fd3

fd3 is typically encountered as shorthand for the Unix-like file descriptor number 3. In these systems, every process starts with three standard file descriptors: 0 for standard input, 1 for standard output, and 2 for standard error. Additional file descriptors, beginning with 3, are created when programs open extra files or establish pipes, sockets, or other resources. The number 3 is not special in itself; it is simply the first available descriptor after the standard streams and can be closed, duplicated, or reassigned as needed.

In shell scripting and common programming practice, fd3 is frequently used to hold an extra, independent I/O stream. For example, a script may create a write-only descriptor 3 to a log file with exec 3>log.txt, then write to it with printf 'message' >&3. A script might also read from a descriptor with exec 3< infile or reference an open descriptor via /proc/self/fd/3. Programs in C can create and manipulate descriptors using open, dup, dup2, or fcntl, enabling redirection, piping, or resource management without disturbing the standard streams.

Notes: The use of the specific number 3 is conventional; fd3 itself is not inherently special beyond serving as an extra descriptor. In contexts outside Unix-like systems, or in non-technical domains, fd3 may have different meanings. When encountered, the surrounding domain usually clarifies the intended interpretation.