From 2e7d15303cd9f208ac698a0ea25c7be056e38c89 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Wed, 25 Mar 2026 10:16:50 +0100
Subject: [PATCH] nixos/sd-image: add type and improve postBuildCommands
 description

The option was missing a type declaration. Use types.lines so multiple
modules can contribute commands that get merged with newlines.

While at it, document the $img shell variable so users do not have to
read the module source to discover it.
---
 nixos/modules/installer/sd-card/sd-image.nix | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/nixos/modules/installer/sd-card/sd-image.nix b/nixos/modules/installer/sd-card/sd-image.nix
index b3251f9c6e1d..223961228ba5 100644
--- a/nixos/modules/installer/sd-card/sd-image.nix
+++ b/nixos/modules/installer/sd-card/sd-image.nix
@@ -180,11 +180,16 @@ in
     };
 
     postBuildCommands = mkOption {
+      type = types.lines;
       example = literalExpression "'' dd if=\${pkgs.myBootLoader}/SPL of=$img bs=1024 seek=1 conv=notrunc ''";
       default = "";
       description = ''
-        Shell commands to run after the image is built.
-        Can be used for boards requiring to dd u-boot SPL before actual partitions.
+        Shell commands to run after the SD image has been assembled.
+
+        The path to the image is available in the {var}`img` shell
+        variable. This hook is typically used for boards that require
+        writing a bootloader (such as u-boot SPL) to a fixed offset
+        before the first partition.
       '';
     };