How to streamingly to read text file with Perl - perl

Question

it can display text in file, however, after i add new text in gedit, it do not show the updated one.
sub start_thread {
my #args = #_;
print('Thread started: ', #args, "\n");
open(my $myhandle,'<',#args) or die "unable to open file"; # typical open call
for (;;) {
while (<$myhandle>) {
chomp;
print $_."\n";
}
sleep 1;
seek FH, 0, 1; # this clears the eof flag on FH
}
}
update video
https://docs.google.com/file/d/0B4hnKBXrOBqRWEdjTDFIbHJselk/edit?usp=sharing
https://docs.google.com/file/d/0B4hnKBXrOBqRcEFhU3k4dUN4cXc/edit?usp=sharing
how to print $curpos for updated data
for (;;) {
for ($curpos = tell($myhandle); $_ = <$myhandle>;
$curpos = tell($myhandle)) {
# search for some stuff and put it into files
print $curpos."\n";
}
sleep(1);
seek(FILE, $curpos, 0);
}

Answer 1

Like I said - it works for me. Changes to your script are minimal - just minimal cleanup.
Script: test_tail.pl
#!/usr/bin/perl
sub tail_file {
my $filename = shift;
open(my $myhandle,'<',$filename) or die "unable to open file"; # typical open call
for (;;) {
print "About to read file...\n";
while (<$myhandle>) {
chomp;
print $_."\n";
}
sleep 1;
seek $myhandle, 0, 1; # this clears the eof flag on FH
}
}
tail_file('/tmp/test_file.txt');
Then:
echo -e "aaa\nbbb\nccc\n" > /tmp/test_file.txt
# wait a bit
echo -e "ddd\neee\n" >> /tmp/test_file.txt
Meanwhile (in a different terminal);
$ perl /tmp/test_tail.pl
About to read file...
aaa
bbb
ccc
About to read file...
About to read file...
About to read file...
ddd
eee

Answer 2

Instead of this:
seek $myhandle, 0, 1; # this clears the eof flag on FH
Can you try something like this:
my $pos = tell $myhandle;
seek $myhandle, $pos, 0; # reset the file handle in an alternate way

Answer 3

The file system is trying to give you a consistent view of the file you are reading. To see the changes, you would need to reopen the file.
To see an example of this, try the following:
1.Create a file that has 100 lines of text in it, a man page, for instance:
man tail > foo
2.Print the file slowly:
cat foo | perl -ne 'print; sleep 1;'
3.While that is going on, in another shell or editor, try editing the file by deleting most lines
Result: The file will continue to print slowly, as if you never edited it. Only when you try to print it again, will you see the changes.

Answer 4

The following would also work:
my $TAIL = '/usr/bin/tail -f'; # Adjust accordingly
open my $fh, "$TAIL |"
or die "Unable to run $TAIL : $!";
while (<$fh>) {
# do something
}