从C程序中的linux命令获取返回码

Question

I am basically trying to check if a particular file exists or not. For that I am using the test command of Unix.

 sprintf(execbuf, "%s if test -r %s ; then true; else exit; fi;",
         execbuf, st->file, NO_FILE);

It works fine, but I do not want to exit if the file is not here, rather it should return FAIL.

I am not able to figure out how to make the program return FAIL. I was thinking of using the exit code from the above command, but still I am not able to figure out how to use that exit code outside the Linux command in the program.

Answer 1

I'd recommend you rather just use the access () call, and not execute external shell commands to figure this out.

Just be aware that such cases are subject to race conditions - the file might exist when you call access() (or execute a shell command that determines whether the file exists), but it might be gone when you actually need it later on. If that's a problem for you, just open() the file, and use the file descriptor later on when you actually need it for I/O.

Answer 2

If you're not married to what your doing right now, then I'd suggest using stat:

#include <sys/stat.h>
#include <stdio.h>

int main (int argc, char** argv[])
{
  struct stat sts;
  if (stat(argv[1], &sts) == -1 && errno == ENOENT)
      printf ("The file %s doesn't exist...\n", argv [1]);
  else
      printf("The file exists\n");

This will tell you if it exists or not. If you dont' want to pass it command line, parameter 1 is a const char*, so just pass it the file name.