change according to shellcheckm add drone.yml

.drone.yml

@@ -0,0 +1,9 @@
+---
+kind: pipeline
+name: shellcheck
+
+steps:
+    - name: shellcheck
+      image: koalaman/shellcheck-alpine:stable
+      commands:
+          - shellcheck backup_cosmicvoyage

backup_cosmicvoyage

@@ -16,20 +16,20 @@ test -d $SHIPS || (echo "ships does not exit" ; exit 1)
 
 test -d $BACKUP || mkdir $BACKUP
 
-OLDBACKUP=`ls -1 $BACKUP/ships-*.tar.gz 2>/dev/null|tail -1`
+OLDBACKUP=$(ls -1 $BACKUP/ships-*.tar.gz 2>/dev/null|tail -1)
 
-cd $SHIPS
+cd $SHIPS || exit
 
-TS=`date -u +%Y-%m-%dZ%H%M`
+TS=$(date -u +%Y-%m-%dZ%H%M)
 
 TGZ="$BACKUP/ships-$TS.tar.gz"
 
 # if you run the script more than once within 1 minute it gets
 # confused
 
-if [ -f $TGZ ]
+if [ -f "$TGZ" ]
 then
-    echo backup file $TGZ already exists, not overwriting
+    echo "backup file $TGZ already exists, not overwriting"
     exit 1
 fi
 
@@ -38,17 +38,17 @@ echo "backing up $SHIPS/* to $TGZ"
 # have to use */* since it would otherwise backup the symlinks and not
 # the target, maybe we could use an option in tar otherwise
 
-tar cvfz $TGZ */* */.??*
+tar cvfz "$TGZ" ./*/* ./*/.??*
 
 # don't keep backup file if nothing has changed
 
-if [ -f "$OLDBACKUP" ] && cmp -s "$OLDBACKUP" $TGZ
+if [ -f "$OLDBACKUP" ] && cmp -s "$OLDBACKUP" "$TGZ"
 then
     echo "nothing has changed, no new backup"
-    rm -f $TGZ
+    rm -f "$TGZ"
 else
     echo "file is different, keeping new file"
 fi
 
-test -f $TGZ && echo backup of file $TGZ was successful
+test -f "$TGZ" && echo "backup of file $TGZ was successful"