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I want cron to only mail stderr output, while simultaneously log stdout+stderr, in correct order, to a file.

I have been fiddling with tee and the likes, but not quite got it right...

It's ok to wrap the command into a shell script

Like

#!/bin/bash
mycommand | maybe_some_shell_redirect_magic >> /var/log/mycommand.log
  • It's difficult to understand what do you want exactly. Please, clarify the question. – Bob Aug 21 '18 at 14:42
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I think it's possible - while keeping line ordering (ie. stderr is copied from within the process where it is written to), even with a POSIX shell. Try :

#!/bin/sh

exec 3>log
command args ... 2>&1 >&3 |while read err; do
  echo "$err" >&2
  echo "$err" >&3
done
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It's not possible to keep the order accurately while separating stdout and stderr. You can get reasonable results with this as long as the timing of stdout and stderr outputs is not too close:

mycommand 2>>/var/log/mycommand.log | tee -a /var/log/mycommand.log

You can test it with this script in place of mycommand:

#!/bin/bash
echo stdout 1
sleep 0.1
echo stderr 1 >&2
sleep 0.1
echo stdout 2
sleep 0.1
echo stderr 2 >&2

If you remove the sleep commands, the order in the log file will be somewhat messed up though.

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