Why a Bash script still outputs to stdout even I redirect it to stderr?

Question

I have a Bash script:

#!/bin/bash
echo this should be visible only in the stderr output but it is not 1>&2
exit 0

Also, I have a custom app that starts this script. The app captures both - stdout and stderr. The stdout is empty and the stderr contains the text provided. So far, so good. But when I start the script from the console (physical terminal):

./my_script.sh

I expect it to not print anything on the display, but it prints. I tried various combinations of redirecting, e.g., >&2, >/dev/stderr, etc. but results are the same. I also tried to redirect stdout to null 1>&2 >/dev/null, but obviously it also sends my text to the null.

Where is this configured? How can I make my terminal to not display stderr?

Answer 1

To stop the script from sending its standard error stream to the terminal, you need to redirect it to /dev/null or a file.

You do this by invoking the script with the standard error stream redirected on the command line or letting the script itself redirect its error stream.

To redirect the error stream to /dev/null on the command line, invoke the script using

./my_script 2>/dev/null

If you want the script to discard its standard error stream automatically, then redirect the stream to /dev/null within the script:

#!/bin/bash

# Discard any diagnostic output, errors, and interactive prompts.
exec 2>/dev/null

echo 'This should not be visible.' >&2
echo 'This should be visible.'

Change /dev/null to the pathname of a file to pass the diagnostic output to that file instead.

The standard output stream is not affected by the redirection of the standard error stream to /dev/null or to a file.

Note that a script should not ordinarily be terminated by exit 0.

Answer 2

To cause this one specific script to discard its standard output:

Given a script:

date
date -r /var/empty/foo

edit it to become a group command. Grouping the commands together in this way permits the standard output and standard error streams of the combined commands to be handled (redirected, piped, discarded, etc.) as though the group was logically a single command.

For example:

{
date
date -r /var/empty/foo
} > /dev/null

will discard the standard output but not alter the routing of the standard error stream.

The following script:

{
date
date -r /var/empty/foo
} > /dev/null 2>&1

will redirect its standard output to /dev/null, and then will redirect its standard error to its standard output, thus discarding both stderr and stdout. That particular syntax is equivalent to:

{
date
date -r /var/empty/foo
} > /dev/null 2>/dev/null