diff --git a/src/mkinitrd/main.c b/src/mkinitrd/main.c
index ded5af0..228b04b 100644
--- a/src/mkinitrd/main.c
+++ b/src/mkinitrd/main.c
@@ -2,13 +2,65 @@
 #include <stdio.h>
 #include <string.h>
 
+int file_size(FILE *file)
+{
+	fseek(file, 0, SEEK_END);
+	int size = ftell(file);
+	fseek(file, 0, SEEK_SET);
+	return size;
+}
+
 int main(int argc, char **argv)
 {
 	if (argc == 1 || (argc > 1 && !strcmp(argv[1], "-h")))
 	{
-		printf("%s <output> [input...]\n", argv[0]);
+		fprintf(stderr, "%s <output> [input...]\n", argv[0]);
 		return argc == 1;
 	}
 
-	FILE *out = fopen(argv[1], "w");
+	FILE *out; /* output file */
+
+	if (!strcmp(argv[1], "-"))
+		out = stdin;
+	else
+		out = fopen(argv[1], "w");
+
+	if (!out)
+	{
+		fprintf(stderr, "Failed to open %s\n", argv[1]);
+		return 1;
+	}
+
+	/* Header */
+	int num_files = argc - 2;
+	struct initrd_global_header header = {
+		.magic = INITRD_MAGIC,
+		.num_files = num_files,
+	};
+
+	fwrite(&header, sizeof(header), 1, out);
+
+	for (int i = 0; i < num_files; i++)
+	{
+		FILE *in = fopen(argv[i + 2], "rb");
+		if (!in)
+		{
+			fprintf(stderr, "Failed to open %s\n", argv[i + 2]);
+			return 2;
+		}
+
+		struct initrd_file_header file = {
+			.size = file_size(in),
+		};
+
+		strncpy(file.name, 64, argv[i + 2]);
+
+		char c;
+		while ((c = getc(in)) != EOF)
+		{
+			putc(c, out);
+		}
+		fclose(in);
+	}
+	fclose(out);
 }