Using Net::OpenSSH tail the message file and grep - linux

Question

I am using Net::OpenSSH
my $ssh = Net::OpenSSH->new("$linux_machine_host")
Using the SSH object, fews commands are executed multiple times for N hours.
At times I need to look for any error messages, such as Timeout, in the var/adm/message file.
My suggestion
$ssh->capture2("echo START >> /var/adm/messages");
$ssh->capture2("some command which will be run in background for n hours");
$ssh->capture2("echo END >> /var/adm/messages");
Then read all lines between START and END and grep for the required error message.
$ssh->capture2("grep -A 100000 "START" /var/adm/messages | grep -B 100000 END");`
Without writing START and END into the message file, can I tail the var/adm/message file at some point and capture any new messages appearing afterwards.
Are there any Net::OpenSSH methods which would capture new lines and write them into a file?

Answer 1

You can read the messages file via SFTP (see Net::SFTP::Foreign):
# untested!
use Net::SFTP::Foreign::Constants qw(:flags);
...
my $sftp = $ssh->sftp;
# open the messages file creating it if it doesn't exist
# and move to the end:
my $fh = $sftp->open("/var/adm/messages",
SSH2_FXF_READ|SSH2_FXF_CREAT)
or die $sftp->error;
seek($fh, 0, 2);
$ssh->capture2("some command which...");
# look for the size of /var/adm/messages now so that we
# can ignore any lines that may be appended while we are
# reading it:
my $end = (stat $fh)[7];
# and finally read any lines added since we opened it:
my #msg;
while (1) {
my $pos = tell $fh;
last if $pos < 0 or $pos >= $end;
my $line = <$fh>;
last unless defined $line;
push #msg, $line;
}
Note that you are not taking into account that the messages file may be rotated. Handling that would require more convoluted approaches.