Why output on perl eval differ between common bash output and redirection STDOUT + STDERR into file?

Question

My code is:

  perl -e'
  use strict; use warnings;

  my $a={};

  eval{ test(); };

  sub test{
     print "11\n";
     if( $a->{aa} eq "aa"){
        print "aa\n";
     }
     else{
        print "bb\n";
     }
  }'

Output on Terminal is:

  11
  Use of uninitialized value in string eq at -e line 9.
  bb

If I redirect in file, the output order differ. Why?

  perl -e'
  ...
  ' > t.log 2>&1

cat t.log:

  Use of uninitialized value in string eq at -e line 9.
  11
  bb

My perl Version:

  This is perl 5, version 18, subversion 4 (v5.18.4) built for x86_64-linux-thread-multi
  (with 20 registered patches, see perl -V for more detail)

Output to STDERR is usually not buffered while other output is buffered. Add a $|++; after the use cases and all of your output will be unbuffered and the sequence will be what you expect. — dgw
– dgw, Commented Apr 13, 2015 at 8:26

ikegami · Accepted Answer · 2015-04-13 14:48:38Z

4

A simpler demonstration of the problem:

$ perl -e'print("abc\n"); warn("def\n");'
abc
def

$ perl -e'print("abc\n"); warn("def\n");' 2>&1 | cat
def
abc

This is due to differences in how STDOUT and STDERR are buffered.

STDERR isn't buffered.
STDOUT flushes its buffer when a newline is encountered if STDOUT is connected to a terminal.
STDOUT flushes its buffer when it's full otherwise.

$| = 1; turns off buffering for STDOUT^[1].

$ perl -e'$| = 1; print("abc\n"); warn("def\n");' 2>&1 | cat
abc
def

Actually, the currently selected handle, which is the one print writes to if no handle is specified, which is STDOUT by default.

edited Apr 13, 2015 at 14:48

answered Apr 13, 2015 at 13:17

ikegami

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Comments

Gerd · Accepted Answer · 2015-04-13 15:41:50Z

0

It's only an autoflush problem NO eval question.

Solution is:

perl -e'
use strict; 
use warnings;

$|++; # <== this autoflush print output

my $a={};
test();

sub test{ 
   print "11\n";
   if( $a->{aa} eq "aa"){
      print "aa\n";
   }
   else{
      print "bb\n";
   }
}' > t.log 2>&1

In some cases on terminal is the same problem:

perl -e'print("abc"); print(STDERR "def\n"); print("ghi\n");'

The only save way to get correct order, is turn on autoflush!

@dgw + ikegami ==> thank's

edited Apr 13, 2015 at 15:41

answered Apr 13, 2015 at 9:10

Gerd

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