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

Question

I have a program (server) and I am looking for a way (script) that will redirect (or better duplicate) all its stdout to file and add timestamp for each entry.

I've done some research and the furthest I could get was thanks to How to add timestamp to STDERR redirection. It redirects stdout but the timestamp added is of the time when the script finishes:

#!/bin/bash
./server | ./predate.sh > log.txt

code of predate.sh:

#!/bin/bash
while read line ; do
    echo "$(date): ${line}"
done

It seems that server output is flushed after exit of the program.(without redirecting it works fine). Also if I try using predate.sh on given example in mentioned thread, it works perfectly. I am aware it would be easy adding a timestamp to the main program but I would rather avoid editing its code.

Answer 1

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)