Creating an action based event in bash

hudac · 05-29-2013, 01:11 PM

Hey,
I couldn't find an answer to my question, and I'm sure it is possible:

How can I create an action based on some event in bash:

Lets say I have some file `text.txt`,
I want to do something, like `grep` on this file
If and only if a line was added to this file,
Or the file size got bigger, or md5sum...

Thanks!

unSpawn · 05-29-2013, 02:31 PM

W/o knowing any requirements (when, how much, how long or whatever else) I'd say that sounds like a job for Inotify.

hudac · 05-29-2013, 03:24 PM

Thanks,

Actually, It suppose to be an action that, lets say, start "gnuplot", every time a line is added to the file `text.txt`
There is no more options given...

Does the question is clearer now ?
Does it gives more information that may offer another solution?
Or should I just use `inotify` ?

Thanks!

unSpawn · 05-29-2013, 04:36 PM

Inotify would work for that, yes. However inotify reacts immediately so unless you script carefully and check for already an running gnuplot process you might find you end up running a gazillion processes if the file gets added to continuously. This example effectively only echoes the file name:

Code:

#!/bin/sh --
LANG=C; LC_ALL=C; export LANG LC_ALL
/usr/bin/pgrep gnuplot >/dev/null 2>&1 && exit 0
/usr/bin/inotifywait -m -q -e CLOSE_WRITE /path/to/text.txt --format '%w%f'\
|while read ITEM; do echo "${ITEM}"; done; exit 0

chrism01 · 05-29-2013, 06:24 PM

You could write a small script to track the new lines being added, based on the cmd 'tail -f test.txt' in bash.
Alternatively, Perl has a module designed for this; see definition & example code http://search.cpan.org/~mgrabnar/Fil...0.99.3/Tail.pm

hudac · 05-30-2013, 12:46 AM

Great, Thanks!