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Introduction to Perl for Unix System Administrators

(Perl without excessive complexity)

by Dr Nikolai Bezroukov

4.4. Perl in Command Line

In modern Unix programming environment Perl can be used as AWK replacement even in simple scripts. Recently Perl became a standard, 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.

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.  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 build up a multi-line script.

There are several more options that can sometimes simplify writing "in-line" Perl scripts or make them more similar to traditional AWL-scripts: 

• -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 } The lines are not 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.
   

• -e  specified the one-line to be executed.  We already discussed it above.
   
• 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 occuring during printing is treated as fatal. To suppress printing use the -n switch. A -p overrides a -n switch.

As a useful example let's look how we can combine power of perl command line with find utilitie 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 add this to your ~/.bashrc or  ~/.kshrc

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

Tom Christiansen posted a list of  on Usenet years ago, and that list is still interesting and useful for any Perl programmer. We those and some more modern one liners on a separate page Perl One Liners which you are encouraged to browse; the old Tom Christiansen list widely available in the file tomc.txt  in various places on the Internet.

As with everything excessive zeal hurts, so you need to exercise judgment and the moment when one liner became counterproductive because of excessive complexity and should be converted into a regular script.