+# deal with the Apt::Periodic::BackupArchiveInterval
+do_cache_backup()
+{
+ BackupArchiveInterval="$1"
+ if [ $BackupArchiveInterval -eq 0 ]; then
+ return
+ fi
+
+ # Set default values and normalize
+ CacheDir="/var/cache/apt"
+ eval $(apt-config shell CacheDir Dir::Cache/d)
+ CacheDir=${CacheDir%/}
+ if [ -z "$CacheDir" ]; then
+ debug_echo "practically empty Dir::Cache, exiting"
+ return 0
+ fi
+
+ Cache="${CacheDir}/archives/"
+ eval $(apt-config shell Cache Dir::Cache::Archives/d)
+ if [ -z "$Cache" ]; then
+ debug_echo "practically empty Dir::Cache::archives, exiting"
+ return 0
+ fi
+
+ BackupLevel=3
+ eval $(apt-config shell BackupLevel APT::Periodic::BackupLevel)
+ if [ $BackupLevel -le 1 ]; then
+ BackupLevel=2 ;
+ fi
+
+ Back="${CacheDir}/backup/"
+ eval $(apt-config shell Back Dir::Cache::Backup/d)
+ if [ -z "$Back" ]; then
+ echo "practically empty Dir::Cache::Backup, exiting" 1>&2
+ return
+ fi
+
+ CacheArchive="$(basename "${Cache}")"
+ test -n "${CacheArchive}" || CacheArchive="archives"
+ BackX="${Back}${CacheArchive}/"
+ for x in $(seq 0 1 $((${BackupLevel}-1))); do
+ eval "Back${x}=${Back}${x}/"
+ done
+
+ # backup after n-days if archive contents changed.
+ # (This uses hardlink to save disk space)
+ BACKUP_ARCHIVE_STAMP=/var/lib/apt/periodic/backup-archive-stamp
+ if check_stamp $BACKUP_ARCHIVE_STAMP $BackupArchiveInterval; then
+ if [ $({(cd $Cache 2>/dev/null; find . -name "*.deb"); (cd $Back0 2>/dev/null;find . -name "*.deb") ;}| sort|uniq -u|wc -l) -ne 0 ]; then
+ mkdir -p $Back
+ rm -rf $Back$((${BackupLevel}-1))
+ for y in $(seq $((${BackupLevel}-1)) -1 1); do
+ eval BackY=${Back}$y
+ eval BackZ=${Back}$(($y-1))
+ if [ -e $BackZ ]; then
+ mv -f $BackZ $BackY ;
+ fi
+ done
+ cp -la $Cache $Back ; mv -f $BackX $Back0
+ update_stamp $BACKUP_ARCHIVE_STAMP
+ debug_echo "backup with hardlinks. (success)"
+ else
+ debug_echo "skip backup since same content."
+ fi
+ else
+ debug_echo "skip backup since too new."
+ fi
+}
+
+# sleep for a random interval of time (default 30min)