How do I log stderr and stdout synchronously, but print stderr to screen only?

Question

This is a task that I try to do pretty often. I want to log both stderr and stdout to a log file. But I only want to print to console stderr.

I've tried with tee, but once I've merge stderr and stdout using "2>&1". I can not print stdout to the screen anymore since both my pipes are merged.

Here is a simple example of what I tried

./dosomething.sh | tee -a log 2>&1. Now I have both stderr and stdout to the log and the screen.

Any Ideas?

Based on some reading on this web site, this question has been asked. Write STDOUT & STDERR to a logfile, also write STDERR to screen

And also a question very similar here: Save stdout, stderr and stdout+stderr synchronously

But neither of them are able to redirect both stdout+stderr to a log and stderr to the screen while stdoud and stderr are synchronously written to the log file.

@Tomalak The OP tried `tee` with `2>&1`. Was there something he's missing? — chrisaycock, Apr 21 '11 at 13:50
I've edited the question to be a bit more precise. Giving an example and refering the the existing questions on stack overflow — Damien, Apr 21 '11 at 14:18

score 5 · Accepted Answer · answered Apr 21 '11 at 14:19

5

I was able to get this working in bash:

(./tmp.sh 2> >(tee >(cat >&2) >&1)) > tmp.log

This does not work correctly in zsh (the prompt does not wait for the process to exit), and does not work at all in dash. A more portable solution may be to write a simple C program to do it.

answered Apr 21 '11 at 14:19

Random832

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I think `dash` and other simple, POSIX-like shells can't express this at all. +1, seems to work. – Fred Foo Apr 21 '11 at 14:22
When I use this I need to press enter to get back to prompt. Typing a command and then enter runs that command. Any way to avoid this? – neildeadman Jun 28 '11 at 09:44

score 1 · Answer 2 · answered Apr 21 '11 at 14:33

1

I managed to get this working with this script in bash.

mkfifo stdout 
mkfifo stderr 

rm -f out 
cat stderr | tee -a out & 
cat stdout >> out & 
(echo "stdout";  
 grep;  
 echo "an other stdout";  
 echo "again stdout";  
 stat) 2> stderr > stdout 

rm -f stdout 
rm -f stderr

The order of the output is preserved. With this script the process ends correctly.

Note: I used grep and stat without parameter to generate stdout.

answered Apr 21 '11 at 14:33

Lynch

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The accepted solution (from Random832) does get executed in the background. See here the command does not print in the rigth order. $ echo line; ((echo line2; grep) 2> >(tee >(cat >&2) >&1)) > tmp.log; echo line3 line line3 $ Usage: grep [OPTION]... PATTERN [FILE]... Try `grep --help' for more information. – Lynch Apr 23 '11 at 16:29

How do I log stderr and stdout synchronously, but print stderr to screen only?

2 Answers2