add build job for disk image file

.github/workflows/build.yml

@@ -9,3 +9,4 @@ jobs:
       - run: sudo apt-get update
       - run: sudo apt-get install -y qemu-user-static
       - run: bin/build
+      - run: sudo bin/img-gen

.gitignore

@@ -2,3 +2,4 @@
 /cache/
 /dist/
 /megabit-armhf.tar.gz
+/megabit-armhf.img.gz

README.md

@@ -39,18 +39,7 @@ blink();
 
 ```sh
 bin/build
-```
-
-### SD カードのフォーマット
-
-- FAT32 (LBA)
-- boot フラグ付ける
-- ボリュームラベル付けない
-
-### SD カードに書き込み
-
-```sh
-tar xf megabit-armhf.tar.gz -C /path/to/sd/
+bin/img-gen
 ```
 
 ### 前提条件
@@ -59,14 +48,8 @@ tar xf megabit-armhf.tar.gz -C /path/to/sd/
 - binfmt-support 2.1.7+
 - Docker
 - Docker Compose
-- curl
-- grep
-- gzip
-- jq
-- ruby
-- ssh-keygen
-- bash
 - qemu-user-static
+- awk, bash, curl, fallocate, grep, gzip, gzip, jq, losetup, mkfs.fat, parted, ruby, ssh-keygen, tar, xargs
 
 ### WiFi の設定
 

bin/img-gen

@@ -0,0 +1,25 @@
+#!/bin/sh
+# デバイスイメージファイルの生成
+# Usage: bin/img-gen
+# Required: gzip awk fallocate parted xargs losetup mkfs.fat tar
+cd -- "$(dirname -- "$0")/.."
+set -eux
+size="$(gzip -l megabit-armhf.tar.gz | awk 'END{printf"%.f",$2*1.02}')"
+fallocate -l "${size}" megabit-armhf.img
+xargs -l parted -s megabit-armhf.img <<EOF
+mklabel msdos
+mkpart primary fat32 0 100%
+set 1 boot on
+EOF
+dev="$(losetup -f)"
+finally="losetup -d '${dev}'"
+trap 'sh -c "${finally}"' EXIT
+losetup -P "${dev}" megabit-armhf.img
+part="${dev}p1"
+mkfs.fat "${part}"
+tmp="$(mktemp -d)"
+finally="rmdir '${tmp}'; ${finally}"
+mount "${part}" "${tmp}"
+finally="umount '${tmp}'; ${finally}"
+tar xf megabit-armhf.tar.gz -C "${tmp}"
+finally="${finally}; gzip -f megabit-armhf.img"