Date: Sun, 20 Sep 2020 22:12:24 -0700 From: David Christensen <dpchrist@holgerdanske.com> To: freebsd-questions@freebsd.org Subject: Re: Error message output Message-ID: <528b2c90-18c4-9e95-a150-67344154c66c@holgerdanske.com> In-Reply-To: <20200920191108.22864e5c.freebsd@edvax.de> References: <20200920191108.22864e5c.freebsd@edvax.de>
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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
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