How to add date string to each line of a continuously written log file

Question

Having a long running program that continuously writes to a logfile - how is it possible, disregarding any buffering issues, to add a date string to each line written to that file using a linux script?

I would imagine something like this:

tail -f logfile | ADD_DATE_TO_EACH_LINE > logfile2

The input would be something like that:

abc
def
ghi
jkl

The output should be similar to that:

2011-06-16 18:30:59 abc
2011-06-16 18:31:00 def
2011-06-16 18:35:21 ghi
2011-06-16 18:40:15 jkl

Answer 1

With perl:

command 2>&1 | perl -pe 'print scalar(localtime()), " ";'

With gawk:

command 2>&1 | awk '{ print strftime(), $0; fflush() }'

Replace command with tail -f logfile for your specific example. Or, perhaps you could just redirect the original program's stdout/stderr to the above pipe.