How to add timestamp while redirecting stdout to file in Bash?

Question

I recently needed exactly that: receive log messages in a serial console (picocom), print them to a terminal and to a file AND prepend the date.

What I now use looks s.th. like this:

picocom -b 115200 /dev/tty.usbserial-1a122C | awk '{ print strftime("%s: "), $0; fflush(); }' | tee serial.txt

the output of picocom is piped to awk
awk prepends the date (the %s option converts the time to the Number of seconds since 1970-01-01 00:00:00 UTC – or use %c for a human-readable format)
fflush() flushes any buffered output in awk
that is piped to tee which diverts it to a file. (you can find some stuff about tee here)