CODEWITHSUNDEEP
perlstdout
KT_
KT_

Reputation: 1087

Perl script called from Perl doesn't seem to flush stdout

I have two Perl scripts — let's call them script.pl and script2.pl.

script2.pl updates to the command line, as in:

print "Progress $progress percent...\r";

In order to get this to work I had to turn on autoflush at the start of script2.pl:

$|++;

Now, the problem arises because I have to have script2.pl called from script.pl, and this — the updating/flushing — only seems to work when script2.pl is run directly, not when it's called from script.pl.

I've tried various ways of calling script2.pl, from system() to backticks and in no case does the updating happen.

Am I missing something about handling stdout output when one Perl script is called from another?

Upvotes: 3

Views: 358

Answers (1)

simbabque
simbabque

Reputation: 54371

Calling with backticks will spawn a new perl process, and for that process, the auto-flushing will be set. That means that whatever output comes out of script2.pl will come out as soon as it gets printed. Of course doing

my $output = `perl script2.pl`;

will mean you are not actually reading it while it's being output, but rather you're asking for all the output to end up in $output when it's done.

The $| in the perl process that runs script2.pl has no influence on script.pl. If you want your script.pl process to have unbuffered output, turn off buffering there.

An alternative might be to not run script2.pl as a separate process, but rather to refactor it to be a module and call it properly, or use do or require to call it once. In either case the change to $| would then happen within the same Perl interpreter, and thus would have an effect.

Of course it's not a good idea if an internal part of your application messes with your application's output buffering. The best way would be to refactor this to be a module without changing the buffering in that module, and in your program where you use it, set the buffering to what you need it to be for that specific program. Always let the consumer decide what they want, even if that's only yourself.

Upvotes: 3

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