perl white magic - command line switches and special variables

Perl White Magic

Jose [email protected]

June 27, 2006

Jose Castro Perl White Magic

http://jose-castro.org

[email protected]

About this talk

What will we be talking about?

Command Line Switches

Special Variables

And others. . .


About this talk

Why are we talking about those things?

These things fasten your coding

These things clean your code

They’re fun!

You can’t expect to be a good programmer if you’re notcurious. . .


About this talk

But. . .

Haven’t we been through this already?

Well. . .

Some have, some haven’t

It’s never too much to repeat these things

You might be seeing different perspectives about these things

We might bump into something you didn’t know about. . .


Problem 1

Problem 1

Count words and lines in a file

#!/usr/bin/perl -wuse strict;my ($lines,$words,@words);while (<>) {

$lines++;@words = split /\W+/, $_;$words += @words;

}

print "lines -> $lines\n";print "words -> $words\n";


Problem 1

But wait, what are these?

$.-n-e


Problem 1

Hum. . .

$. Current input line (record) number-n Loop around program-e Eval script


Problem 1

-n

LINE:while (<>) {

... # your program goes here}

-e

Used to enter one line of program.


Problem 1

Problem 1


#!/usr/bin/perl -wuse strict;my ($lines,$words,@words);while (<>) {

$lines++;@words = split /\W+/, $_;$words += @words;

}

print "lines -> $lines\n";print "words -> $words\n";


Problem 1

Problem 1


#!/usr/bin/perl -wuse strict;my ( $words,@words);while (<>) {

@words = split /\W+/, $_;$words += @words;

}

print "lines -> $.\n";print "words -> $words\n";


Problem 1

Problem 1


#!/usr/bin/perl -wnuse strict;our( $words,@words);

@words = split /\W+/, $_;$words += @words;

END {print "lines -> $.\n";print "words -> $words\n"; }


Problem 1

Problem 1


#!/usr/bin/perl -wnuse strict;our( $w ,@w );

@w = split /\W+/ ;$w += @w;

END {print "lines -> $.\n";print "words -> $w\n"; }


Problem 1

Problem 1


#!/usr/bin/perl -wnuse strict;my @w = split /\W+/;our $w += @w;

END {print "lines -> $.\n";print "words -> $w\n"; }


Problem 1

Problem 1


#!/usr/bin/perl -wnuse strict;my @w = split /\W+/;our $w += @w;

END {print "lines -> $.\n";print "words -> $w\n"; }__END__perl -wn -e ’$w+=split/\W+/;END{

print"lines -> $.\nwords -> $w\n"}’


About this talk

Before coming here. . .

I rehearsed this presentation. . .

He was quite amused!


About this talk

See?


Problem 2

Problem 2

Adding a commit each 100 lines (rollback information would betoo much)

insert into table values (’1.1.0’ ,’10:30’ ,’11:30’);insert into table values (’1.1.1’ ,’08:15’ ,’08:30’);insert into table values (’1.1.2’ ,’11:00’ ,’11:45’);insert into table values (’1.1.3’ ,’07:50’ ,’09:20’);...


Problem 2

Problem 2

Adding a commit each 100 lines

#!/usr/bin/perl -wuse strict;

my $line;while (<>) {

$line++;unless ( $line % 100 ) {

print "commit;\n";}print;

}


Problem 2

Look! More stuff!

$. Current input line (record) number-e Eval script-i-p


Problem 2

Hum. . .

$. Current input line (record) number-e Eval script-i In-place editing-p Loop around program, printing lines


Problem 2

-i

Files processed by the ”<>” construct are to be edited in-place. Itdoes this by renaming the input file, opening [. . . ]

-p

LINE:while (<>) {

... # your program goes here} continue {

print or die "-p destination: $!\n";}


Problem 2

Problem 2



my $line;while (<>) {

$line++;unless ( $line % 100 ) {

print "commit;\n";}print;

}


Problem 2

Problem 2



while (<>) {

unless ( $. % 100 ) {print "commit;\n";

}print;

}


Problem 2

Problem 2


#!/usr/bin/perl -wpuse strict;


}


Problem 2

Problem 2


#!/usr/bin/perl -wpuse strict;


}

__END__perl -wp -e ’unless ($.%100) {print"commit;\n"}’


Problem 2

Problem 2


perl -wp -e ’unless ($.%100) {print"commit;\n"}’old_file > new_file

mv new_file old_file


Problem 2

Problem 2


perl -wpi -e ’unless ($.%100) {print"commit;\n"}’old_file


About this talk

Things you should know about switches. . .

If they could speak, they would probably say:

We’re here to help.

Don’t run. We are your friends.


About this talk

These are not switches. . .


Problem 3

Problem 3

Trim lines to 80 columns


while (<>) {chomp;substr($_, 80) = "";print "$_\n";

}


Problem 3

Wow! More?

-i In-place editing-p Loop around program, printing lines-e Eval script-l

$/$\


Problem 3

Sounds good. . .

-i In-place editing-p Loop around program, printing lines-e Eval script-l Automatic line-ending processing

$/ Input record separator$\ Output record separator


Problem 3

$/

This influences Perl’s idea of what a ”line” is.

$\Specify what is printed at the end of the print.


Problem 3

Problem 3



while (<>) {chomp;substr($_, 80) = "";print "$_\n";

}


Problem 3

Problem 3


#!/usr/bin/perl -wuse strict;$\ = "\n";while (<>) {

chomp;substr($_, 80) = "";print $_ ;

}


Problem 3

Problem 3


#!/usr/bin/perl -wpuse strict;BEGIN { $\ = "\n"; }

chomp;substr($_, 80) = "";


Problem 3

-l[octnum]

Enables automatic line-ending processing.It has two separate effects.First, it automatically chomps $/ from $_:

chomp;

Second, it assigns ”$\” to have the value of ”octnum” or of ”$/”,if octnum is ommited:

$\ = $/;


Problem 3

Problem 3


#!/usr/bin/perl -wpuse strict;BEGIN { $\ = "\n"; }



Problem 3

Problem 3


#!/usr/bin/perl -wpuse strict;BEGIN { $\ = $/ ; }



Problem 3

Problem 3


#!/usr/bin/perl -wpluse strict;

substr($_, 80) = "";


Problem 3

Problem 3


#!/usr/bin/perl -wpluse strict;

substr($_, 80) = "";

__END__perl -wpl -e ’substr($_, 80) = ""’


About this talk

If this is your first time. . .

If you’re felling dizzy, remember:

You’re not the only one!


About this talk

Trust me!


Problem 4

Problem 4

Study commands from .bash history


my %commands;while (<>) {

my @args = split ’ ’, $_;$commands{$args[0]}++;

}

for (keys %commands) {print "$_ -> $commands{$_}\n";

}


Problem 4

I was expecting this. . .

-n Loop around program-e Eval script-a

@F


Problem 4

Surprising. . .

-n Loop around program-e Eval script-a Autosplit mode

@F Fields of each line (only with -a)


Problem 4

-a

An implicit split command to the @F array is done as the firstthing inside the implicit while loop produced by the -n or -p.

perl -ane ’print pop(@F), "\n";’

is equivalent to

while (<>) {@F = split(’ ’);print pop(@F), "\n";

}


Problem 4

Problem 4



my %commands;while (<>) {


}

for (keys %commands) {print "$_ -> $commands{$_}\n";

}


Problem 4

Problem 4


#!/usr/bin/perl -wnuse strict;

our %commands;


END {for (keys %commands) {

print "$_ -> $commands{$_}\n";} }


Problem 4

Problem 4


#!/usr/bin/perl -wnause strict;

our %commands;

$commands{$F [0]}++;

END {for (keys %commands) {

print "$_ -> $commands{$_}\n";} }


Problem 4

Problem 4


#!/usr/bin/perl -wnause strict;our %commands;

$commands{$F[0]}++;

END {for (keys %commands) {print "$_ -> $commands{$_}\n";}}


Problem 4

Problem 4


#!/usr/bin/perl -wnause strict;our %commands;

$commands{$F[0]}++;

END {for (keys %commands) {print "$_ -> $commands{$_}\n";}}__END__perl -wna -e ’$c{$F[0]}++;END{print"$_ -> $c{$_}\n"

for keys %c}’


About this talk

Other sources.

You should also:

Read!

Read!

Read!


About this talk

There are lots of good books out there!


Problem 5

Problem 5

List users on /etc/passwd


while (<>) {my @user = split /:/, $_;print "$user[0]\n";

}


Problem 5

What are we using this time?

-n Loop around program-e Eval script-a Autosplit mode-l Automatic line-ending processing

@F Fields of each line (only with -a)-F


Problem 5

Hum. . .

-n Loop around program-e Eval script-a Autosplit mode-l Automatic line-ending processing

@F Fields of each line (only with -a)-F Specifies the pattern to split on (only with -a)


Problem 5

Problem 5



while (<>) {my @user = split /:/, $_;print "$user[0]\n";

}


Problem 5

Problem 5


#!/usr/bin/perl -wnuse strict;

my @user = split /:/, $_;print "$user[0]\n";


Problem 5

Problem 5


#!/usr/bin/perl -wnluse strict;

my @user = split /:/, $_;print $user[0] ;


Problem 5

Problem 5


#!/usr/bin/perl -wnla -F:use strict;

print $F [0] ;


Problem 5

Problem 5



print $F[0];


Problem 5

Problem 5



print $F[0];

__END__perl -F: -wnlae ’print $F[0]’


About this talk

Too many switches?. . .

Here’s what I’d like to tell you:

You’ll get over it!

Don’t think of this as a nuisance.


About this talk

It only looks strange on the first times. . .


Problem 6

Problem 6

Create a line for /etc/passwd


my @params;while (my $p = shift) {

push @params, $p;}

print join ":", @params;


Problem 6

Let’s see what we have this time.

-e Eval script@ARGV

$,


Problem 6

Hum. . .

-e Eval script@ARGV Contains the command-line arguments for the script

$, Output field separator for the print operator


Problem 6

Problem 6



my @params;while (my $p = shift) {

push @params, $p;}

print join ":", @params;


Problem 6

Problem 6



print join ":", @ARGV ;


Problem 6

Problem 6



$, = ":";

print @ARGV ;


Problem 6

Problem 6



$, = ":";

print @ARGV;


Problem 6

Problem 6



$, = ":";

print @ARGV;

__END__perl -e ’$,=":";print @ARGV’


About this talk

If this is all starting to look too much. . .

Remember:

There’s no need to worry.

You’ll get along just fine.


About this talk

It’s just a matter of practise!


Problem 7

Problem 7

List Perl modules installed

#!/usr/bin/perl -wuse strict;use ExtUtils::Installed;

for (ExtUtils::Installed->new()->modules) {print "$_\n";

}


Problem 7

These things never end, do they? :-)

-e Eval script-l Automatic line-ending processing

-M


Problem 7

And I like it.

-e Eval script-l Automatic line-ending processing

-M Use module


Problem 7

Problem 7


#!/usr/bin/perl -wuse strict;use ExtUtils::Installed;

for (ExtUtils::Installed->new()->modules) {print "$_\n";

}


Problem 7

Problem 7


#!/usr/bin/perl -wluse strict;use ExtUtils::Installed;

for (ExtUtils::Installed->new()->modules) {print $_ ;

}


Problem 7

Problem 7



print for (ExtUtils::Installed->new()->modules)


Problem 7

Problem 7



print for (ExtUtils::Installed->new()->modules)__END__perl -MExtUtils::Installed -wle ’print for

ExtUtils::Installed->new()->modules’


About this talk

Do remember all it takes is practice. . .

Important:

Be curious.

Try everything.


About this talk

Well. . .Maybe not everything. . .


About this talk

And with time. . .


About this talk

You’ll do the most magnificent tricks!


That’s it for today

This is what we went through today

$. $/ $\ $, @F -n -e -i -p -l -a -F -M @ARGV

But there is much more. . .


That’s it for today

This is what we went through today

$. $/ $\ $, @F -n -e -i -p -l -a -F -M @ARGV

But there is much more. . .

$& $‘ $’ $+ $^N @+ $* $| $" $; $#$% $= $- @- $~ $^ $: $^L $Â $? $!%! $Ê $@ $$ $< $> $( $) $0 $[ $]$^C $^D $^F $^H $Î $^M $^0 $^P $^R $^S $^T

$^V $^W $^X$ARGV @INC @_ %INC %ENV $SIG

-0 -C -c -d -D -h -I -m -P -s-S -t -T -u -U -v -V -W -X -x


But that’s not all

See also

perldoc perlvar

perldoc perlrun


Thank you

And remember. . .

Use Perl!


You’re felling spleepy

And. . .

Go to Portugal!


perl white magic - command line switches and special variables

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