prepend timestamp and custom text before stdout and stderr

run_tests.sh

#!/bin/bash
COMMAND=./test.sh
STDERR=./test.err
STDOUT=./test.out

# prepend timestamp followed by given string before each line of the output
IFS=''  # preserve whitespaces
timestamp(){ while read line; do echo `date "+%Y-%m-%d %H:%M:%S"` $1 $line; done }

{ $COMMAND 2>&3 | timestamp "[$COMMAND]:" > $STDOUT; } \
    3>&1 | timestamp "[$COMMAND] ERROR:" > $STDERR &

test.sh

#!/bin/bash
sleep 1
echo "stderr sample" 1>&2

sleep 1
echo "stdout..."
echo "  ...sample"

sleep 1
echo "    stderr sample 2" 1>&2