G'day ovedpo15,

Three reasons why I wouldn't recommend this approach:

• You are effectively deleting information from a log. Except for appending information to a log, I would consider it to be very bad practice to modify a log's contents in any other manner.
• Your progress is based on the number of paths processed. Information about how long each took to process (i.e. from the timestamps) is lost.
• You will run into all sorts of problems if you try to `tail -f logfile` when its contents are in flux.

Having said that, if you want to proceed with this — perhaps you just want snapshots of the log instead of continual progress updates — you could use the core module Tie::File. Here's an example (with show_log_to_date() simulating snapshots):

#!/usr/bin/env perl

use strict;
use warnings;

use Tie::File;

my $DEBUG_FILE = 'pm_11134904_log_progress.log';
tie my(@DEBUG_RECORDS), 'Tie::File', $DEBUG_FILE
    or die "Can't tie $DEBUG_FILE: $!";

debug("$0 started.");

for my $step ('A' .. 'C') {
    debug("Start step $step");

    if ($step eq 'B') {
        my @paths = 1 .. 5;
        my $overwrite_last = 0;

        for my $path (@paths) {
            sleep 1;
            my $progress = 100 * $path / @paths;
            my $msg = "Processing step $step Progress: $progress%";
            debug($msg, $overwrite_last++);
            show_log_to_date();
        }
    }
    else {
        debug("Processing step $step ...");
    }

    debug("End step $step");
}

debug("$0 completed.");

untie @DEBUG_RECORDS;

show_log_to_date();

sub debug {
    my ($msg, $overwrite_last) = @_;

    my $log_entry = join ' ', "".localtime, $msg;
    pop @DEBUG_RECORDS if $overwrite_last;
    push @DEBUG_RECORDS, $log_entry;

    return;
}

sub show_log_to_date {
    print '-' x 40, "\n";
    system cat => $DEBUG_FILE;
}
[download]

Output from a sample run:

$ ./pm_11134904_log_progress.pl
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:40 2021 Processing step B Progress: 20%
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:41 2021 Processing step B Progress: 40%
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:42 2021 Processing step B Progress: 60%
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:43 2021 Processing step B Progress: 80%
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:44 2021 Processing step B Progress: 100%
----------------------------------------
Sun Jul 11 10:20:39 2021 ./pm_11134904_log_progress.pl started.
Sun Jul 11 10:20:39 2021 Start step A
Sun Jul 11 10:20:39 2021 Processing step A ...
Sun Jul 11 10:20:39 2021 End step A
Sun Jul 11 10:20:39 2021 Start step B
Sun Jul 11 10:20:44 2021 Processing step B Progress: 100%
Sun Jul 11 10:20:44 2021 End step B
Sun Jul 11 10:20:44 2021 Start step C
Sun Jul 11 10:20:44 2021 Processing step C ...
Sun Jul 11 10:20:44 2021 End step C
Sun Jul 11 10:20:44 2021 ./pm_11134904_log_progress.pl completed.
[download]

Some other points regarding the specific code you posted:

• You are opening and closing the logfile every time you call debug(). Unless you have a very good reason for doing otherwise, open once near the start of the program, pass a filehandle to debug(), close once near the end of the program.
• The 'open ... or return 0;' made me cringe: that's no way to handle I/O problems. See open for many examples of how to do this; my code above has an example; also autodie is often a good alternative method (I use this a lot).
  • Use of a lexical filehandle and the 3-argument form of open is very good; it's just the error handling that has a problem.
• I see 'return 0' used twice in debug(); however, although not shown, I would expect debug() is called in void context, i.e. debug(...) not my $debug_return_value = debug(...). Just use return; in any subroutine that's called in void context.

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