timestamp_trick.sh

# FIRST VERSION - Outputs UTS formatted timestamp.

# -------------------------------------------------------------------------------------
# handy pipe redirect that appends a datestamp to every line of output. Just paste this into
# the top of a typical BASH script.

s_datestamp() {
    while read -r line; do
        timestamp=$(date -u '+%Y-%m-%d %H:%M:%S')

        # by nature BASH might run process subst twice when using >&2 pipes. This is a lazy
        # way to avoid dumping two timestamps on the same line:
        if [[ "$line" != \[${timestamp%% *}* ]]; then
            echo "[$timestamp] $line"
        else
            echo "$line"
        fi
    done
}

exec 1> >(s_datestamp)
exec 2> >(s_datestamp)
# -------------------------------------------------------------------------------------

# SECOND VERSION - Outputs time since the script started.

# -------------------------------------------------------------------------------------
# handy pipe redirect that appends a datestamp to every line of output. Just paste this into
# the top of a typical BASH script.

procstarttime=$(date -u '+%s')

s_timestamp() {
    while read -r line; do
        curtime=$(date -u '+%s')
        deltatime=$(( curtime - procstarttime ))
        timestamp="$(printf "%03d:%02d" $(( deltatime / 60 )) $(( deltatime % 60 )))"

        # by nature BASH might run process subst twice when using >&2 pipes. This is a lazy
        # way to avoid dumping two timestamps on the same line:
        if [[ "$line" != \[${timestamp%% *}* ]]; then
            echo "[$timestamp] $line"
        else
            echo "$line"
        fi
    done
}

exec 1> >(s_timestamp)
exec 2> >(s_timestamp)
# -------------------------------------------------------------------------------------

Hi, for line 9, the timestamp has a shorter option for iso with seconds:
date -Is

Also recommend using while IFS= read -r line so that prepended whitespace isn't removed.

Hi
(echo 1 && sleep 3 && echo 2) | while IFS= read -r line; do echo "[$(date -Is)] $line"; done