How to read a dynamically changing file

Shell Programming and Scripting
1

I want to read a constantly changing file and do some operation on text found in that file.
Actually that is log file of linux system and whenever i find a matching string in that file i want to create a text file with timestamp. is it possible to read that file?

here is sample output of log file

Apr 14 16:03:38 localhost kernel: usb 1-2.1: new high speed USB device using uhci_hcd and address 9
Apr 14 16:03:38 localhost kernel: usb 1-2.1: New USB device found, idVendor=12d1, idProduct=1037
Apr 14 16:03:38 localhost kernel: usb 1-2.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
Apr 14 16:03:38 localhost kernel: usb 1-2.1: Product: Ideos
Apr 14 16:03:38 localhost kernel: usb 1-2.1: Manufacturer: Huawei Incorporated

Apr 14 16:02:21 localhost kernel: usb 1-1: new full speed USB device using uhci_hcd and address 8
Apr 14 16:02:21 localhost kernel: usb 1-1: New USB device found, idVendor=0951, idProduct=1643
Apr 14 16:02:21 localhost kernel: usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
Apr 14 16:02:21 localhost kernel: usb 1-1: Product: DataTraveler G3
Apr 14 16:02:21 localhost kernel: usb 1-1: Manufacturer: Kingston

I want to create a file whenever "New USB device found" text is found in this file. create a file with timestamp in \usr\temp folder.

2

You can get the information directly from the source by creating a custom udev rule. You can use them to run simple commands whenever a device is detected. See man udev for details and examples, and also check out the udev rules your system already has ( which do things like populate /dev/disk/by-path/ ) which may be under /etc/udev.d/ or /lib/udev/rules.d/

On my system, custom rules are kept in /etc/udev.d/ while system ones are kept in /lib/udev/rules.d/

3

@ Corona688: thanks for reply. but in my case i can't use udev as it is not allowed to be dependent on it. for me there are 2 ways to check it. one write perl code ( which I can't as I know nothing about perl). 2nd use bash scripting (which i'm trying to do)

4

Try:

tail -f file
5

@Scrutinizer: sorry, you didn't get my point.
I know tail -f or tailf can be used to read from some file but i want to put that thing in if condition.
like keep reading this file and when u get string "USB connected". do some action. Now I'm getting problem while using this command in if condition.

while loop (never ending)
              if ( $ (tailf filename) == "USB Connected" ) ----> this is point where I'm getting problem.
              then
              generate alert
              fi

hope you understand my query

6

I got your point, but you had not posted what you tried up to this point. tail -f cannot be used that way since it is never ending. Try this approach:

tail -f logfile |
while IFS= read -r line
do
  ...
done > test.out
7

@Scrutinizer: can you please explain what is "IFS" in this loop.is it the string that I'm trying to search? and why i need text.out? can't i go without it? and sorry If my question wasn't clear enough.

simply, I just need a way to keep a check all the time on a log file so that whenever some specific text comes into that file, i get alert regarding that text. any function call? or any loop, condition? any way in scripting?

Thanks for your time and efforts.

8

IFS is is the Internal Field Separator. In the example I set that to "" so that leading white space does not get removed. test.out out is just an arbitrary file name, you can choose whatever file you like or use a pipeline symbol instead to pipe it into something that sends out the alert..