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I wish to prepend a timestamp to each line of output from a command. For example:


would become

[2011-12-13 12:20:38] foo
[2011-12-13 12:21:32] bar
[2011-12-13 12:22:20] baz

...where the time being prefixed is the time at which the line was printed. How can I achieve this?

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2 Answers 2

up vote 62 down vote accepted

moreutils includes ts which does this quite nicely:

command | ts '[%Y-%m-%d %H:%M:%S]'

It eliminates the need for a loop too, every line of output will have a timestamp put on it.

$ echo -e "foo\nbar\nbaz" | ts '[%Y-%m-%d %H:%M:%S]'
[2011-12-13 22:07:03] foo
[2011-12-13 22:07:03] bar
[2011-12-13 22:07:03] baz

You want to know when that server came back up you restarted? Just run ping | ts , problem solved :D.

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How have I not known about this?!?!?! This complements tail -f amazingly! tail -f /tmp/script.results.txt | ts – Bruno Bronosky Mar 5 at 22:51

Firstly, if you are expecting these timestamps to actually represent an event, bear in mind that since many programs perform line buffering (some more aggressively than others), it is important to think of this as close to the time that the original line would have been printed rather than a timestamp of an action taking place.

You may also want to check that your command doesn't already have an inbuilt feature dedicated to doing this. As an example, ping -D exists in some ping versions, and prints the time since the Unix epoch before each line. If your command does not contain its own method, however, there are a few methods and tools that can be employed, amongst others:

POSIX shell

Bear in mind that since many shells store their strings internally as cstrings, if the input contains the null character (\0), it may cause the line to end prematurely.

command | while IFS= read -r line; do printf '[%s] %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$line"; done

GNU awk

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


command | perl -pe 'use POSIX strftime; print strftime "[%Y-%m-%d %H:%M:%S] ", localtime'


command | python -c 'import sys,time;sys.stdout.write("".join(( " ".join((time.strftime("[%Y-%m-%d %H:%M:%S]", time.localtime()), line)) for line in sys.stdin )))'
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One problem here is that many programs turn on even more output buffering when their stdout is a pipe instead of the terminal. – cjm Dec 13 '11 at 18:02
@cjm - True. Some output buffering can be alleviated by using stdbuf -o 0, but if the program is manually handling its output buffering, it won't help (unless there is an option to disable/reduce the size of the output buffer). – Chris Down Dec 13 '11 at 22:46

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