Recursive reading of a file (like tail -f)

Question

I'm trying to recursively read the contents of a file that is simultaneously being written to; esentially emulating tail -f in AppleScript. Any examples I could be pointed to?
My AppleScript application issues a 'do shell script' that returns data as the script executes. So I can wait for the script to finish and display the results, how my app currently works (less than ideal), or I can have the shell script write to a temp file and then iteratively read and display the contents of the file as it fills up with the shell script's output (ideal).
Problem is, I don't know how to tell if the temp file has finished being written to, and therefore when to stop reading into my display field.
Trying to get my head around this - any thoughts, tips and/or examples would be much appreciated -- thanks

Answer 1

There's several ways of doing this. The most obvious solution is to have your script write out some text that indicates the end of its processing, to which your AppleScript can react and know the script is finished. For example, if the script wrote out the string "done" at the end, it's trivial for AppleScript to check whether the last line in the file is 'done' or not.
The alternative appoach is to capture the PID of the shell script and periodically check to see if that PID is still running. When it no longer appears in the process table you know you're done. For example:
<pre class=command>set thePID to do shell script "/path/to/length/script &> /path/to/outputfile $!"</pre>
the $! is a shell flag that returns the PID of the process you just started. Now you can do something like:
<pre class=command>set thePID to do shell script "/path/to/script > /var/tmp/outputfile 2>&1 & echo $!"
-- now thePID will hold the PID o the /path/to/script process
set charsRead to 0
try
repeat
do shell script "ps -p " & thePID
set theProgress to read file ":private:var:tmp:outputfile" as text
set numChars to number of characters in theProgress
display dialog (text (charsRead + 1) through numChars of theProgress) giving up after 1
set charsRead to count theProgress
end repeat
end try</pre>
This launches the script and captures the PID. It then repeats through a loop that checks if the PID is still running. If it is, it reads the file and displays the chunk of the file that hasn't yet been seen (using the 'charsRead' variable to keep track of how much has been seen.
If the PID is not running, the 'ps' shell script returns an error, which terminates the try block.