Answer by Sandeep_black for What are file descriptors, explained in simple terms?

More points regarding File Descriptor:

1. File Descriptors (FD) are non-negative integers (0, 1, 2, ...) that are associated with files that are opened.

2. 0, 1, 2 are standard FD's that corresponds to STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO (defined in unistd.h) opened by default on behalf of shell when the program starts.

3. FD's are allocated in the sequential order, meaning the lowest possible unallocated integer value.

4. FD's for a particular process can be seen in /proc/$pid/fd (on Unix based systems).