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Dumpdir

Incremental archives keep information about contents of each dumped directory in special data blocks called dumpdirs.

Dumpdir is a sequence of entries of the following form:

 
C filename \0

where C is one of the control codes described below, filename is the name of the file C operates upon, and `\0' represents a nul character (ASCII 0). The white space characters were added for readability, real dumpdirs do not contain them.

Each dumpdir ends with a single nul character.

The following table describes control codes and their meanings:

`Y'

filename is contained in the archive.

`N'

filename was present in the directory at the time the archive was made, yet it was not dumped to the archive, because it had not changed since the last backup.

`D'

filename is a directory.

`R'

This code requests renaming of the filename to the name specified with the `T' command, that immediately follows it.

`T'

Specify target file name for `R' command (see below).

`X'

Specify temporary directory name for a rename operation (see below).

Codes `Y', `N' and `D' require filename argument to be a relative file name to the directory this dumpdir describes, whereas codes `R', `T' and `X' require their argument to be an absolute file name.

The three codes `R', `T' and `X' specify a renaming operation. In the simplest case it is:

 
R`source'\0T`dest'\0

which means "rename file `source' to file `dest'".

However, there are cases that require using a temporary directory. For example, consider the following scenario:

  1. Previous run dumped a directory `foo' which contained the following three directories:
     
    a
    b
    c
    
  2. They were renamed cyclically, so that:
     
    `a' became `b'
    `b' became `c'
    `c' became `a'
    
  3. New incremental dump was made.

This case cannot be handled by three successive renames, since renaming `a' to `b' will destroy the existing directory. To correctly process it, GNU tar needs a temporary directory, so it creates the following dumpdir (newlines have been added for readability):

 
Xfoo\0
Rfoo/a\0T\0
Rfoo/b\0Tfoo/c\0
Rfoo/c\0Tfoo/a\0
R\0Tfoo/a\0

The first command, `Xfoo\0', instructs the extractor to create a temporary directory in the directory `foo'. Second command, `Rfoo/aT\0', says "rename file `foo/a' to the temporary directory that has just been created" (empty file name after a command means use temporary directory). Third and fourth commands work as usual, and, finally, the last command, `R\0Tfoo/a\0' tells tar to rename the temporary directory to `foo/a'.

The exact placement of a dumpdir in the archive depends on the archive format (see section Controlling the Archive Format):


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