Question

I have to a do C program that uses the unix environment. I have already purchased the “Advancing Programming in the Unix Environment” book and it has helped out a lot so far. However, some of my questions have gone unanswered and I’m looking for some help.

I’m trying to write a program that can verify if the first and second arguments entered if a copy program exist. If the first argument does not exist, then an error message and exit must occur. If the second argument does exist, then an overwrite prompt must be displayed. I’m not exactly sure how to verify if a file already exists or not basically.

I have seen a few people saying that you can do (!-e) or something like that to verify the file existing/not existing.

If anyone could help me, I’d really appreciate it.

Answer 1

The access() function is designed to tell you if a file exists (or is readable, writeable or executable).

#include <unistd.h>
int result;
const char *filename = "/tmp/myfile";
result = access (filename, F_OK); // F_OK tests existence also (R_OK,W_OK,X_OK).
                                  //            for readable, writeable, executable
if ( result == 0 )
{
   printf("%s exists!!\n",filename);
}
else
{
   printf("ERROR: %s doesn't exist!\n",filename);
}