Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :

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4.4. Perl in Command Line

Version 0.82

Perl options that simply writing oneliners

Selected One-liners

On modern servers bath time to load Perl interpreter vs time to load AWK interpreter are both negligible and that means that Perl can be used as AWK replacement even in simple scripts. Like AWK, Perl is installed by default in all major Unixes including AIX, HP-UX, Linux and Solaris. It is usually available in /usr/bin

Default availability dramatically changed role of Perl in Unix system scripting and routine text processing. For most administrators it is much easier to use Perl that any of the older alternatives. The main consideration here is the power of the language and availability of a very good, built-in debugger. Neither bash nor AWK has built-in debugger installed by default. And that alone tips the scales in Perl favor, as each administrator has a lot of things to do to spend time guessing where he made mistake using multiple echo/print statements.

That's why Perl is gradually displacing older Unix utilities such as cut, sed , wc , and, of course, AWK. Often you can replace quite complex set of pipe stages that use classic UNIX utilities with one Perl loop. Don't interpret me wrong, pipes are a blessing and extremely powerful tool that each UNIX admins should use to the max, but if you can avoid unnecessary complexity, why stick to old practices.

Perl is also amazingly Unix-friendly language that give the programmer full access to the Unix API.

Perl options that simply writing one-liners

The simplest way to start is to remember -e option (execute), which instructs Perl interpreter that the next argument is a Perl statement to be compiled and run. If -e is given, Perl will not look for a script filename in the argument list in will take the argument that follows -e as the text of the script. Make sure to use semicolons where you would in a normal program. For example

perl -e 'print "Hello world of Perl command line";'

Multiple -e commands may be given to simplify building a multi-line script.

There are several more useful "in-line" oriented options that Perl interpreter accepts:

• -n causes Perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed -n or awk: while (<>) { ... # your script goes here } You need to supply you own print statement(s) to have output printed, Nothing is printed by default. See -p to have lines printed. If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file.
  

• -p causes Perl to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed:

  while (<>) { 
     ... # your script
  } continue { 
     print or die "-p destination: $!\n";
  } 

• -a turns on autosplit mode when used with either -n or -p. An implicit split command to the @F array is done as the first thing inside the implicit while loop produced by the -n or -p. For example:

  perl -ane 'print pop(@F), "\n";' 

  is equivalent to

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

  It is important to know that an alternate delimiter for split may be specified using option -F. 

Tips:

• If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves on to the next file. Note that the lines are printed automatically. An error occurring during printing is treated as fatal.
• To suppress printing use the -n switch. A option -p overrides the options -n.

As a useful example let's look how we can combine power of Perl command line with find utility to produce a very simple but still useful command global string/pattern replace utility for multiple files :

find . -type f -exec perl -i -pe 's/something/another/g' {} \;

To make a command named lower that converts all filenames in the current directory to lower case, you can add the following function to your ~/.bashrc or ~/.kshrc

function lower { 
   perl -e 'for (@ARGV) { rename $_, lc($_) unless -e lc($_); }' * 
}

As with everything excessive zeal hurts. You need to exercise judgment and not to miss the moment when one liner became counterproductive because of excessive complexity. In this case it should be converted into a regular script.

Selected One-liners

Tom Christiansen posted a list on useful one liners on Usenet years ago, and that list is still interesting and useful for any Perl programmer.  For more recent and broad Softpanorama collection of one liners see  Perl One Liners for more information. Here are some one liners that I have found especially useful:

1. Replace a pattern pattern1 with another (pattern2)  globally inside the file and create a backup

   perl -i.bak -pe 's/pattern1/pattern2/g' inputFile

2. unix2dos (conversion from Unix to Windows format; useful on SLES 10 and SLES 11 as it does not include it by default):

   perl -i.bak -pe 's/\n/\r\n/' filename 

3. Select only lines between  a pattern pattern1 and pattern2

   perl -i.old -ne 'print unless /pattern1/ .. /pattern2/' filename

   for example

   perl -i.old -ne 'print unless /^START$/ .. /^END$/' filename

4. Print file with line numbers

   perl -ne '$i++;print"$i\t$_";' filename

5. Print balance of quotes in each line (useful for finding missing quotes in Perl and other scripts)

   perl -ne '@F=split;for $s (@F){ $j++ if $s eq q(")}; $i++;print"$i\t$j\t$_";' filename

6. Emulation of Unix cut utility in Perl. Option -a ( autosplit mode) converts each line into array @F. The pattern for splitting a whitespace (space or \t) and unlike cut it accommodates consecutive spaces of \t mixes with spaces. Here's a simple one-line script that will print out the fourth word of every line, but also skip any line beginning with a # because it's a comment line.

   perl -naF 'next if /^#/; print "$F[3]\n"'

   Here is example on how to print first and the second from the last columns:

   perl -lane 'print "$F[0]:$F[-2]\n"'

7. To capitalize the first letter on the line and convert the other letters to small case.  Here are two variants, one simple and one idiomatic:

   perl5 -pe 's/(\w)(.*)$/\U$1\L$2/'

   perl5 -pe 's/\w.+/\u\L$&/'

   The second one belongs to Matz Kindahl  and simultaneously belongs to the class of Perl idioms. It is difficult to understand so the first version is preferable. Beware Perl authors who prefer the second variant to the first ;-)

Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater’s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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Last modified: March 12, 2019