Shell Programming and Scripting

I have following script to send email when a USB is attached.

Code:

#!/bin/bash

NUMBER=`/bin/cat /u/numberoflines`
LINES=`/usr/bin/wc -l < /var/log/messages`

DIFFERENCE=$(($LINES-$NUMBER))

if [ $DIFFERENCE != 0 ]; then
tail -n $DIFFERENCE /var/log/messages |while read line
do
if $( echo $line | grep --quiet 'New USB device found' )
then
        Email text/message
        EMAIL='myeamilid'
        SUBJECT="New USB device found"
        EMAILMESSAGE="/tmp/emailmessage.txt"
        echo "New USB device found etc ..." > $EMAILMESSAGE
        /bin/mail -s "$SUBJECT" "$EMAIL" < $EMAILMESSAGE

fi
done
echo "$LINES" > /u/numberoflines
fi

Problem with this script is that sometimes it works properly and i can see it using grep command "ps -ef | grep scriptname" but sometimes it doesn't. Unable to understand why it doesn't work sometimes. Please guide.

Hi, just a couple of things that come to mind. Why do you have $( ... )? Does that even work. That should not need to be there (it would only work if there really is no output from those commands inside the command substitution), plus I would replace

Code:

echo $line ...

with

Code:

if printf "%s\n" "$line" | grep ...

. The fact that it sometimes does or does not work, may be due to buffering somewhere, so it works but not yet, not in real time. You can test this by having an extra tail -f and check what is written to the log at what point in time..

Why are you using tail and not tail -f? The tail buffer can also be a factor, it can be that the amount written exceeds what tail can display...

Another possibility is that by the time the script starts to do the tail the number of lines has changed and so some lines will never get read...

If you for some reason you need to use tail instead of tail -f, why don't you run the tail buffer straight through grep instead of line for line?

Does your system use udev? Because that would really be ideal for this application.

It has udev, as revealed in his other threads on the subject. I think he's trying to make it portable, but I'm not sure he realizes non-Linux systems aren't going to have the same log messages anyway.

I continue to agree that using udev for its intended purpose would be the simplest and most reliable way to do this.