CODEWITHSUNDEEP
linuxbashcompressiontar
user420095
user420095

Reputation:

How to update tar (NOT append)

I want to update an existing tar file with newer files.

At GNU, I read:

4.2.3 Updating an Archive

In the previous section, you learned how to use ‘--append’ to add a file to an existing archive. A related operation is ‘--update’ (‘-u’). The ‘--update’ operation updates a tar archive by comparing the date of the specified archive members against the date of the file with the same name. If the file has been modified more recently than the archive member, then the newer version of the file is added to the archive (as with ‘--append’).

However, When I run my tar update command, the files are appended even though their modification dates are exactly the same. I want to ONLY append where modification dates of files to be tarred are newer than those already in the tar...

tar -uf ./tarfile.tar /localdirectory/ >/dev/null 2>&1

Currently, every time I update, the tar doubles in size...

Upvotes: 4

Views: 24364

Answers (5)

OrangeDog
OrangeDog

Reputation: 38836

By default tar strips the leading / from member names, but it does this after deciding what needs to be updated.

Therefore if you are archiving an absolute path, you either need to cd / and use relative paths, or add the -P/--absolute-names option.

cd /
tar -uf "$OLDPWD/tarfile.tar" localdirectory/ >/dev/null 2>&1

tar -cPf tarfile.tar /localdirectory/ >/dev/null 2>&1
tar -uPf tarfile.tar /localdirectory/ >/dev/null 2>&1

However, the updated items will still be appended. A tar (tape archive) file cannot be modified except by appending.

Upvotes: 1

Zbyszek
Zbyszek

Reputation: 21

If you do not want to use the -P switch tar -u... works correctly if the current directory is the parent directory of the one we are going to update, and the path to this directory in the tar command will not be an absolute path.
For example:
We want to update catalog /home/blabla/Dir. We do it like that:

cd /home/blabla  
tar -u -f tarfile.tar Dir

In general, the update must be made from the same place as the creation, so that the paths agree.
It is also possible:

cd /home/blabla/Dir
tar -u -f /path/to/tarfile.tar .

Upvotes: 2

physkets
physkets

Reputation: 182

You may simply create (instead of update) the archive each time:

tar -cvpf tarfile.tar *

This will solve the problem of your archive doubling in size each time. But of course, it is generating the whole archive every time.

Upvotes: 1

jørgensen
jørgensen

Reputation: 10579

The update you describe implies that the file within the archive is replaced. If the new copy is smaller than what's in the archive, it could be directly rewritten. If the new copy however is larger, tar would have to zero the existing archive entry and append. Such updates would leave runs of '\0's or other unused bytes, so any normal computer user would want that such sections are removed, which would be done by "moving up" bytes comprising the archive contents towards the start of the file (think C's memmove).

Such an in-place move operation however, which would involve seek-read-seek-write cycles, is costly, especially when you look at it in the context of tapes — which tar was designed for originally —, i.e. devices with a seek performance that is not comparable to harddisks. You'd wear out the tape rather quickly with such move ops. Oh and of course, WORM devices don't support this move op either.

Upvotes: 3

fge
fge

Reputation: 121840

Warning! When speaking about "dates" it means any date, and that includes the access time.

Should your files have been accessed in any such way (a simple ls -l is enough) then tar is right to do what it does!

You need to find another way to do what you want. Probably use a sentinel file and see if its modification date is less than the files you wish to append.

Upvotes: 0

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