Writing a bash loop to check differences in file size in loop

Question

I am writing a continuous bash loop to check for new mail in the /var/spool/mail file. This script needs to be continuous and compares the size of the previous loop. If the size increases then it needs to echo something. This is what I have now. It is not echoing anything when I run it. I am not sure if it is picking up the difference in the size of the file. To run it you need to input ./"the name of the script" /var/spool/mail/user

#!/bin/bash
checkUsage()
{
while true
do
sleep 10
fileSize=$(stat -c%s $1)
        sleep 5;
        fileSizeNew=$(stat -c%s $1)

        if [ "$fileSize" -lt "$fileSizeNew" ]
        then
           echo -e  "[Notice : ] $USER you have mail!!!"
           exit 
        fi

done
}
checkUsage $1

Answer 1

Try this

#!/bin/bash
checkUsage()
{
fileSize=$(stat -c%s $1)
while true;     do
        sleep 10
        fileSizeNew=$(stat -c%s $1)
        if [ "$fileSize" -lt "$fileSizeNew" ]; then
                echo -e  "[Notice : ] $USER you have mail!!!"
           exit 
        fi

done
}
checkUsage $1

The problem with your current code is that you're only really checking for a difference in file size during the 5 seconds of the second sleep. Meaning that you'd re-declare fileSize every 10 seconds, then wait 5 seconds and re-declare fileSizeNew and compare them (Mind the 5 second gap).

In my example, you declare fileSize once because it's sitting outside of the while loop, so every 10 seconds we'll take a look at the new size and compare it to the old - ever lasting original file size.