Perl one-liners

Perl One Liners for the Shell

Dave Oswald

● Conferences Committee Chairman

● Salt Lake Perl Mongers Organizer

● Software Engineer

● Platform & Infrastructure Engineering Manager

perl [switches] [--] [programfile] [arguments]

Switches -0[octal] specify record separator (\0, if no argument)

-a autosplit mode with -n or -p (splits $_ into @F)

-C[number/list] enables the listed Unicode features

-c check syntax only (runs BEGIN and CHECK blocks)

-d[:debugger] run program under debugger

-D[number/list] set debugging flags (argument is a bit mask or alphabets)

-e program one line of program (several -e's allowed, omit programfile)

-E program like -e, but enables all optional features

-f don't do $sitelib/sitecustomize.pl at startup

-F/pattern/ split() pattern for -a switch (//'s are optional)

-i[extension] edit <> files in place (makes backup if extension supplied)

-Idirectory specify @INC/#include directory (several -I's allowed)

-l[octal] enable line ending processing, specifies line terminator

-[mM][-]module execute "use/no module..." before executing program

-n assume "while (<>) { ... }" loop around program

-p assume loop like -n but print line also, like sed

-s enable rudimentary parsing for switches after programfile

-S look for programfile using PATH environment variable

-t enable tainting warnings

-T enable tainting checks

-u dump core after parsing program

-U allow unsafe operations

-v print version, patchlevel and license

-V[:variable] print configuration summary (or a single Config.pm variable)

-w enable many useful warnings

-W enable all warnings

-x[directory] ignore text before #!perl line (optionally cd to directory)

-X disable all warnings

perldoc perlrun

Introducing the tools

perl -e ''

-e

perl -e 'print "Hello World!\n"'

Hello World!

#!/usr/bin/env perlprint 'Hello World!\n';

Hello World!

echo 'Hello World'

while (defined(local $_ = <>)) {

...

print;}

$ perl myscript file1.txt file2.txt

Internally <> reads from each file in @ARGV, falling back to STDIN.

A concept in Perl is only valid if it can be expressed as a one-liner.

-- Documentation for Inline::C

perl -pe '...'

while (defined(local $_ = <>)) {

...

print;}

$ perl myscript file1.txt file2.txt

while (defined(local $_ = <>)) { chomp ... $_ .= "\n"; print;}

$ perl myscript file1.txt file2.txt

perl -lpe '...'

Reverse the characters in every word in a dictionary

perl -ple '$_ = reverse' ~/scripts/2of12inf.txt

Count by fives

seq 1 10 | perl -lpe '$_ *= 5'

-l

chomps input, adds newline to output.

-l

chomps input, adds newline to output.

$/ = $\ = "\n"

-p

Creates a loop that iterates over lines of input, carries out any actions in the code block, and

prints each line.

$.

perl -pe '$_ = "$. " . $_' 2of12inf.txt

$.

Special variable contains the line number of the most recently read line of input.

Lines start at 1.

Sometimes printing every line of input is not wanted.

-n

perl -ne 'print "$.: $_" if m/^th/' 2of12inf.txt

Let’s get a count at the end.

This means we need something to happen only one time, after the loop is done.

perl -ne 'if (m/^th/) {$c++; print} END{print "$c\n"}'

grep '^th' 2of12inf.txt |wc -l

-n iterates over lines of a file without printing.

You can print explicitly.

-n is useful as a filter similar to GNU grep.

END{ ... }

Can be used to defer an action to after the iterating is done.

Input can come from STDIN rather than a file.This allows pipes to work.

seq 1 10 |perl -ne '$_*=5; print "$_\n" if $_ % 3 == 0'

perl -l0pe '...' filename

-l0

Specify an octal ASCII value as input/output record separator.

-0

Set output record separator to octal value

-0oct and -loct

Useful for working with null-terminated lines or input.

-a

df |perl -ane 'next if $. == 1; print "$F[-1] $F[-2]\n"'

-a

Auto-split each line on whitespace into @F

Let’s split on comma and change to colon

-F/pattern/

perl -F/,/ -ne '$"=":"; print "@F";' csv

-F/pattern/

Specifies an alternate split pattern.

$" = "..."

Specifies an alternate “join” delimiter when an array is interpolated into a string.

perl -pe 'tr/,/:/' csv

-i

perl -i.bak -pe 'tr/,/:/' csv

-i

In-place edit a file.

Optionally create a backup of original.

perl -i.bak -pe 'tr/:/,/' csv

Fixed.

Handling multiple files distinctly

eof

perl -ne 'print; print "EOF\n" if eof' 2of12inf.txt

Use eof to detect end of input files.

-M

Load a module at startup

seq 1 10 |perl -MList::Util=shuffle -ne 'push @S, $_; END{print for

shuffle @S}'

seq 1 10 |perl -MList::Util=shuffle -ne\

'push @S, $_; END{print for shuffle @S}'

-M

Loads a module at startup.

Optionally specify an import list.

May invoke -M more than once.

perl -Mstrict -Mwarnings -e '$foo=10'

(Strict violation)

What modules are loaded when I loadData::Dumper?

perl -MData::Dumper -e 'print "$_\n" for keys %INC'

Where is Perl finding Data::Dumper?

perl -MData::Dumper -e 'print qq/$INC{"Data/Dumper.pm"}\n/'

qq/.../

Same as "..."

Used to specify an alternate quoting mechanism

q/.../ '...'qq/.../ '...'qr/.../ m/.../qx/.../ `...`

q{...} '...'qq{...} '...'qr{...} m/.../qx{...} `...`

%INC

Hash containing list of modules loaded, and their file paths.

-E

perl -MData::Dumper -E 'say $INC{"Data/Dumper.pm"}'

-E

Allow use of say keyword.

say is like print, but auto-appends newline.

-E

Also enables all feature keywords for the Perl version in use.

-I

perl -I/path/to/lib -MGreet -E 'say greet()'

perl -I./lib -E 'say for @INC';

-I

Specify the first path to search for a module.

Multiple -I switches are fine.

-V

perl -V

perl -V:u64type

perl -MConfig -E 'say $Config{u64type}'

Find all mentions of a feature in Perl POD "*delta.pod" articles for current version of Perl.

How do I know when feature X was added to Perl?

How do I know when feature X was added to Perl?

grep -ri 'signatures' $(find $(perl -E 'say for @INC') -name '*delta.pod')

Perl's documentation

Perl's documentation

● perlrun– Explanation of all command-line switches.

● perlvar– Explanation of Perl's "special variables"

● perlsyn– Description of syntax

Perl is part of Linux's rich set of tools.

daoswald@gmail.com

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