diff --git a/src/cli/cli.odin b/src/cli/cli.odin
index 0ae01c7..08a6c9b 100644
--- a/src/cli/cli.odin
+++ b/src/cli/cli.odin
@@ -10,7 +10,7 @@ Options :: struct {
 	compression: u16 `args:"name=compression"                 usage:"Compression level, 0-12"`,
 	size:        u64 `args:"name=size"                        usage:"Chunk size in bytes"`,
 	input:       ^os.File `args:"name=input,file=r,required"  usage:"Path to input directory"`,
-	output:      ^os.File `args:"name=output,file=wc"         usage:"Path to ouput directory"`,
+	output:      ^os.File `args:"name=output,file=r"          usage:"Path to ouput directory"`,
 	// overflow:    [dynamic]string,
 }
 
diff --git a/src/file/header.odin b/src/file/header.odin
index 5b71762..5550bb5 100644
--- a/src/file/header.odin
+++ b/src/file/header.odin
@@ -19,7 +19,6 @@ Header :: struct #packed #all_or_none {
 	size_per_chunk:   u64,
 	total_size:       u64,
 	number_of_assets: u32,
-	// asset_index:      []AssetIndex,
 }
 
 AssetIndex :: struct #packed #all_or_none {
@@ -61,10 +60,7 @@ compute_header :: proc(
 		}
 
 		offset_in_bytes := size_of(AssetIndex) * cast(u64)i
-		offset_pointer := mem.ptr_offset(
-			&bytes[0],
-			offset_in_bytes,
-		)
+		offset_pointer := mem.ptr_offset(&bytes[0], offset_in_bytes)
 
 		ai := cast(^AssetIndex)offset_pointer
 		copy(ai.relpath[:PATH_SIZE], f.relpath)
@@ -87,3 +83,48 @@ compute_header :: proc(
 	assert(len(bytes) % size_of(AssetIndex) == 0) // clean multiple
 	return header, mem.slice_data_cast([]AssetIndex, bytes) // transmute slice with proper size
 }
+
+write_header :: proc(
+	output: ^os.File,
+	header: Header,
+	asset_index: []AssetIndex,
+	allocator := context.allocator,
+) {
+	// remove :: proc(name: string) -> Error {…}
+	// create :: proc(name: string) -> (^File, Error) {…}
+	// write_at :: proc(f: ^File, p: []u8, offset: i64) -> (n: int, err: Error) {…}
+
+	output_path := os.name(output)
+
+	chunk_1, c1_err := os.join_path({output_path, "CHUNK1"}, allocator)
+	if c1_err != nil {
+		fmt.eprintln("error: failed to allocate path join:", c1_err)
+		os.exit(1)
+	}
+
+	// Remove old chunk 1
+	if os.exists(chunk_1) {
+		rm_err := os.remove(chunk_1)
+		if rm_err != nil {
+			fmt.eprintln("error: failed to remove old chunk:", rm_err)
+			os.exit(1)
+		}
+	}
+
+	// Create chunk 1
+	chunk_file_1, cf1_err := os.create(chunk_1)
+	if cf1_err != nil {
+		fmt.eprintln("error: failed to create chunk:", cf1_err)
+		os.exit(1)
+	}
+
+	// Write header
+	header := header // shadow parameter
+	header_bytes := mem.ptr_to_bytes(&header)
+	os.write_at(chunk_file_1, header_bytes, 0)
+
+	// Write asset index
+	asset_index := asset_index // shadow parameter
+	index_bytes := mem.slice_data_cast([]u8, asset_index) // transmute slice with proper size
+	os.write_at(chunk_file_1, index_bytes, size_of(Header))
+}
diff --git a/src/main.odin b/src/main.odin
index d1f126b..46d14f3 100644
--- a/src/main.odin
+++ b/src/main.odin
@@ -25,7 +25,9 @@ main :: proc() {
 	entries := file.list_dir_recursive_by_path("./src")
 	fmt.println("entries:", entries)
 
-	header, asset_index_array := file.compute_header(entries)
+	header, asset_index := file.compute_header(entries)
 	fmt.println("header:", header)
-	fmt.println("aia:", asset_index_array)
+	fmt.println("aia:", asset_index)
+
+	file.write_header(opts.output, header, asset_index)
 }