IO Redirection and Pipes

Part of the Bash course

Every command you run in Bash is, underneath, hooked up to three separate channels, whether you ever notice them or not. Think of a mail sorting machine: letters go in through one slot, successfully sorted mail comes out one tray, and damaged or undeliverable mail gets kicked out into a separate tray so it does not get mixed in with the good mail.

The three channels have names. stdin ("standard in") is where a command reads its input from, by default your keyboard. stdout ("standard out") is where normal output goes, by default your screen. stderr ("standard error") is a separate channel just for error messages, also shown on your screen by default, which is why the two normally look indistinguishable until you start redirecting them. ...