Change first byte of file in Linux?

Question

How can I change the first byte of multiple files in Linux? Willing to use perl/awk/sed/whatever. Must work correctly on binary files (i.e. not change any other bytes).

Answer 1

You can do something like echo -ne \\xFF | dd conv=notrunc bs=1 count=1 of=YOURFILE, replacing FF with your hex value. Try it first though :)

Answer 2

compile this with gcc -o w1stb w1stb.c and use it like ./w1stb <file> <byte>:

#include <stdio.h>
int main(int argc, const char *argv[]) {

   int i;
   FILE* f;
   unsigned char b;

   if (argc < 3) {
       printf("usage: w <filename> <byte>\n");
       return 1;
   }

   i = atoi(argv[2]);

   if (i < 0) {
       printf("error, negative byte\n");
       return 2;
   }

   if (i > 255) {
       printf("error, to big byte\n");
       return 3;
   }

   f = fopen(argv[1], "w");
   if (!f) {
       printf("error, can't open file\n");
       return 4;
   }

   b = (unsigned char)i;
   fwrite(&b, 1, 1, f);
   fclose(f);

   return 0;
}