Fix description of the CACHEDIR.TAG file.

doc/tar.1

@@ -13,7 +13,7 @@
 .\"
 .\" You should have received a copy of the GNU General Public License
 .\" along with this program.  If not, see <http://www.gnu.org/licenses/>.
-.TH TAR 1 "July 11, 2022" "TAR" "GNU TAR Manual"
+.TH TAR 1 "January 15, 2024" "TAR" "GNU TAR Manual"
 .SH NAME
 tar \- an archiving utility
 .SH SYNOPSIS
@@ -876,7 +876,14 @@ Exclude backup and lock files.
 .TP
 \fB\-\-exclude\-caches\fR
 Exclude contents of directories containing file \fBCACHEDIR.TAG\fR,
-except for the tag file itself.
+except for the tag file itself.  The \fBCACHEDIR.TAG\fR file must be
+a regular file whose content begins with the following 43 characters:
+.IP
+.RS
+.EX
+Signature: 8a477f597d28d172789f06886806bc55
+.EE
+.RE
 .TP
 \fB\-\-exclude\-caches\-all\fR
 Exclude directories containing file \fBCACHEDIR.TAG\fR and the file itself.

doc/tar.texi

@@ -8218,9 +8218,19 @@ that match the following shell globbing patterns:
 @findex exclude-caches
 When creating an archive, the @option{--exclude-caches} option family
 causes @command{tar} to exclude all directories that contain a @dfn{cache
-directory tag}. A cache directory tag is a short file with the
-well-known name @file{CACHEDIR.TAG} and having a standard header
-specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
+directory tag}. A cache directory tag is a regular file with the
+well-known name @file{CACHEDIR.TAG} whose first 43 octets
+consist of the following ASCII header string:
+
+@example
+Signature: 8a477f597d28d172789f06886806bc55
+@end example
+
+Any remaining bytes are not relevant, in particular the signature
+string does not have to be followed by an LF or CR/LF pair@footnote{The
+cache directory tagging specification is set forth in
+@url{https://bford.info/cachedir/}}.
+
 Various applications write cache directory tags into directories they
 use to hold regenerable, non-precious data, so that such data can be
 more easily excluded from backups.