UNIX for Dummies Questions & Answers

I am writing a script that periodically reads in data from a text file. The only issue is that, that text file is periodically updated (appended to) by another script. I am using perl in UNIX environment. How can I check if that text file is being accessed, so I can wait until it is no longer being accessed before trying to access it. Know what I mean? Help woud be apprecated.

Ricky

The 'fuser' command shows if a file is in use and by who

hereby the SUN man pages of fuser

http://docs.sun.com/?q=fuser&p=/doc/...9vfiln4&a=view

use flock function on the file
if it returns a false, that means the file is being updated
so wait and try again later
if it returns a true, go ahead and carry out your operations

asif

I'm not sure if you use an error subroutine in case the file cannot be opened, closed, etc, but here's one possible way to code this:

Code:

open (file_label, "file_path") || Error('open', 'file');
flock (file_label, 1) || Error('lock', 'file');
my @array_name = <file_label>;
close (file_label) || Error ('close', 'file');

sub Error {
 print "Content-type: text/html\n\n";
 print "The server can't $_[0] the $_[1] : $! \n";
 exit;
}

This will request shared access to a file in order to read from it and then will store the whole file in an array.

Replace the '1' in the flock() command with a '2' if you want to request exclusive access to a file, in order to write to it.

added code tags for readability

Thanks to all,
Rickylui.

LSOF is a good way to tell if the file is being accessed.


Here is a link I got from a google search.

http://ftp.cerias.purdue.edu/pub/too.../hpux/B.11.11/


From the man page:

Instead of a formatted display, lsof will produce output that can be
parsed by other programs. See the -F, option description, and the
OUTPUT FOR OTHER PROGRAMS section for more information.

In addition to producing a single output list, lsof will run in repeat
mode. In repeat mode it will produce output, delay, then repeat the
output operation until stopped with an interrupt or quit signal.


Hope this helps.

If you only read file, you should not worry about file locking and
accessing -- OS will take care of it. Anyway, when you write to the file from shell it is no reliable way from the OS point to determine, that all writes are done.
You can accomplish this programmatically: main shell create a flag file at the end of all writes, and subshell delete this flag file, as soon as it finished it's own processing.