File Descriptor

Core Idea

A file descriptor is a small integer handle a process uses for an open file, pipe, socket, terminal, or device.

Standard File Descriptors

Redirection Examples

command >out.txt
command 2>error.txt
command >combined.txt 2>&1
command 2>/dev/null

> is shorthand for redirecting file descriptor 1. 2> redirects file descriptor 2. 2>&1 makes stderr point where stdout points at that moment.

Why Order Matters

command >combined.txt 2>&1

This sends stdout to combined.txt, then sends stderr to the same destination.

command 2>&1 >out.txt

This first sends stderr to the old stdout, then sends stdout to out.txt. The result is different.

Inspecting Open Descriptors

For your current shell process:

ls -l /proc/$$/fd

$$ is the shell process id.

Common Confusions

• File descriptors are per process.
• A descriptor can refer to something that is not a regular file, such as a pipe or socket.
• 2>&1 duplicates a destination; it does not mean “put descriptor 2 inside descriptor 1”.
• Redirecting stderr to /dev/null hides errors. Do it only when you understand them.

Proof Check

Run a command that produces both normal output and an error, redirect stdout and stderr separately, then inspect both files.

Docs Pointers

• Run man bash, then search for REDIRECTION.
• Run man proc and inspect /proc/self/fd.
• Read I/O, Stream Redirection, Devices And `/dev, and Sockets.