Shell Programming and Scripting

By tradition most processes are started with three open files: Standard input at FD 0, standard output at FD 1, and standard error at FD 2. When you run a process in a shell without redirecting anything, stdin reads from the keyboard, stdout writes to the terminal screen, and stderr also writes to the terminal screen.

echo "hello world" prints "hello world" to standard output.

For error messages, we want to print to standard error, so that the error messages will show up even if standard out has been redirected, and so that error messages don't get shoved where they don't belong, like in the middle of a big flat file you're transforming or something.

So we print to stderr, by redirecting FD 1 into FD 2.

Thank you Corona688. Can you please tell in what instances we should go for this re-direction.? Im still uncertain on this because though we re-direct FD 1 into FD 2 still the output is going to be displayed in the terminal window.

When printing error messages. Yes, it does. Do you want error messages ending up anywhere but the console? Usually not. You don't want them in whatever data you're saving from the process, which it will end up in if you just print it to stdout...