How to redirect stdout to a file, and stdout+stderr to another one?

Problem is that when you redirect your output, it's not available anymore for the next redirect. You can pipe to tee in a subshell to keep the output for the second redirection:

( cmd | tee -a file2 ) >> file1 2>&1

or if you like to see the output in terminal:

( cmd | tee -a file2 ) 2>&1 | tee -a file1

---

To avoid adding the stderr of the first tee to file1, you should redirect the stderr of your command to some file descriptor (e.g. 3), and later add this to stdout again:

( 2>&3 cmd | tee -a file2 ) >> file1 3>&1
# or
( 2>&3 cmd | tee -a file2 ) 3>&1 | tee -a file1

(thanks @fra-san)

With zsh:

cmd >& out+err.log > out.log

In append mode:

cmd >>& out+err.log >> out.log

In zsh, and provided the mult_ios option has not been disabled, when a file descriptor (here 1) is redirected several times for writing, then the shell implements a built-in tee to duplicate the output to all targets.

You could : tag stdout (using an UNBUFFERED sed, ie: sed -u ...), have stderr also go to stdout (untagged, as it didn't go through that tagging sed), and thus be able to differentiate the 2 in the resulting logfile.

The following: is slow (It can be seriously optimized, by using for exemple a perl script instead of the while ... ; do ... ;done, for exemple, that will spawning subshells & commands at every lines!), weird (it seems I need the 2 {} stages to in one rename stdout and then in the other one add the "falled through" stderr to it), etc. But it is : a "proof of concept", that will try to keep the output's order the most of stdout & stderr as much as possible:

#basic principle (some un-necessary "{}" to visually help see the layers):
# { { complex command ;} | sed -e "s/^/TAGstdout/" ;} 2>&1 | read_stdin_and_redispatch

#exemple:
# complex command = a (slowed) ls of several things (some existing, others not)
#  to see if the order of stdout&stderr is kept

#preparation, not needed for the "proof of concept", but needed for our specific exemple setup:
\rm out.file out_AND_err.file unknown unknown2 
touch existing existing2 existing3

#and the (slow, too many execs, etc) "proof of concept":
uniquetag="_stdout_" # change this to something unique, that will NOT appear in all the commands outputs... 
                     # avoid regexp characters ("+" "?" "*" etc) to make it easy to remove with another sed later on.

{
   { for f in existing unknown existing2 unknown2 existing3 ; do ls -l "$f" ; sleep 1; done ;
   } | sed -u -e "s/^/${uniquetag}/" ;
} 2>&1 | while IFS="" read -r line ; do
    case "$line" in
       ${uniquetag}*) printf "%s\n" "$line" | tee -a out_AND_err.file | sed -e "s/^${uniquetag}//" >> out.file ;; 
        *)            printf "%s\n" "$line"       >> out_AND_err.file ;;   
    esac; 
done;

# see the results:
grep "^" out.file out_AND_err.file