Glossary

stderr (standard error)

Stream 2 — the separate channel for errors and diagnostics.

Standard error is stream 2, where programs write error messages and progress notes. It goes to your terminal by default, like stdout, which is why the two appear mixed together — but they are separate channels.

The practical consequences: 2> /dev/null silences errors without touching results; 2>&1 merges errors into wherever stdout is going; and a pipe carries only stdout, so errors must be merged in explicitly (cmd 2>&1 | grep …) if you want to filter them.

See it in practice

$ find / -name "*.conf" 2> /dev/null

Results stay visible while permission-denied noise is discarded. Stream 2 redirected independently of stream 1.

Where you'll meet it