Linux Segmentation fault (core dumped)

I need to simulate the effect of the grep -f command between 2 files The code gets compiled with no errors, but when I run it instead of the output it displays Segmentation fault (core dumped) How can I that? Also if you can help with a better way to open f1 and f2 without using aux1 and aux2 to provide the name of the file that would be great. Thanks in advance.

//I know I have to check if the code has enough arguments and if not I should do something with stderr (that's I've been told to do in class but I don't know how)

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
  FILE *file1;
  FILE *file2;
  char aux1[100]="",aux2[100]="";
  strcpy(aux1,argv[1]);
  strcpy(aux2,argv[2]);
  file1 = fopen("aux1","r");
  file2 = fopen("aux2","r");

  char s1[100]="",s2[100]="";
  char aux[100]="";

  while(fscanf(file1,"%s",aux)!=EOF)
  {
    strcat(s1,aux);
    strcat(s1," ");
  }
  s1[strlen(s1)-1]='\0';

  while(fscanf(file2,"%s",aux)!=EOF)
  {
    strcat(s2,aux);
    strcat(s2," ");
  }
  s2[strlen(s2)-1]='\0';

  printf("%s\n",strstr(s2,s1));

  return 0;
}

Here is one big mistake:

strcpy(aux1, argv[1]); 
file1 = fopen("aux1", "r");

You aren't opening the file passed, and without checking whether it opened, you lurch right into a segfault by using a NULL file pointer.

It should be

file1 = fopen(aux1, "r");     // removed " marks
if(file1 == NULL)
    exit(1);

You also asked how to avoid using aux1 which can be done with

if(argc < 2)
    exit(1);
fopen(argv[1], "r");