How can I use a Perl variable in my Log::Log4perl config file?

Question

I would like to use a Perl variable from my script in a Log::Log4perl config file. I read the documentation and found that I can use a subroutine, but I would like to do it a little bit simpler, if possible.

I want to set the filename for my appender:

log4perl.appender.av_std_LOGFILE.filename="whateverfilename.log"

But doing this this way, it is a fixed value.

I have the filename in a variable within my script and would like to use this at runtime:

log4perl.appender.av_std_LOGFILE.filename=\
 sub { return &av_getLogfileName(); }

Where this is the subroutine:

sub av_getLogfileName
{
    return $av_std_LOGFILE;
}

This works, but I would like to avoid the sub inside my script since the return value is very simple.

The documentation says:

Each value starting with the string sub {... is interpreted as Perl code to be executed at the time the application parses the configuration...

So I tried something like this, but it did not work:

log4perl.appender.av_std_LOGFILE.filename=\
    sub { print "$av_std_LOGFILE"; }

Is there a way to get result of the variable without the sub inside my script?

You said $av_std_LOGFILE is set in a script a couple times and a module a couple times. Which is it? — ThisSuitIsBlackNot
– ThisSuitIsBlackNot, Commented May 12, 2016 at 15:44
Also, you shouldn't call subroutines with & unless you know what that does. return &av_getLogfileName(); should be return av_getLogfileName(); (or better yet, return av_get_logfile_name();, mixing snake case and camel case is really hard to read and snake case is generally preferred in Perl). — ThisSuitIsBlackNot
– ThisSuitIsBlackNot, Commented May 12, 2016 at 15:51

ThisSuitIsBlackNot · Accepted Answer · 2016-05-12 15:22:17Z

5

print returns 1 on success, so

sub { print "$av_std_LOGFILE"; }

returns 1, not the value of $av_std_LOGFILE. You also have to fully qualify variable names in hooks, which means you'll have to make $av_std_LOGFILE a package global.

Change your hook to:

sub { return $main::av_std_LOGFILE; } # double quotes are unnecessary

and set $av_std_LOGFILE in your script like this (before calling Log::Log4perl::init):

our $av_std_LOGFILE = '/path/to/logfile';

Generally, you should avoid global variables, so I would prefer using a subroutine.

edited May 12, 2016 at 15:22

answered May 12, 2016 at 15:05

ThisSuitIsBlackNot

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9 Comments

averlon Over a year ago

thanks; I get error messages: Can't open (Datei oder Verzeichnis nicht gefunden) at /home/fx42252/perl5/lib/perl5/Log/Log4perl/Appender/File.pm line 124. at /home/fx42252/perl5/lib/perl5/Log/Log4perl/Appender/File.pm line 129.

ThisSuitIsBlackNot Over a year ago

Either you're not setting $av_std_LOGFILE in package main, or you're setting it after you call Log::Log4perl::init. If you're setting the variable inside a module, you need to use the module's package name, e.g. if your module code is package Foo::Bar; our $av_std_LOGFILE = '/foo'; then your hook would be sub { return $Foo::Bar::av_std_LOGFILE; }

averlon Over a year ago

lets skip the expression "module". I am using perl up to now to replace a bash script. So there is not realy a structure like package, module, ..... What I understood so far is, by using perl that way, it is all "main::". Not sure. Anyhow: You can be sure the variable is set at the time init is used.

averlon Over a year ago

our $av_std_LOGFILE='test.log'; $av_std_DEBUG && say "debug \$av_std_LOGFILE: $av_std_LOGFILE"; Log::Log4perl::Config->utf8( 1 ); Log::Log4perl::init( "av_log4perl.conf" ); my $logger = get_logger();$logger->error("Blah"); that is all of the script- just for testing.

averlon Over a year ago

see my edits of the original question which now shows the whole script.

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