Run external command and process its log file in parallel - perl

Question

I need to run external tool from within my Perl code. This command works for a pretty long time, prints almost nothing to STDOUT but creates a log file.
I would like to run it and in parallel read and process its log file. How can I do it in Perl?
Thanks in advance.

Answer 1

If you use something like File::Tail to read the log file, then you can do a simple fork and exec to run the external command. Something like the following should work:
use strict;
use warnings;
use File::Tail;
my $pid = fork;
if ( $pid ) {
# in the parent process; open the log file and wait for input
my $tail = File::Tail->new( '/path/to/logfile.log' );
while( my $line = $tail->read ) {
# do stuff with $line here
last if $line eq 'done running'; # we need something to escape the loop
# or it will wait forever for input.
}
} else {
# in the child process, run the external command
exec 'some_command', 'arg1', 'arg2';
}
# wait for child process to exit and clean it up
my $exit_pid = wait;
If there are problems running the child process, the exit return code will be in the special variable $?; see the documentation for wait for more information.
Also, if the logging output does not provide a clue for when to stop tailing the file, you can install a handler in $SIG{CHLD} which will catch the child process's termination signal and allow you to break out of the loop.