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

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are you sure you mean byte? (a byte is a unit of data that is eight binary digits long). If you change the first byte of a file Im guessing you will corrupt the file. Are you referring to changing the first char of the filename? – DataPimp Mar 17 '10 at 17:18
No, I meant what I said. I need to invalidate the data for a bunch of torrents that my client has marked as completed, but the tracker doesn't agree. I figure the best way is to change the data as little as possible and force my client to re-check the data. Then it'll download at most one piece from each torrent, and tell the tracker the torrent is done. – jnylen Mar 17 '10 at 17:20
up vote 10 down vote accepted

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 :)

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Nice and simple, works fine according to my testing. Is that easily adaptable to changing a byte other than the first? I played around with it a little but couldn't get it to work. – jnylen Mar 17 '10 at 20:10
@jnylen It should be adaptable to changing an arbitrary byte other than the first as well, the seek=<nblocks> option to dd is what you're looking for. – Kjetil Jørgensen Mar 19 '10 at 10:51
@Kjetil: thanks, that worked... I'd vote your comment up if I had 2 more reputation points :p – jnylen Mar 20 '10 at 19:14

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);

   return 0;
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