Is there a Unix utility to prepend timestamps to stdin?

Question

ts from moreutils will prepend a timestamp to every line of input you give it. You can format it using strftime too.

$ echo 'foo bar baz' | ts
Mar 21 18:07:28 foo bar baz
$ echo 'blah blah blah' | ts '%F %T'
2012-03-21 18:07:30 blah blah blah
$ 

To install it:

sudo apt-get install moreutils

Could try using awk:

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

You may need to make sure that <command> produces line buffered output, i.e. it flushes its output stream after each line; the timestamp awk adds will be the time that the end of the line appeared on its input pipe.

If awk shows errors, then try gawk instead.

annotate, available via that link or as annotate-output in the Debian devscripts package.

$ echo -e "a\nb\nc" > lines
$ annotate-output cat lines
17:00:47 I: Started cat lines
17:00:47 O: a
17:00:47 O: b
17:00:47 O: c
17:00:47 I: Finished with exitcode 0

Distilling the given answers to the simplest one possible:

unbuffer $COMMAND | ts

On Ubuntu, they come from the expect-dev and moreutils packages.

sudo apt-get install expect-dev moreutils

How about this?

cat somefile.txt | perl -pne 'print scalar(localtime()), " ";'

Judging from your desire to get live timestamps, maybe you want to do live updating on a log file or something? Maybe

tail -f /path/to/log | perl -pne 'print scalar(localtime()), " ";' > /path/to/log-with-timestamps

Just gonna throw this out there: there are a pair of utilities in daemontools called tai64n and tai64nlocal that are made for prepending timestamps to log messages.

Example:

cat file | tai64n | tai64nlocal