Allow backing-up empty volumes (#50)

* Add -a flag to not error on empty volume

* Merge -a flag into -f flag

* Clean up if condition

* Modify readme for new -f behavior

* Update README

* Update usage

---------

Co-authored-by: Jarek Lipski <loomchild@loomchild.net>

Author: YeapGuy

README.md

@@ -59,7 +59,7 @@ will clean and restore volume named `some_volume` from `some_archive.tar.bz2` ar
 > Don't forget the `-i` switch for interactive operation.
 
 > [!NOTE]
-> Restore will fail if the target volume is not empty (use `-f` flag to override).
+> Backup will fail if the source volume is empty. Restore will fail if the target volume is not empty. You can use the `-f` flag to override this.
 
 ### Restore from a file (deprecated)
 

volume-backup.sh

@@ -6,15 +6,15 @@ usage() {
   >&2 echo "Options:"
   >&2 echo "  -c <algorithm> chooose compression algorithm: bz2 (default), gz, xz, pigz, zstd and 0 (none)"
   >&2 echo "  -e <glob> exclude files or directories (only for backup operation)"
-  >&2 echo "  -f force overwrite even if target volume is not empty during restore"
+  >&2 echo "  -f Allow backing up empty volume or restoring into non-empty volume, overwriting its contents"
   >&2 echo "  -x <args> pass additional arguments to the Tar utility"
   >&2 echo "  -v verbose"
 }
 
 backup() {
-    if [ -z "$(ls -A /volume)" ]; then
-       >&2 echo "Volume is empty or missing, check if you specified a correct name"
-       exit 1
+    if [ -z "$FORCE" ] && [ -z "$(ls -A /volume)" ]; then
+        >&2 echo "Volume is empty or missing, check if you specified a correct name"
+        exit 1
     fi
 
     if ! [ "$ARCHIVE" == "-" ]; then
@@ -71,10 +71,6 @@ while getopts "h?vfc:e:x:" OPTION; do
         TAROPTS+=(--exclude $OPTARG)
         ;;
     f)
-        if [ "$OPERATION" != "restore" ]; then
-          usage
-          exit 1
-        fi
         FORCE=1
         ;;
     v)