Stderr redirection unexpectedly redirects some BASH built-ins as well

Question

I'm trying to hide all the stderr from users running an interactive BASH script but keep the errors in a log file. However, simple stderr redirection quite unexpectedly hides some BASH output as well that should go to stdout instead. Have tried this on two systems and got the same results (one has GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu) and the other is on MacOS X).

I have a lingering suspicion that it could be caused by the exec replacing the shell... but another built-in (times) works as expected and outputs to stdout!

An example:

#!/bin/bash

exec 2>>file_log

echo This will be printed to stdout, as expected

ls ThereIsNoSuchFileOnEarth # this will go to “file_log”, as expected

read –p 'User would never see this prompt and it would go to file_log. Totally unexpected.' –r -e test

species=”Daleks Raxacoricofallapatorians Judoon”

select enemy in $species;
do
     # …code omitted as the user would never see the list. It would go into file_log again!
done

Answer 1

The prompts from read and select are supposed to go to Standard Error, as they are prompts presumably for user interaction and not actual output. This allows you to run tool.sh > tool.out and still use read and select to collect information from the user without "contaminating" the actual output.

Standard Output is meant to be the typical output of a program, ideally such that it could be piped into Standard Input of another with neither fuss, nor muss.

This is also why curl, for example, presents its download progress on Standard Error rather than Standard Output; you can curl http://www.example.com/path/to/file > file and only the contents of file is displayed (and therefore redirected into file, while informational data are displayed using standard error.

Answer 2

If you use exec to redirect stderr for the whole shell, this affects the whole shell.

You either cannot use exec to redirect the whole stderr, or you need to call something like:

read .... 2> /dev/tty

to have the builtins in question create readable output for stderr.