Использовать stat для проверки файла на существование.

#include <stdio.h>
#include <unistd.h>
//unistd.h is needed for access() function
#include <sys/stat.h>
#include <sys/types.h>
#include <time.h>
#include <string.h> 
struct stat b;
int help();
int main(int argc, char **argv)
{
	if(argc==3) if(!strcmp(argv[1],"-n")||!strcmp(argv[1],"--number"))
		{
		if(access(argv[2],0)==-1){
		printf("File %s does not exist or access to the directory, where file is located is denied.\n",argv[2]);
		return 1;
		}
		else if(access(argv[2],4)==-1){
		stat(argv[2],&b);
		printf("You don't have rights to read this file.\n"
		"Rights for this file:\n"
		"Access mode: %o\n"
		"Ownership: UID: %d GID: %d\n"
		"Last access time: %s\nLast status change time: %s\nLast change time: %s\n",b.st_mode,b.st_uid,b.st_gid,ctime(&b.st_atime),ctime(&b.st_mtime),ctime(&b.st_ctime));
		return 1;
		}
		else {
			stat(argv[2], &b);
			if(S_ISDIR(b.st_mode)){
				printf("File %s is a directory.\n",argv[2]);
				return 1;
			}
		}
		FILE *f=fopen(argv[2],"r");
		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==1||!strcmp(argv[1],"-h")||!strcmp(argv[1],"--help")) help();
	else{
		if(!strcmp(argv[1],"-v")||(!strcmp(argv[1],"--version"))){
		printf("cloncat version 0.3\n");
		return 0;
	}
	else{
	if(access(argv[1],0)==-1){
		printf("File %s does not exist or access to the directory, where file is located is denied.\n",argv[1]);
		return 1;
	}
	else if(access(argv[1],4)==-1){
		stat(argv[1],&b);
		printf("You don't have rights to read this file.\n"
		"Rights for this file:\n"
		"Access mode: %o\n"
		"Ownership: UID: %d GID: %d\n"
		"Last access time: %s\nLast status change time: %s\nLast change time: %s\n",b.st_mode,b.st_uid,b.st_gid,ctime(&b.st_atime),ctime(&b.st_mtime),ctime(&b.st_ctime));
		return 1;
	}
	else {
		stat(argv[1], &b);
		if(S_ISDIR(b.st_mode)){
			printf("File %s is a directory.\n",argv[1]);
			return 1;
		}
	}
		FILE *f=fopen(argv[1],"r");
		while(feof(f)==0){
			int a;
			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 putchar(a);
				fseek(f,pos,SEEK_SET);
			}
			else putchar(a);
		}
	}
}
}
int help(){
		printf("usage: cloncat filename\n"
		"-h or --help for help\n"
		"-v or --version displays version and exit\n");
		return 0;
	}

FILE *f=fopen(argv[1],"r"); 
feof(f)==0//если файла нет, то f==NULL..и 3,14..

struct stat s;
int         fd;

fd = open("data/xyz.foo", O_RDONLY);
if (-1 == fd) {
    perror("open");
    exit(1);
    /* Или если очень хочется… */
    switch (errno) {
    case EACCES :
        fputs("You don't have rights to read this file\n", stderr);
        break;
    case ENOENT :
        fputs("File does not exist\n", stderr);
        break;
    }
    exit(1);
}

fstat(fd, &s);
if (s.st_mode & S_IFDIR) {
    fputs("File is a directory\n");
    exit(1);
}

int
main(int argc, char **argv)
{
        struct stat st;
        if(argc > 1) {
                if (stat(argv[1], &st) == -1) {
                        printf("%s",strerror(errno));// или другая оработка.
                        return 1;
                }
        //здесь ваш код полезный код
        } else {
                printf("не хватило параметров!!");// или другая оработка.
                return 1;
        }
        
        return 0;
}

for (int i = 0; i < argc; ++i)
{
   ....
   } else if (!strcmp(argv[i], "--option1") { 
      ....
   } else if (!strcmp(argv[i], "--option_with_arg") { 
      if (i == argc - 1) {
         usage();
      }
      // читаем argv[i + 1];
      ...
   }
   ...
}

fstat(fileno(f), &s)

a(){
int b;
while(b<20)++b;
}

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int 
main(int argc, char **argv)
{
        char buf;
        int fd;
        
        fd = open(argv[1],O_RDONLY);//можно попробовать с O_WRONLY, O_RDWR
        
        if (fd == -1) {
                switch (errno) {
                case EISDIR:
                       fprintf(stderr, "%s - ссылается  на  каталог,"
                               " а тип доступа подразумевает запись\n",
                               argv[1]);
                       break;
                default:
                       fprintf(stderr, "не удалось олткрыть файл  - %s\n",
                               strerror(errno));
                }               
                return 1;
        }
        //тут может быть ваш цикл
        if (read(fd, &buf, 1) == -1) {     
                switch (errno) {
                case EISDIR:
                        fprintf(stderr, "%s - ссылается  на  каталог,"
                               " а  тип доступа подразумевает чтение\n",
                               argv[1]);
                        break;
                default:
                        fprintf(stderr, "не удалось прочитать - %s\n",
                                strerror(errno));
                }
                
                return 1;
        }
        
        printf("%c\n", buf);
        return 0;
}

#include <stdio.h>
#include <unistd.h>
//unistd.h is needed for access() function
#include <sys/stat.h>
#include <sys/types.h>
#include <time.h>
#include <string.h> 
int help();
int main(int argc, char **argv){
	if(argc==3) if(!strcmp(argv[1],"-n")||!strcmp(argv[1],"--number")){
		if(access(argv[2],0)==-1){
		printf("File %s does not exist or access to the directory, where file is located is denied.\n",argv[2]);
		return 1;
		}
		else if(access(argv[2],4)==-1){
			struct stat b;
			stat(argv[2],&b);
			printf("You don't have rights to read this file.\n"
			"Rights for this file:\n"
			"Access mode: %o\n"
			"Ownership: UID: %d GID: %d\n"
			"Last access time: %sLast status change time: %sLast change time: %s",b.st_mode,b.st_uid,b.st_gid,ctime(&b.st_atime),ctime(&b.st_mtime),ctime(&b.st_ctime));
			return 1;
			}
		else {
			struct stat b;
			stat(argv[2], &b);
			if(S_ISDIR(b.st_mode)){
				printf("File %s is a directory.\n",argv[2]);
				return 1;
			}
		}
		FILE *f=fopen(argv[2],"r");
		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==1||!strcmp(argv[1],"-h")||!strcmp(argv[1],"--help")) help();
	else{
		if(!strcmp(argv[1],"-v")||(!strcmp(argv[1],"--version"))){
		printf("cloncat version 0.3\n");
		return 0;
	}
	else{
	if(access(argv[1],0)==-1){
		printf("File %s does not exist or access to the directory, where file is located is denied.\n",argv[1]);
		return 1;
	}
	else if(access(argv[1],4)==-1){
		struct stat b;
		stat(argv[1],&b);
		printf("You don't have rights to read this file.\n"
		"Rights for this file:\n"
		"Access mode: %o\n"
		"Ownership: UID: %d GID: %d\n"
		"Last access time: %sLast status change time: %sLast change time: %s",b.st_mode,b.st_uid,b.st_gid,ctime(&b.st_atime),ctime(&b.st_mtime),ctime(&b.st_ctime));
		return 1;
	}
	else {
		struct stat b;
		stat(argv[1], &b);
		if(S_ISDIR(b.st_mode)){
		printf("File %s is a directory.\n",argv[1]);
		return 1;
		}
	}
		FILE *f=fopen(argv[1],"r");
		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 filename\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;
	}

void
function_name(const char *name)
{
     printf("main name is %s\n",name);
}
int 
main(int argc, char **argv)
{
        function_name(argv[0]);
        return 0;
}

void fchk(const char *fn){ 
	if(access(fn,0)==-1){
		printf("File %s does not exist or access to the directory, where file is located is denied.\n",fn);
		return 1;
		}
		else if(access(fn,4)==-1){
			struct stat b;
			stat(fn,&b);
			printf("You don't have rights to read this file.\n"
			"Rights for this file:\n"
			"Access mode: %o\n"
			"Ownership: UID: %d GID: %d\n"
			"Last access time: %sLast status change time: %sLast change time: %s",b.st_mode,b.st_uid,b.st_gid,ctime(&b.st_atime),ctime(&b.st_mtime),ctime(&b.st_ctime));
			return 1;
			}
		else {
			struct stat b;
			stat(fn, &b);
			if(S_ISDIR(b.st_mode)){
				printf("File %s is a directory.\n",fn);
				return 1;
			}
	}
}