shell_escape.c

#include <stdlib.h>

char *shell_escape(char *, size_t);

int main(int argc, char **argv) {
  puts(shell_escape("'; echo shell'", 14));
}

char *shell_escape(char *data, size_t n) {
  size_t i, j, single_quotes = 0;
  char *p, *o;

  for(i = 0, p = data; i < n && *p != (char *)NULL; i++, p++) {
    if(*p == '\'') {
      single_quotes++;
    }
  }

  o = (char *)malloc(3*single_quotes + i + 3);

  if(o == (char *)NULL) {
    exit(EXIT_FAILURE);
  }

  for(i = 0, j = 1, p = data; i < n && *p != (char *)NULL; i++, j++, p++) {
    if(*p == '\'') {
      o[j] = '\'';
      o[j+1] = '\\';
      o[j+2] = '\'';
      o[j+3] = '\'';
      j += 3;
    } else {
      o[j] = *p;
    }
  }

  o[0] = '\'';
  o[3*single_quotes + i + 1] = '\'';
  o[3*single_quotes + i + 2] = '\0';

  return o;
}