What is a file descriptor?

In a nutshell

In Unix-like operating systems, a file descriptor is an integer that uniquely identifies an open file or stream (e.g., socket). It’s generated when a file is created or opened and is used by the calling process to refer to the file in subsequent calls.

Reserved file descriptors

View file descriptors used by a process

Run

lsof -p <process-id>

If your process uses stdin, stdout, or stderr, you should see an entry with 0, 1, or 2 as the FD (file descriptor) value.