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Contents Bulletin Scripting in shell and Perl Network troubleshooting History Humor

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:

Tips:

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 ;-)



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