Ensure to keep directories content if present

This way the script can be called before installation or at the end
(preseed/late_command).

debian/part.btrfs.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-# For Debian install
+# For Debian install in late_command or in a TTY during installation.
 # Complete the Btrfs partitionning
 #
 # Source: https://wiki.101010.fr/doku.php?id=debian:installation_btrfs
@@ -8,22 +8,20 @@
 # Remount the partition with "compress" option
 mount -o remount,defaults,compress=lzo /target
 
-# Create all subvolumes
-btrfs subvolume create /target/home
-btrfs subvolume create /target/opt
-btrfs subvolume create /target/root
-btrfs subvolume create /target/srv
-btrfs subvolume create /target/tmp
-btrfs subvolume create /target/usr
-btrfs subvolume create /target/var
-
-# Specific actions for "etc" subvolume
-## In case the installation is based on LUKS/cryptsetup, we need to keep
-## the generated cryptsetup's configuration.
-mv /target/etc /target/etc.temp
-btrfs subvolume create /target/etc
-mv /target/etc.temp/* /target/etc
-rmdir /target/etc.temp
+# Manage subvolumes creation
+for DIR in etc home opt root srv tmp usr var; do
+	## Move existent directory to a temp directory
+	test -d "/target/${DIR}" && mv -- /target/"${DIR}" /target/"${DIR}".temp
+	## Create subvolume
+	btrfs subvolume create /target/"${DIR}"
+	## If the directory already existed
+	if [ -d "/target/${DIR}.temp" ]; then
+		## Move the content to the new subvolume
+		mv -- /target/"${DIR}".temp/.* /target/"${DIR}".temp/* /target/"${DIR}"
+		## Remove the temp directory
+		rmdir -- /target/"${DIR}".temp
+	fi
+done
 
 # Ensure everything is ok
 btrfs subvolume list -p /target