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

Redirections are taken in order. Try this:

Given a script -

$: cat tst
echo a
sleep 2
echo 1 >&2
echo b
sleep 2
echo 2 >&2
echo c
sleep 2
echo 3 >&2
echo d

I get the following

$: ./tst 2>&1 1>stdout | sed 's/^/echo $(date +%Y%m%dT%H%M%S) /; e'
20180925T084008 1
20180925T084010 2
20180925T084012 3

And as much as I dislike awk, it does avoid the redundant subcalls to date.

$: ./tst 2>&1 1>stdout | awk "{ print strftime(\"%Y%m%dT%H%M%S \") \$0; fflush() }"; >stderr

$: cat stderr
20180925T084414 1
20180925T084416 2
20180925T084418 3

$: cat stdout
a
b
c
d