grub-set-default: add page (#9915)

pages/linux/grub-set-default.md

@@ -0,0 +1,12 @@
+# grub-set-default
+
+> Set the default boot entry for GRUB.
+> More information: <https://manned.org/grub-set-default>.
+
+- Set the default boot entry to an entry number, name or identifier:
+
+`sudo grub-set-default {{entry_number}}`
+
+- Set the default boot entry to an entry number, name or identifier for an alternative boot directory:
+
+`sudo grub-set-default --boot-directory {{/path/to/boot_directory}} {{entry_number}}`