how does print know what filehandle to use, when in a diamond in-place edit loop?

tphyahoo has asked for the wisdom of the Perl Monks concerning the following question:

See code below... is the current selected filehandle (usually STDOUT, but not in the magic while(<>)) some perl default variable? I looked in perlvar but didn't see anything. How does this inplace edit magic work? Where is it documented?


use strict;
use warnings;
use Test::More qw(no_plan);

#create a.txt contining "a"
open F, "> a.txt";
print F "a";
close F;

#fails because no substitution was made
#prints to STDOUT
process_file(); 


local $^I = ".bak";

#now it works.
#prints to "a.txt" as we want. But how does it know to print there?
write_and_read_file(); #ok

sub process_file {
    
    @ARGV = qw(a.txt);
    
    local $/; # slurp it
    #changes a.txt so contains "b"
    #the diamond operator operates on @ARGV by default
    while (<>) {
        #$_ now contains the entire file, alter at will
        s/a/b/g;
        #How does the print operator know what file handle to print to
+? 
        #Was this selected somewhere by magic?
        #Where is this documented / explained?
        print;
    }

    #need to reset @ARGV, I guess it got wiped out somewhere in that w
+hile.    
    @ARGV = qw(a.txt);
    while (<>) {
        is($_,"b");
    }
    
}
[download]

Outputs:

bnot ok 1
#     Failed test (diamond.pl at line 35)
#          got: 'a'
#     expected: 'b'
Undefined subroutine &main::write_and_read_file called at diamond.pl l
+ine 15, <> line 1.
1..1
# Looks like you failed 1 tests of 1.
# Looks like your test died just after 1.
[download]

(Inspired by Re: Shell trick to edit many files with perl)

Comment on how does print know what filehandle to use, when in a diamond in-place edit loop? Select or Download Code

Replies are listed 'Best First'.
Re: how does print know what filehandle to use, when in a diamond in-place edit loop? by merlyn (Sage) on Aug 09, 2005 at 15:40 UTC
`#How does the print operator know what file handle to print to +? #Was this selected somewhere by magic? #Where is this documented / explained?` [download] perldoc perlrun says: From the shell, saying $ perl -p -i.orig -e "s/foo/bar/; ... " is the same as using the program: #!/usr/bin/perl -pi.orig s/foo/bar/; which is equivalent to #!/usr/bin/perl $extension = '.orig'; LINE: while (<>) { if ($ARGV ne $oldargv) { if ($extension !~ /\/) { $backup = $ARGV . $extension; } else { ($backup = $extension) =~ s/\/$ARGV/g; } rename($ARGV, $backup); open(ARGVOUT, ">$ARGV"); select(ARGVOUT); $oldargv = $ARGV; } s/foo/bar/; } continue { print; # this prints to original filename } select(STDOUT); except that the -i form doesn't need to compare $ARGV to $ +oldargv to know when the filename has changed. It does, however, +use ARGVOUT for the selected filehandle. Note that STDOUT is +restored as the default output filehandle after the loop. [download] -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l] [select]
Re: how does print know what filehandle to use, when in a diamond in-place edit loop? by gellyfish (Monsignor) on Aug 09, 2005 at 15:38 UTC
The inplace edit case uses the special filehandles `ARGV` (for input) and `ARGVOUT` (for output) these are document in perlvar /J\	[reply] [d/l] [select]
Re: how does print know what filehandle to use, when in a diamond in-place edit loop? by ysth (Canon) on Aug 11, 2005 at 10:05 UTC
You can get the current selected filehandle by: `select(my $selected = select(my $dummy)); print $selected;` [download] You may get the name of a glob (e.g. "main::ARGVOUT") or a globref, depending on what was passed to select before.	[reply] [d/l]