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.