Shell Programming and Scripting

Hi,
I have a loop like this -

Code:

while read item
 do
   // fire insert query
done < itemList.txt

The itemList.txt has say, 1000 records. Now what I do is that rhough another program, I make the itemList.txt EMPTY, but still the INSERT query keeps firing the sequence of records.

Does it mean that while loop reads the entire content in-memory and later does not bother about the file ???

I do not want this behavior. Please help

Hi,

"while" reads one line at a time "sequentially" until it encounters EOF.
So if another process modifies the file before the while reads/processes the entire data, the behaviour of while loop will change.

Regards,
gaurav.

Hi,
Even I thought so. But I see that is not the behavior. I tried even this

Code:

cat itemList.txt | 
while read item
do
 set $item
 ## do operation with $item
done

In both the cases when my other program makes the itemList.txt as empty, the while loop continues to do the operation till the end. I am using CSHELL on Solaris 8

hello,

This seems to be implementation dependant. When a process opens a file for reading/writing, the operating system loads a chunk of the file in memory for the process to read/write. So when the process has done reading with that chunk, the OS loads another chunk. This is implementation dependant.

I tried the opposite, as the while loop was executing, I added a few more entries to the end of the file(cat >> file) and while read them all. So while has a dynamic behaviour which is OS dependant.

So I can conclude that it depends on two factors - 1. size of the file 2.OS policy.
However this behaviour will cause haphazard results. Its unpredictable way to achieve things.

Regards,
gaurav.

Can you try the other way and see if you empty the file, the the while loop stops immediately or does it first finishes its old execution fully and then stops ?

I've created a file with 1.000.000 lines and run a while read line loop. When the loop was processing the ~150.000 line, I deleted the processed file from another terminal and the while loop did not stop, it just continued until 1.000.000th line.
HTH

Thanks pseudocoder ! Exactly the same scenario is happening. So, how do we define this behavior ? Is it correct ?