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
A tool exists for that exact purpose, it’s ts (see man ts)
For example, with your logfile:
tail -f logfile | ts '%Y-%m-%d %H:%M:%S'
Also works with any software writing on the standard output, of course:
./blabla | ts '%Y-%m-%d %H:%M:%S'
If needed, you can add sub-second precision, with %.S instead of %S
I've seen elsewhere a suggestion, if timeliness is important, that one should avoid calling other programs from BASH and use a built-in:
printf '%(%F %T)T\n'
%T is the same format strings from date (see man date), AFAICT.
Example:
output="Err, Miss Crabapple, I just ate my worm!"; \
printf '\n%(%F %s)T'; printf "%s \n" " $output"
will output:
2021-03-28 1616894012 Err, Miss Crabapple, I just ate my worm!
I've just noticed a comment on that link has a suggestion to use gawk for timestamp top speeds.