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The warnings that M. Siebennmann hit (since this has come up again this year) were going to standard error.

If you write to standard error in a cron job, mail gets sent to root. All of the cron jobs using fgrep and egrep were suddenly mailing root, every single run, even when nothing went wrong.

Yea, this seems like a good place to use stderr. Many ci/cd systems as well as subprocess libraries will concatenate stdout and stderr, but it is better than printing the warnings to stdout.

Maybe FD 3 should be stdwarning.

Yeah, a third output would be nice to have. For example, grep could use stdout to print the actual matches it finds, stderr for any files it was unable to open, and a third stream to show the progress of the search.

Output to Stderr can also break things, particularly in dot files or from applications that tend to output meaningful information to stderr.

Yep. Unix' "everything is a stream of bytes" hoist by its own petard, again.