dup, dup2 - duplicate file descriptor

Syntax	#include <unistd.h> int dup(int fildes); int dup2(int fildes, int fildes2);
Description	fildes is a file descriptor obtained from a creat() , open(), dup(), fcntl, or pipe() system call. dup() returns a new file descriptor having the following in common with the original file descriptor: the same open file or pipe the same file position indicator the same access mode (read, write or read/write) fildes2 is a non-negative integer that is less than {OPEN-MAX}. dup2 causes fildes2 to point to the same file as fildes. If fildes2 already points to an open file other than fildes, the open file is first closed; however, if fildes2 points to fildes or if fildes is not a valid file descriptor, fildes will not be first closed. The dup() and dup2() functions provide an alternative interface to the service provided by fcntl() using the F_DUPFD command. The call: fid = dup (fildes); is equivalent to: fid = fcntl (fildes, F_DUPFD, 0); The call fid = dup2 (fildes, fildes2); is equivalent to: close (fildes2); fid = fcntl (fildes, F_DUPFD, fildes2); except for the following: If fildes is a valid file descriptor and is equal to fildes2, dup2() returns fildes2 without closing it.
Return val.	Non-negative integer (the file descriptor)
		if successful.
	-1	if an error occurs; errno is set to indicate the error.

Errors	dup() and dup2() will fail if:
	EBADF	fildes is not a valid open file descriptor or the argument fildes2 is negative or greater than or equal to {OPEN_MAX}.
	EINTR Extension EINVAL EMFILE	dup2() was interrupted by a signal. fildes and fildes2 designate BS2000 files. (End) The number of file descriptors in use by the process would exceed {OPEN_MAX}, or no fildes2 file descriptors are available.
Notes See also	dup() and dup2() are executed only for POSIX files. close(), fcntl(), open(), unistd.h.