How to add timestamp to STDERR redirection

Question

In bash/ksh can we add timestamp to STDERR redirection?

E.g. myscript.sh 2> error.log

I want to get a timestamp written on the log too.

Answer 1

How about timestamping the remaining output, redirecting all to stdout?

This answer combines some techniques from above, as well as from unix stackexchange here and here. bash >= 4.2 is assumed, but other advanced shells may work. For < 4.2, replace printf with a (slower) call to date.

: ${TIMESTAMP_FORMAT:="%F %T"} # override via environment
_loglines() { 
    while IFS= read -r _line ; do 
      printf "%(${TIMESTAMP_FORMAT})T#%s\n" '-1' "$_line";
    done;
}
exec 7<&2 6<&1
exec &> >( _loglines )
# Logit

To restore stdout/stderr:

exec 1>&6 2>&7

You can then use tee to send the timestamps to stdout and a logfile.

_tmpfile=$(mktemp)
exec &> >( _loglines | tee $_tmpfile )

Not a bad idea to have cleanup code if the process exited without error:

trap "_cleanup \$?" 0 SIGHUP SIGINT SIGABRT SIGBUS SIGQUIT SIGTRAP SIGUSR1 SIGUSR2 SIGTERM
_cleanup() { 
    exec >&6 2>&7
    [[ "$1" != 0 ]] && cat "$_logtemp"
    rm -f "$_logtemp"
    exit "${1:-0}"
}