add DELETE_OUTDATED to prune old remote bkfiles

README.md

@@ -204,12 +204,38 @@ If you want to cleanup the backup file locally after uploading it upstream, set
 `CLEAR_BKFILE` environment variable to a non-empty value. `zfsbk.sh` will maintain the local
 backup regardless of this variable if the upload failed.
 
+    # delete generated bkfile locally after successful upload
+    UPLOAD_PATH="..." CLEAR_BKFILE=yep /usr/local/sbin/zfsbk.sh mybk 7
+
+Ask `zfsbk` to remove an old sequence remotely before uploading a new one:
+
+    # remove all bkfiles of old sequence *remotely* before uploading new one
+    UPLOAD_PATH="..." DELETE_OUTDATED="yep" /usr/local/sbin/zfsbk.sh mybk 7
+
+This allows you to cap the size taken by backups at the remote.
 
 # Managing multiple ZFS Pools
 
 Simply run these scripts once for each pool, setting the _ZPOOL_ environment variable
 accordingly.
 
+# Summary of control variables
+
+Here's all environment variables controlling the behavior or `zfssnap` and `zfsbk`:
+
+## zfssnap
+
+* `ZPOOL` [str] name of the ZFS pool to operate on.
+* `EXCLUDES` [str] space-separated list of dataset names to exclude from snapshots. Overrides defaults.
+* `EXTRA_EXCLUDES` [str] space-separated list of dataset names to exclude from snapshots in addition to defaults.
+
+## zfsbk
+
+* `UPLOAD_PATH` [str] Upload generated bkfile to this remote location; 'scp://' or 'rsync://'
+* `CLEAR_BKFILE` [any] If non-empty, remove copy of generated bkfile after successful remote upload.
+* `DELETE_OUTDATED` [any] If non-empty, remove old backup sequence remotely before uploading first bkfile of a new sequence.
+
+
 # Ancillaries
 
 ## Warranty