How do I extract a single chunk of bytes from within a file?

user_92265 0
Viewed 63513

On a Linux desktop (RHEL4) I want to extract a range of bytes (typically less than 1000) from within a large file (>1 Gig). I know the offset into the file and the size of the chunk.

I can write code to do this but is there a command line solution?

Ideally, something like:

magicprogram --offset 102567 --size 253 < input.binary > output.binary
linux file split
6 Answers

Even faster

dd bs=<req len> count=1 skip=<req offset> if=input.binary of=output.binary
user_24276670

I have had the same problem, trying to cut parts of a RAW disk image. dd with bs=1 is unusable, therefore I have made a simple C program for the task.

// usage:
//  ./cutfile srcfile destfile offset length
//  ./cutfile my.image movie.avi 4524 20412452
// compile, presuming it is saved as cutfile.cc:
//  gcc cutfile.cc -o cutfile -std=c11 -pedantic -W -Wall -Werror 
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int main(int argc, char *argv[])
{
  if(argc != 5) {
      printf("error, need 4 arguments!\n");
      return 1;
  }


  const unsigned blocksize = 16*512;  // can adjust
  unsigned char buffer[blocksize];

  FILE *f = fopen(argv[1], "rb");
  FILE *fout = fopen(argv[2], "wb");
  long offset = atol(argv[3]);
  long length = atol(argv[4]);
  if(f==NULL || fout==NULL) {
      perror("cannot open file");
      return 1;
  }
  fseek(f, offset, SEEK_SET);

  while(length > blocksize) {
      fread(buffer, 1, blocksize, f);
      fwrite(buffer, 1, blocksize, fout);
      length -= blocksize;
  }
  if(length>0) { // copy rest
      fread(buffer, 1, length, f);
      fwrite(buffer, 1, length, fout);
  }    

  fclose(fout);
  fclose(f);
  return 0;
}
user_31466850
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