GNU tar with --exclude-cache fails to exclude directories containing a CACHEDIR.TAG file

Question

According to the GNU tar manual page, directories containing a CACHEDIR.TAG file can be excluded by specifying a command-line option, such as --exclude-cache .

This doesn't seem to happen.

$ mkdir -p t/a/b/c
$ touch t/a/b/foo t/a/b/CACHEDIR.TAG
$ tar --exclude-caches-all -cvf /dev/null t
t/
t/a/
t/a/b/
t/a/b/c/
t/a/b/CACHEDIR.TAG
t/a/b/foo

Answer 1

A look at the GNU tar source code (and then at the Texinfo documentation) reveals that the file needs to contain a special signature. Adding this signature to the file fixes the problem.

$ mkdir -p t/a/b/c
$ touch t/a/b/foo
$ echo Signature: 8a477f597d28d172789f06886806bc55 >t/a/b/CACHEDIR.TAG
$ tar --exclude-caches-all -cvf /dev/null t
t/
t/a/
tar: t/a/b/: contains a cache directory tag CACHEDIR.TAG; directory not dumped