Error message output

[David Christensen]

On 2020-09-20 10:11, Polytropon wrote:
> I have a general question. Is it still considered useful to
> output error messages of a script to standard error?
> 
> Example:
> 
> 	if [ something not okay ]; then
> 		echo "the error message" > /dev/stderr
> 		exit 1
> 	fi
> 
> While progress messages will per default go to standard output,
> error messages should be printed to standard error. The reason:
> If a program is silenced to > /dev/null, error messages will
> still be visible (no "silent failing"); if a user wants to
> explicitely mute all messages, > /dev/null 2>&1 has to be
> specified for the redirection. The judgement if a message is
> a regular progress message, an information about some slightly
> problematic case, or a real fatal error depends on the programmer.

I have been migrating my programming style towards a data flow paradigm, 
which includes "command-line filters".  So, an "ideal" command-line 
program or script would:

* Use stdin for the input data.

* Use stdout for the output data.

* Use configuration files, command-line options and arguments, received 
signals and direct tty reads for out-of-band/ non-data input.

* Use stderr, log files, and the exit value for out-of-band/ non-data 
output.


This model doesn't work for all programs, but it is nice when it does.


A mouse and/or graphical environment adds even more possibilities.


> For example:
> 
> 	echo "${FILE] processed, ${RECS} records counted."
> 	 -> standard output

If the above message represents the output data of the program, I would 
send it to stdout -- wc(1), for example.


Otherwise, I would send it to stderr -- dd(1), for example.


In the latter case, the message might be enabled or disabled by a 
configuration file setting and/or command-line option.


> 	echo "${DIR} already checked, skipping."
> 	 -> standard output (non-fatal error"

As above.


> 	echo "${DEV} is read only, aborting."
> 	exit 1
> 	 -> standard error (fatal error)

Yes, but don't you need to redirect echo(1) output to stderr?

     echo "writing to stderr" >&2


In some cases, it could be useful to print a warning to stderr and 
prompt the user to retry; again per configuration settings/ options.


> 	echo "Cannot start: Input filename missing."
> 	usage()
> 	exit 1
> 	 -> standard error (fatal error)

As above.


> At least that's what I've learned centuries ago.
> 
> Is that still valid?

As the author of a program, you decide what is valid.


David