8.3.3 Hard Links

Normally, when tar archives a hard link, it writes a block to the archive naming the target of the link (a ‘1’ type block). In that way, the actual file contents is stored in file only once. For example, consider the following two files:

$ ls -l
-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden

Here, ‘jeden’ is a link to ‘one’. When archiving this directory with a verbose level 2, you will get an output similar to the following:

$ tar cvvf ../archive.tar .
drwxr-xr-x gray/staff        0 2007-10-30 15:13 ./
-rw-r--r-- gray/staff        4 2007-10-30 15:11 ./jeden
hrw-r--r-- gray/staff        0 2007-10-30 15:11 ./one link to ./jeden

The last line shows that, instead of storing two copies of the file, tar stored it only once, under the name ‘jeden’, and stored file ‘one’ as a hard link to this file.

It may be important to know that all hard links to the given file are stored in the archive. For example, this may be necessary for exact reproduction of the file system. The following option does that:

‘--check-links’

‘-l’

Check the number of links dumped for each processed file. If this number does not match the total number of hard links for the file, print a warning message.

For example, trying to archive only file ‘jeden’ with this option produces the following diagnostics:

$ tar -c -f ../archive.tar -l jeden
tar: Missing links to 'jeden'.

Although creating special records for hard links helps keep a faithful record of the file system contents and makes archives more compact, it may present some difficulties when extracting individual members from the archive. For example, trying to extract file ‘one’ from the archive created in previous examples produces, in the absence of file ‘jeden’:

$ tar xf archive.tar ./one
tar: ./one: Cannot hard link to './jeden': No such file or directory
tar: Error exit delayed from previous errors

The reason for this behavior is that tar cannot seek back in the archive to the previous member (in this case, ‘one’), to extract it(23). If you wish to avoid such problems at the cost of a bigger archive, use the following option:

‘--hard-dereference’

Dereference hard links and store the files they refer to.

For example, trying this option on our two sample files, we get two copies in the archive, each of which can then be extracted independently of the other:

$ tar -c -vv -f ../archive.tar --hard-dereference .
drwxr-xr-x gray/staff        0 2007-10-30 15:13 ./
-rw-r--r-- gray/staff        4 2007-10-30 15:11 ./jeden
-rw-r--r-- gray/staff        4 2007-10-30 15:11 ./one

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