diff --git a/scripts/_common.sh b/scripts/_common.sh
new file mode 100644
index 0000000..ae0a063
--- /dev/null
+++ b/scripts/_common.sh
@@ -0,0 +1,126 @@
+#!/bin/bash
+
+#=================================================
+# COMMON VARIABLES AND CUSTOM HELPERS
+#=================================================
+
+php_upload_max_filesize="250M"
+
+install_yunohost_logo() {
+    local logo="$install_dir/resources/branding/Logo_Globinours_maxi.png"
+
+    install -D -m 0644 \
+        "$logo" \
+        "/usr/share/yunohost/applogos/$app.png"
+    yunohost user permission update "$app.main" --logo "$logo"
+}
+
+migrate_legacy_data() {
+    local legacy_data="$install_dir/data"
+    local legacy_storage="$install_dir/storage"
+    local legacy_media="$install_dir/public/media"
+
+    if [[ -d "$legacy_data" && ! -L "$legacy_data" ]]; then
+        ynh_print_info "Migrating the legacy data directory to $data_dir..."
+        cp -a "$legacy_data/." "$data_dir/"
+
+        if [[ -f "$legacy_data/refuge.sqlite" ]]; then
+            local migrated_database="$data_dir/refuge.sqlite.migrating"
+            sqlite3 "$legacy_data/refuge.sqlite" ".backup '$migrated_database'"
+
+            local integrity_check
+            integrity_check="$(sqlite3 "$migrated_database" "PRAGMA integrity_check;")"
+            if [[ "$integrity_check" != "ok" ]]; then
+                rm -f "$migrated_database"
+                ynh_die --message="The migrated Globinours database failed its integrity check. The legacy data was left untouched."
+            fi
+
+            mv -f "$migrated_database" "$data_dir/refuge.sqlite"
+            rm -f "$data_dir/refuge.sqlite-wal" "$data_dir/refuge.sqlite-shm"
+        fi
+    fi
+
+    if [[ -d "$legacy_storage" && ! -L "$legacy_storage" ]]; then
+        ynh_print_info "Migrating the legacy storage directory to $data_dir/storage..."
+        cp -a "$legacy_storage/." "$data_dir/storage/"
+    fi
+
+    if [[ -d "$legacy_media" && ! -L "$legacy_media" ]]; then
+        ynh_print_info "Migrating the legacy public media directory to $data_dir/media..."
+        cp -a "$legacy_media/." "$data_dir/media/"
+    fi
+}
+
+prepare_persistent_paths() {
+    for path_to_replace in "$install_dir/data" "$install_dir/storage" "$install_dir/public/media"; do
+        if [[ -d "$path_to_replace" && ! -L "$path_to_replace" ]]; then
+            ynh_die --message="Refusing to replace the physical data directory $path_to_replace before it is migrated."
+        fi
+        if [[ -e "$path_to_replace" || -L "$path_to_replace" ]]; then
+            ynh_safe_rm "$path_to_replace"
+        fi
+    done
+
+    ln -s "$data_dir" "$install_dir/data"
+    ln -s "$data_dir/storage" "$install_dir/storage"
+    ln -s "$data_dir/media" "$install_dir/public/media"
+
+    repair_and_verify_permissions
+}
+
+repair_and_verify_permissions() {
+    ynh_print_info "Checking Globinours persistent-data permissions..."
+
+    chown -R "$app:www-data" "$install_dir"
+    chown -R "$app:$app" "$data_dir"
+    chmod -R u=rwX,g=rX,o= "$install_dir"
+    chmod -R u=rwX,g=,o= "$data_dir"
+
+    chown "$app:www-data" "$data_dir"
+    chmod 0710 "$data_dir"
+    chown -R "$app:www-data" "$data_dir/media"
+    chmod -R u=rwX,g=rX,o= "$data_dir/media"
+
+    if ! ynh_exec_as_app test -r "$data_dir" || ! ynh_exec_as_app test -w "$data_dir"; then
+        ynh_die --message="The Globinours system user cannot read and write its persistent data directory."
+    fi
+    if ! runuser -u www-data -- test -x "$data_dir" || ! runuser -u www-data -- test -r "$data_dir/media"; then
+        ynh_die --message="Nginx cannot access the Globinours public media directory."
+    fi
+    if [[ -f "$data_dir/refuge.sqlite" ]]; then
+        local integrity_check
+        integrity_check="$(ynh_exec_as_app sqlite3 "$data_dir/refuge.sqlite" "PRAGMA quick_check;")"
+        if [[ "$integrity_check" != "ok" ]]; then
+            ynh_die --message="The Globinours database failed its integrity check."
+        fi
+    fi
+}
+
+inventory_persistent_data() {
+    local inventory_file="$1"
+
+    : > "$inventory_file"
+    if [[ -f "$data_dir/refuge.sqlite" ]]; then
+        printf '%s\n' "refuge.sqlite" >> "$inventory_file"
+    fi
+    for persistent_subdir in association grants medical-documents private-media media; do
+        if [[ -d "$data_dir/$persistent_subdir" ]]; then
+            find "$data_dir/$persistent_subdir" -type f -printf '%P\n' |
+                sed "s#^#$persistent_subdir/#" >> "$inventory_file"
+        fi
+    done
+    sort -u -o "$inventory_file" "$inventory_file"
+}
+
+verify_persistent_inventory() {
+    local inventory_file="$1"
+    local missing_file
+
+    while IFS= read -r persistent_file; do
+        [[ -z "$persistent_file" ]] && continue
+        if [[ ! -f "$data_dir/$persistent_file" ]]; then
+            missing_file="$persistent_file"
+            ynh_die --message="Persistent data disappeared during the operation: $missing_file"
+        fi
+    done < "$inventory_file"
+}