standard error
also: stderr, file descriptor 2, FD 2
Standard error (stderr) is the default output stream where a program sends error messages and diagnostic output, separate from normal output (stdout).
Standard error is one of three standard I/O streams available to every Linux process. While stdout carries normal program output, stderr is reserved for error messages, warnings, and diagnostic information. This separation allows users and scripts to handle regular output differently from errors.
In the shell, stderr is referred to as file descriptor 2, while stdout is file descriptor 1. You can redirect stderr independently using 2>, for example: command 2> errors.log sends only errors to a file while stdout goes to the terminal.
A common example: ls /nonexistent 2> /dev/null runs the command but discards error messages. You can also combine both streams with command > output.log 2>&1, sending both stdout and stderr to the same file.