Использовать stat для проверки файла на существование.

if (f == NULL) { 
                switch (errno) { 
                case EISDIR: 
                    fprintf(stderr, "File %s is directory.\n",argv[1]); 
                    break; 
                default: 
                    fprintf(stderr, "cannot open file- %s\n", 
                               strerror(errno)); 
                }                
                return 1; 
        } 

/*     cloncat.c
  
Copyright 2010 Alexandr Dorif <dorif11@gmail.com>
 
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
  
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
   
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA.*/
#include <stdio.h>
#include <errno.h>
#include <time.h>
#include <string.h> 
int help();
int fchk(const char *fn);
int main(int argc, char **argv){
	if(argc==3) if(!strcmp(argv[1],"-n")||!strcmp(argv[1],"--number")){
		FILE *f=fopen(argv[2],"r");
		if(f==NULL)fchk(argv[2]);
		int a,i;
		i=1;
		printf("1:");
		while(feof(f)==0){
			a=fgetc(f);
			if(a=='\n'){
			long pos;
			pos=ftell(f);
			fseek(f,1,SEEK_CUR);
			if(fgetc(f)==EOF){
				putchar(a);
				fclose(f);
				return 0;
				}
			else printf("\n%d: ",++i);
			fseek(f,pos,SEEK_SET);
			}
		else putchar(a);
		}
	}
		else help();
	else if(argc>3||argc==1||!strcmp(argv[1],"-h")||!strcmp(argv[1],"--help")) help();
	else{
		if(!strcmp(argv[1],"-v")||(!strcmp(argv[1],"--version"))){
		printf("cloncat version 0.4\n");
		return 0;
	}
	else{
		FILE *f=fopen(argv[1],"r");
		if (f==NULL)fchk(argv[1]);
		long pos;
		fseek(f,0,SEEK_END);
		pos=ftell(f)-1;
		fseek(f,0,SEEK_SET);
		for(int i;i<=pos;++i)putchar(fgetc(f));
		fclose(f);
		return 0;
	}
}
}
int help(){
		printf("usage: cloncat filefn\n"
		"-h or --help for help\n"
		"-v or --version displays version and exit\n"
		"-n or --number number the output lines\n");
		return 0;
	}
int fchk(const char *fn){  
                switch(errno) {  
					case EISDIR:  
						fprintf(stderr, "File %s is directory.\n",fn);  
						break;
                    case ENOENT:
						fprintf(stderr, "File %s does not exist.\n",fn);
						break;
					case EACCES:
						fprintf(stderr, "Access to file %s denied.\n",fn);
						break;
					default:  
                    fprintf(stderr, "cannot open file- %s\n",strerror(errno));
                    break;
                }                 
                return 1;
			}