A backup script that can be used in conjunction with rdup needs to be able to grok the output listed below (obviously). Writing such a script isn't really that difficult. You can look at the examples already used by rdup. See rdup-snap for instance. In case you want to write your own scripts you will have keep in mind the following things.

The line numbers given all refer to rdup-snap version 0.6.0 in rdup distribution. You can see the file online

Sample rdup output:

+d 0751 1000 1000 11 0 /home/miekg/bin
+h 0700 1000 1000 44 21 /home/miekg/bin/acx2 -> /home/miekg/bin/cx
+h 0700 1000 1000 51 28 /home/miekg/bin/cx-hardlink -> /home/miekg/bin/cx
+l 0777 1000 1000 24 18 /home/miekg/bin/t -> tt
+- 0775 1000 1000 21 174 /home/miekg/bin/wifi

Add or remove

In rdup's default output a '+' as the first character signals the object (file, directory or link) named in this line should be added to the backup. This case is the most complicated one, so it will be handled in its own section.

When a line starts with a '-' the object should be removed, in the case of a directory the whole directory (and everything below it) is removed. All other cases are handled with a simple unlink (rm).

See lines 179 - 187 in rdup-snap.

Adding objects

When objects are added you need to look at the type of the object, which is either a directory, a normal file, a symlink or a hardlink.

Removing existing files

When we've reached this point we have to look at our files in the backup directory (depending on the setup we may have copied the backup from yesterday to today and the script is working on those files).

If the object we want to add already exists in the backup, we need to remove it and replace it with the current one. Usually we can just unlink the object, unless it is a directory and that case we need to rm -rf it. See lines 122 - 135 in rdup-snap.

Next we need to split on the actual type: files, directory, symlink and hardlink.

Files

Basically just copy the from the file system over to the backup file system. In rdup-snap File::copy is used for this. See line 145.

Sample output:

+- 0775 1000 1000 21 174 /home/miekg/bin/wifi

Directory

The directory size as printed by rdup is always zero as there no content for a directory. Just mkdir $directory, see line 151 in rdup-snap.

Example of a directory output:

+d 0751 1000 1000 11 0 /home/miekg/bin

Note: when a backup is made by mortal users (i.e. non root) the directory permissions need to be set in such a way that rdup can still access this directory in case it needs to place more files in this directory. If this is detected a chmod u+rwx is issued.

Symbolic link

A symbolic link can be created as-is. The target need not yet to exist in the backup file system. The target of the link might even fall outside the directories that are backed up.

In any case the parsing of the output of rdup is slight different. The file size (%s) of a symlink is not relevant, so rdup overloads it to mean the path length of the symlink name. The path length (%n) is extended to include the link target also. So in this sample output:

+l 0777 1000 1000 24 18 /home/miekg/bin/t -> tt

The 24 is the combined length of /miekg/miekg/bin/t -> tt and 18 is the length of /home/miekg/bin/t. The symlink is thus a substring from the pathname up to the 18th character. From the 18+4th till the last position is the target name of the symlink. See the lines 114 - 120 for this parsing in rdup-snap.

With the link name and the target name we can re-create the symlink in our backup file system.

See line 166 for the symlink creation.

Hard link

A hardlink look almost like a symlink in rdup's output, except for the 'h' as type. The link name and target name parsing is exactly the same as for symlinks, as you might spot from this sample output:

+h 0700 1000 1000 44 21 /home/miekg/bin/acx2 -> /home/miekg/bin/cx

The only problem with hardlinks is that you can not create them when the target is not available. To fix this all lines with type 'hardlink' are saved up (line 170) and the link creation is done after all files and directories are put in the backup file system. This post processing can be found at lines 71 - 76.

Note that hardlink support is new and will be available in the (yet) to be released rdup version 0.6.0