Why Redirection Order Matters: Mastering stdout and stderr in Shell

In Unix‑like systems, I/O redirection lets you change where a command’s output goes, and the shell works with three standard streams: stdout (standard output) – file descriptor 1 stderr (standard error) – file descriptor 2 stdin (standard input) – file descriptor 0

Analysis of the first command

nc -zv 10.0.0.1 443 2>&1 >> /dev/null

The fragment 2>&1 redirects standard error to the current location of standard output, but this happens before the subsequent > /dev/null redirection. At that moment stdout is still pointing to the terminal, so both stderr and stdout are printed on the screen.

Analysis of the second command

nc -zv 10.0.0.1 443 > /dev/null 2>&1

Here > /dev/null is applied first, sending stdout to the null device. Then 2>&1 redirects stderr to whatever stdout now points to—also /dev/null. Consequently no output appears on the terminal.

Conclusion

Redirection order is decisive. To discard all output, first redirect stdout to /dev/null, then redirect stderr to stdout. Proper use of I/O redirection makes shell scripts more robust, easier to debug, and prevents unwanted output from polluting logs or user interfaces.