how to beep on tail -f event

Solution 1:

Just define beep as following:

beep() { read a || exit; printf "$a\007\n"; beep; }

Then, you can use your command:

tail -f development.log | grep "something rare" | beep

Solution 2:

GNU screen has a built-in feature to beep when a given window changes: see the relevant section of the man page.

Headline summary:

$ screen
$ tail -f yourfile.log    # inside the screen session
<C-a> M    # "Window 0 (bash) is now being monitored for all activity."

As pointed out in comments, this will beep on every new log entry, not just those that match "something rare", so this doesn't do quite what the OP asked for. Still a useful trick to know IMHO.

You can get the best of both worlds by opening two screen windows (<C-a> c to open a window, <C-a> <C-a> to toggle between two windows):

1. monitored, with tail -f yourfile.log | grep 'something rare'
2. unmonitored, with a plain tail -f yourfile.log

Then you can sit watching the log scroll past in window 2, and you'll get beeped from window 1 when "something rare" occurs.

screen is awesomely versatile - I highly recommend reading up on it.