Add timestamp to cat output

$ awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0; }' /dev/ttyUSB0 

$ awk '{ print strftime("%H:%M:%S"), $0; }' /dev/ttyUSB0 

$ awk '{ cmd = "date +\"%a, %b %d %Y %T.%6N\""; cmd | getline dt; close(cmd); print dt, $0; fflush(); }' /dev/ttyACM0 

save to file

$ stdbuf -oL awk '{ print strftime("%H:%M:%S"), $0; }' /dev/ttyUSB0 > file_name 

or

$ stdbuf -oL awk '{ print strftime("%H:%M:%S"), $0; }' /dev/ttyUSB0 | tee file_name

會保留舊紀錄 ( -a )

$ stdbuf -oL -eL ./at_command_tool_x86 2>&1 | awk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0; fflush() }' | tee -a output.log

ref :
Add timestamp to cat output from shell script