fd1

fd1 is a shorthand used in computing to denote the first file descriptor, which on most Unix-like systems corresponds to standard output.

File descriptors are non-negative integers that index into a per-process table of open files, pipes, and other I/O resources. By convention, 0 is standard input, 1 is standard output, and 2 is standard error. The descriptor 1, or fd1, is what a process uses when it writes to the terminal or to any destination if stdout is redirected.

In shells and programming, redirection operators manipulate fd1. For example, in a POSIX shell, command > file redirects stdout to file. You can also duplicate or reassign stdout using additional descriptors, such as exec 3>&1 to save a copy of stdout to fd3, and later exec 1>&3 to restore it.

In software design, fd1 is an implementation detail for low-level I/O. Many languages expose higher-level streams or file objects instead of raw descriptors, and not all platforms use 0, 1, and 2 in the same way. Windows uses a different handle model, though analogous concepts exist.

See also: file descriptor, standard streams, I/O redirection.