stderr

I've run programs before that huff and puff and produce ... an error message. If the output is going into some archive, or is input into another program, I usually don't find out about the error until too late, and I think, "Gee. I wish it had beeped or something."

In well-designed programs, errors are announced through a separate channel from normal output.

On Linux, default output goes to "standard output" (stdout), while errors go to "standard error" (stderr). These are two separate channels. Both go to the screen by default, but you can redirect them separately.

Try this. First, snarf-and-barf this script into the file hello:

#!/bin/sh

echo hello, world
ls A_NONEXISTENT_FILE

Run hello and you'll see this:

hello, world
ls: A_NONEXISTENT_FILE: No such file or directory

Next, use the command-line-editing keys to redirect the output into a file. What happens?

hello > OUT

You can capture the errors with the construction "2>"

hello 2> ERR

You can redirect both separately.

hello > OUT 2> ERR

Try this and think about what happens.

hello > OUT 2> OUT

Try redirecting either channel to the bit-bucket, /dev/null. Then make up experiments of your own.

Oh, Happy Thanksgiving!