UNIX for Dummies Questions & Answers

Hi,
I have a landing area where some files keep on coming after irregular intervals of time. From this landing area, I need to move files to another directory for processing. For this, I am using a for loop to find certain kinds of files in the landing area.

Now my question is, suppose I start my script which moves the files at a certain moment and by the time the script completes, more files come into the landing area.... will the new files be moved into the processing directory too??

According to me it should not....but how exactly does the for loop work? how does it know what files to take?

Thanks
Neelaksh

They are not.

for VAR in SOMETHING

SOMETHING is only evaluated ONCE at the beginning of the for loop.

So if if SOMETHING would select a number of files, then only those files which meet the selection at the time the for statement is executed will be taken into consideration.

The for has no knowledge of files which "arrive" at a later time and would have met the condition if they had been present at the time the for is executed.

okk.. I thought so too....what happens in this scenario...

I am talking about close to 15,000 files totalling about 40 Gb in size. What if during the execution of the for statement, new files are added into the directory???

The number and total size of the files is irrelevant.

All new files will not be affected by whatever happens inside the for loop.

If you want them to be affected as well you could put the for loop e.g. inside a while loop.

Suppose your for loop works on files with the extention ".txt".

while [ `ls *.txt > /dev/null 2>&1; echo $?` -eq 0 ]
do
for FILE in `ls *.txt`
do
do_whatever_needs_to_be_done
done
done

The while loop makes sure that as long as *.txt files are present in the directory the for loop will be executed over and over.

Ofcourse this process will come to an end at sometime as well, when no more *.txt are present. New *.txt files arriving shortly after are not affected either.

1.) Is there any possibility that the snapshot goes wrong?? by snapshot i mean the list of files in the directory at the time of execution?

2.) Suppose i execute the script, the for loop evaluates the 'ls *.dat' command.... and while it is moving those files... i delete a few of them... will the script thrown an error??

Like the "for" has no knowledge of files being added it also has no knowledge of files being removed.

So if at the time of evaluating the "for" some files are present, which are later on removed, the "for" will still try to perform the actions which are mentioned inide the loop. If the files are gone this will result in error messages.

okk... thanks for answering my queries