Learning Linux Shell Scripting - Second Edition

File descriptors

All I/O–including files, pipes, and sockets - are handled by the kernel via a mechanism called the file descriptor. A file descriptor is a small, unsigned integer, which is an index into a file-descriptor table maintained by the kernel, and used by the kernel to reference open files and I/O streams. Each process inherits its own file-descriptor table from its parent. The first three file descriptors are 0, 1, and 2. File descriptor 0 is standard input (stdin), 1 is standard output (stdout), and 2 is standard error (stderr). When you open a file, the next available descriptor is 3, and it will be assigned to the new file.