' To do this, you will need more free space on your disk. In fact, you'll need space twice the size of the created archive. To avoid this limitation, split the archive as it is being created.
To create a tar archive that splits itself on the fly use the following set of commands:
First create the archive:tar -czf /dev/stdout $(DIRECTORY_OR_FILE_TO_COMPRESS) | split -d -b $(CHUNK_SIZE_IN_BYTES) - $(FILE_NAME_PREFIX)
To extract the contents:cat $(FILE_NAME_PREFIX)* >> /dev/stdout | tar -xzf /dev/stdin
The above shown set of commands works on the fly. You don't need additional free space for temporary files.
A few notes about this exercise:
- 'tar -L' prompts you on every chunk created. Compression can not be used with -L option. The above command is not interactive and does not prompt for anything. Compression can be used.
- The number of separate files is 100. This is because we use numerical suffixes – 'split -d.' If the specified chunk size is small you will get 'split: Output file suffixes exhausted' error. Try with bigger chunk size or with alphabetic suffixes.
- 'cat' will concatenate the files properly if they are not renamed. This is due to the fact that the sort order is retained by the appended chunk suffixes.
- Replace 'tar -z' with 'tar -j' for bzip2 compression or try your favourite compression program. Almost all 'tar' and 'split' options should be possible.
- The resulting chunk files are not valid tar archives. They can not be extracted separately. If you want such functionality use 'split-tar,' which also needs more free space.
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Isn't it easier to just omit the "f"?
tar cz $(DIRECTORY_OR_FILE_TO_COMPRESS) | split -d -b $(CHUNK_SIZE_IN_BYTES) – $(FILE_NAME_PREFIX)
cat $(FILE_NAME_PREFIX)* | tar xz
You are right. Using /dev/stdin and /dev/stdout is to be more clear.
- Klaus Lichtenwalder says:
December 14th, 2007 at 2:49 pm
Just a few nitbits… If you want to use stdin/stdout with tar, it's simply a -
e.g.: tar cf – . | (cd /elsewhere; tar xf -)
cat always appends its arguments to stdout, so
cat $(prefix)* | command
is sufficient. I don't know and (honestly) don't care if gnu-tar sends its output to stdout if no f argument given, every other unix uses the default tape device (which is /dev/rmt) if no f argument given (I have to work with Solaris and AIX too…).
Viktor Balogh's HP-UX blog
This is how to tar a bunch of files and send it over network to another machine over SSH, in one turn:# cd /etc; tar cf - passwd | ssh hp01a01.w1 "cd /root;tar xf - passwd"
Note that with tar you must always use relative path, anyway the files on the target system will be extracted with fullpath and the original files will be overwritten. GNU tar also offers some options which allow the user to modify/transform the paths when files are extracted. You can find the GNU tar on HP-UX under the name gtar, you can download it from the HP-UX porting center:# which gtar /usr/local/bin/gtar
If you have a 'tar' archive that was made with absolute paths, use 'pax' to extract it to a different directory:# pax -r -s '|/tmp/|/opt/|' -f test.tar
If you unpack this archive with other user privileges (non-root) all uid and gid will be replaced with the uid and gid from this user. Keep that in mind, if you make backups/restore, practically always do any backup/restore with UID 0.
The use of tar with find isn't apt to work if there are lots of files. Instead use pax(1):# find . -atime +7 | pax -w | gzip > backup.tgz
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