How to append a timestamp to each line as it comes out of grep?

Question

I have an infinite stream of data coming out of a logger, which I am piping to grep. I would like to save the output of the grep to a file, but also include a timestamp at the beginning of each line (the time at which the line appeared). Is there an easy way to accomplish this? Assume I cannot change the output of the logger process.

Answer 1

You can append a static timestamp using sed and date:

... | sed "s/^/$(date) /" >> output.txt

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Alternatively, if you require a realtime timestamp, use gawk's strftime function:

... | gawk '{ print strftime(), $0 }'

You can define your favourite formatting:

... | gawk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0 }'

And if buffering is a problem, don't forget to flush each line:

... | awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0; fflush() }'

Alternatively, use unbuffer:

unbuffer ... | awk '{ print strftime("%Y-%m-%d %H:%M:%S"), $0 }'

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If you don't have gawk, you have a couple of other options:

(a) Install ts (from moreutils):

... | ts '%F %T'

(b) Use perl:

... | perl -pe 's/^/localtime . " "/e'

or with formatting:

... | perl -MPOSIX -pe 's/^/strftime("%Y-%m-%d %H:%M:%S", localtime) . " "/e'

Don't forget that you can use gmtime instead of localtime if you need GMT formatted to your locale.

(c) Ask a question.