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I want to do something like this:

watch tail -f | wc -l
#=> 43
#=> 56
#=> 61
#=> 44
#=> ...

It counts new lines of tail each second

/ Linux, CentOs

To be more clear. I have got something like this:

tail -f /var/log/my_process/*.log | grep error

I am reading some error messages. And now I want to count them. How many ~ errors I have got in a second. So one line in a log is one error in a proccess.

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What are you trying to do? If you want to watch appended lines, it is enough to use tail -f /path/to/file alone. If you want to watch file lines, you can use watch wc -l /path/to/file. – Khaled Oct 22 '12 at 12:30
@Khaled, I need to watch how many new lines were appended – fl00r Oct 22 '12 at 12:31
I use tail with regex for a number of files, and I want to see how many lines are appended to all of this files through a time – fl00r Oct 22 '12 at 12:35
Hmpfh. tail -f /var/log/my_process/*.log |grep error > /tmp/error.lines & ; watch wc /tmp/error.lines ? Then do math. – cjc Oct 22 '12 at 18:02
@cjc it works, thanks! :) you should at it as ab answer – fl00r Oct 22 '12 at 18:13
up vote 16 down vote accepted

I've recently discovered pv, and it's really cool, you could do something like

tail -f logfile | pv -i2 -ltr > /dev/null

  • -i2 = count every 2 seconds
  • -l = count lines
  • -t = print time
  • -r = show rate
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Here's a quick and dirty method. You basically want to break the tail and the watch wc into separate parts, and do something like:

tail -f /var/log/my_process/*.log |grep error > /tmp/error.lines &
watch wc /tmp/error.lines

at which point, you can do math to get an errors/sec number. But, if you're just doing this for an one-off examination of your error rate, quick-and-dirty might be good enough.

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You could try something like this:

tail -f /var/log/my_process/*.log | perl -pe '$_ = "$. $_"'

It prints the line number in front of each line.

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1) That's not what the OP is looking for and 2) nl will do this, not requiring the call out to perl. – EEAA Oct 22 '12 at 17:58

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