Shell Programming and Scripting

Hello all, I'm new to shell scripting and want to make a script that I can write to log the users logging on and off the a unix system.

I have had a good look over the past few days to crack it, I think I am getting close.

I want a script that runs an infinite loop to check every 5 seconds to report on who logs into and who logs out of the unix system.

This is a example of the output I would like:

I am using
who | awk '{print $1}' | sort > temp1
to get the users logged on and saving that into a temp file.

and have a while loop set up like this which will run each 5 seconds.

while true
do
.
.
.
.
.
sleep 5
done

The problem I'm having is, I can't work out how to compare the current users logged in to the users that have previously logged in.

I thought of making a second temp file that stores the current users then using:
cmp temp1 temp2
to look at the difference. problem is it gives me an output saying "file differs at character 42, line 6" and that isn't helping me much as I would like to then display that users name but not sure how to print that position in the file (or just a line for that matter).

The test to see if the user has logged on or off is easy, as it is only a couple of if statements.

like if user is in temp1 but not in temp2 then the user has logged out and vice versa.

I have been looking at this for a while now and am going around in circles. any code fragments, help or written shell scripts would be greatly appreciated.

Thanks in advance

Noodle

Use the 'comm' utility. The -13 and the -23 options will give you what you want. Check the man page for details.

you are quite the life saver, thank you very much.

Also is there a quick command you can use for a an if statement to see if the files differ?

if [statement here].

I had a look at the cmp utility and the exit status was 0 is the files were identical and 1 if the files were diferent. but I couldn't get the exit status printed to screen.

Thanks in advance

Hm. Use the diff command. The exit code for diff is 0 if the files are identical, and 1 if they are not.

OK, this is what I have got now, not srue where I have gone wrong. it says I have a syntax error on line 30; ';;' unexpected? any ideas? I thought these were needed for the case to work

Code:

       if list then list [elif list then list] ... [else list] fi
              If the exit status of the first list is zero, the second list is
              executed; otherwise the list following the elif, if any, is exe-
              cuted with similar consequences.  If all the lists following the
              if and elifs fail (i.e., exit with non-zero  status),  the  list
              following the else is executed.  The exit status of an if state-
              ment is that of non-conditional list that  is  executed;  if  no
              non-conditional list is executed, the exit status is zero.

thanks for the reply, but i'm still a little stuck.
Vgersh99 would you mind rewriting my if statements in there correct syntax for me as i still can't get them working right.

Thanks in advance