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

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)

moreutils ts

Absolute date and time is the default:

$ sudo apt-get install moreutils$ (echo a;sleep 1;echo b;sleep 3;echo c;sleep 2;echo d;sleep 1) | ts | tee myfile$ cat myfileApr 13 03:10:44 aApr 13 03:10:45 bApr 13 03:10:48 cApr 13 03:10:50 d

or counting from program start with ts -s:

$ (echo a; sleep 1; echo b; sleep 3; echo c; sleep 2; echo d; sleep 1) | ts -s00:00:00 a00:00:01 b00:00:04 c00:00:06 d    

or deltas for benchmarking with ts -i:

$ (echo a; sleep 1; echo b; sleep 3; echo c; sleep 2; echo d; sleep 1) | ts -i00:00:00 a00:00:01 b00:00:03 c00:00:02 d$ (echo a; sleep 1; echo b; sleep 3; echo c; sleep 2; echo d; sleep 1) | ts -i '%.s'0.000010 a0.983308 b3.001129 c2.001120 d

See also: How to monitor for how much time each line of stdout was the last output line in Bash for benchmarking?

Tested on Ubuntu 18.04, moreutils 0.60.

For Me Your Code is working perfectly fine

Check this is what I tried

test.sh

#!/bin/bashwhile true; do  echo "hello"done

predate.sh

#!/bin/bashwhile read line; do  echo $(date) ":" $line;    done

then

./test.sh  | ./predate.sh

gives me

Tue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : helloTue Jan 14 17:49:47 IST 2014 : hello

This can be redirected to some file using ">" or ">>" for append