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perl note

POD 格式 http://www.pgsqldb.org/mwiki/index.php/PerlCulture_PlainOldDoc

1. match content from ... to ...

match !! START !! to !! END !!

 while (<FILE>) {
    if (/!! START !!/ .. /!! END !!/) {
      # process line
    }
  }

Input fh: $/; Output fh: $\

ref http://www.perl.com/pub/2004/06/18/variables.html

2.show the first ten line of the file
  while (<FILE>) {
    print if 1 .. 10;
  }
3. slurp the whole file

1. my $file = do { local $/; <FILE };>

2. my $file = do { open my $fh, '<:mmap', $inFile or die ""; <$fh> }

4.open many file
      my @open_fh = map { open my $fh, '<', $_; $fh } @file;
5.get fixed size data

read a file 2Kb at a time and you can do this.

  {
    local $/ = \2048;

    while (<FILE>) {
      # $_ contains the next 2048 bytes from FILE
    }
  }
6.noName

The string that is printed between the arguments passed to print is stored in a variable called $, (because you use a comma to separate arguments)

  my @arr = (1, 2, 3);
  {
    local $, = ',';

    print @arr;
  }

This code prints the string 1,2,3.

The string that separates the elements of an array when expanded in a double quoted string is stored in $";

  my @arr = (1, 2, 3);
  {
    local $" = '+';

    print "@arr";
  }

This code prints 1+2+3".

7. add more detail error info
      use Carp ();

      local $SIG{__WARN__} = \&Carp::cluck;
8.heredoc

substitle

        my $interpolation = "We will interpolated variables";
        print <<"END";
        With double quotes, $interpolation, just like normal HEREDOCS.
        END

show what i type

     print <<'END';
     With single quotes, the variable $foo will *not* be interpolated.
     (You have probably seen this in other languages.)
     END

Execute it as command

        my $shell_output = <<`END`;
        echo With backticks, these commands will be executed in shell.
        echo The output is returned.
        ls | wc -l
        END>
9.}{ magic

ls |perl -lne 'print $_; }{ print "$. Files"'

which is converted internally to this code:

      LINE: while (defined($_ = <ARGV>)) {
          print $_; }{ print "$. Files";
      }
10. @INC %INC

@INC: contains a lists of modules from which perl modules and libraries can be load;

%INC: cache the names of files and modules that were successfully loaded and compiled by use(), require() and do();

      perl -le 'print join "\n", @INC'

      perl -MLWP -e 'print map {"$_ => $INC{$_}\n"} sort keys %INC'
11.use do require

Before load a file with use or require, Perl checks whether it's already in the %INC, if %INC has, require just returns without doing a thing. do does unconditional loading--no lookup in the %INC hash;

If require() fails to load the file, either beacuse it couldn't find the file in question or the code failed to compile, or it didn't return TRUE, then the program will die(). To prevent this the require() statement can be enclosed into an eval() execption-handling blocks;

use() load and compile files containing perl code, but it works with modules only and is executed at compile time. use translate :: int /; use () is equivalent: BEGIN { require Module; Module->import(LIST);}

If do() can't read the file, it return undef and set $! to report error. If do() can read the file but can't compile it, it returns undef and puts error msg in $@. If the file is successfully compiled, do() return the value of the last expression evaluated.

12.load module
      use FindBin;
      use lib "$FindBin::Bin";
      use PrivateModule;   ## add private module
13. regex
      m//o;
      s///e;
14.perl -[dD]

perl -dt indicated the debugger that threads will be used in the code being debugged.

perl -Dx list compiled syntax tree

perl -Dr list comipled regular experssion

16.don't quote large string unless absolutely necessary
  my $copy = "$large_string";

makes 2 copies of $large_string (one for $copy and another for the quote)

  my $copy = $large_string;

only makes one copy.

17.use map and grep selectively

map and grep expect a LIST argument, so doing this:

    @wanted = grep { /pattern/ } <FILE>;

will cause the entire file to be slurp. for large files, it's better to loop:

     while ( <FILE> ) {
                   push (@wanted, $_) if /pattern/;
         }      
18. Expand function call inside string

call the function inside the braces used to dereference a reference. If we have more than on return value, we can construct and dereference an anonymous array, in this case, we call the function in list context.

   print "The time value are @{ [localtime] }.\n";

call function in scalar context

   print "The time value are ${ \(scalar localtime) }.\n";
   print "The time value are ${ my $x = localtime; \$x }.\n";

if your function already returns a reference, you don't need to create the reference yourself.

   sub timestamp { my $t = localtime; \$t }
   print "The time value are ${ timestamp() }.\n";   

The "Interpolation" module can also get the result.

   use Interpolation E => 'eval';
   print "The time values are $E{localtime()}.\n";
19. expand tabls in a string
   1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
20. expand variable in text strings

The first /e evaluates $1 on the replacement side and turns it in into $foo, the second /e evalutes starts with $foo and replaces it with its value.

   $foo = 'Fred';
   $bar = 'Barney';
   $string = 'Say hello to $foo and $bar';
   $string =~ s/(\$\w+)/$1/eeg;   
21. running command

There are three basic ways of running external commands:

   system $cmd;            # using system()
   $output = `$cmd`;               # using backticks (``)
   open (PIPE, "cmd |");   # using open()

With "system()", both STDOUT and STDERR will go the same place as the script's STDOUT and STDERR, unless the "system()" command redirects them. Backticks and "open()" read only the STDOUT of your command.

22. Back file on command line
      perl -pi'old/*.orig' -e 's/bar/baz/' fileA> # backup to 'old/fileA.orig'

      perl -pi -e 's/bar/baz/' fileA> # overwrite current file

      perl -pi'*' -e 's/bar/baz/' fileA> # overwrite current file

      perl -pi'.orig' -e 's/bar/baz/' fileA> # backup to 'fileA.orig'

      perl -pi'*.orig' -e 's/bar/baz/' fileA> # backup to 'fileA.orig'

a fancy way copy file

   perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...

or perl -p -i'.orig' -e 1 file1 file2 file3...

23. effective way delete file

find . -mtime +7 -print | perl -nle unlink

This is faster than using the -exec switch of find because you don't have to start a process on every filename found.

24. open

Use Indirect Filehandles make namespace management easier; and it's also has lexical scope.

open my $fh, '<', $file or die $!;

three op for |

got $command output

open my $fh, '-|', $command or die $!;

put output to $command

open my $fh, '|-', $command or die $!;

filename contain whitespace, Leading whitespace is protected by inserting a "./" in front of a filename that starts with whitespace. Trailing whitespace is protected by appending an ASCII NUL byte ("\0") at the end of the string.

     $file =~ s#^(\s)#./$1#;
     open(FH, "< $file\0")   || die "can't open $file: $!";

STDOUT add pager

    $pager = $ENV{PAGER} || "(less || more)";
    open(STDOUT, "| $pager")
        || die "can't fork a pager: $!";
25. parse reference

Perl follows these two simple rules while parsing such expressions: ( $$array[1] == ${$array}[1] ) (1) Key or index lookups are done at the end, and (2) the prefix closest to a variable name binds most closely.

26. reference scalar

passing references to scalars typically turns out not to be an optimization at all.

27. hash You can preallocate space for a hash by assigning to the keys() function. This rounds up the allocated buckets to the next power of two:
       keys(%users) = 1000;                # allocate 1024 buckets
28. repeat read DATA
    my $pos = tell DATA;
    while ( 1 ) {
        print while ( <DATA> );
        seek DATA, $pos, 0;
    }
29. Slice
    #!/usr/bin/perl
    #
    use strict;
    use warnings;

    use Smart::Comments;

    my @item = qw(first second third);
    my $hash = {
        world => {
            first  => 1,
            second => 2,
            third  => 3,
        },
    };

    ### $hash
    @{$hash->{world}}{@item} = ( 4, 5, 6);
    ### $hash
29. Remove ^M tr/\015//d
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