System Administration Introduction to Scripting, Perl Session 4

1
System AdministrationIntroduction to Scripting,
PerlSession 4 Sat 17 Nov 2007

• References
• chapter 1, The Unix Programming Environment,
  Kernighan Pike, ISBN 0-13-937681-X
• Perl man pages
• Albert Lingelbach, Jr.alingelb_at_yahoo.com

2
Review

• Scripts perl syntax comment, statement
• Scalar variables
• Special operators .
• Math operators - / precedence,
  associativity
• Numerical comparisons lt gt lt gt !
• String comparisons eq ne lt gt le ge
• Control flow if, while, for
• Boolean logic !
• Subroutines, arguments/parameters, functions

3
backquotes

• it is often necessary to execute a command and
  capture the results (standard output)?
• sh and perl use the same syntax for this enclose
  the command in backquotes (, usually in the
  upper left of the keyboard) and assign the result
  to a variable
• example
• userid who am i
• print userid

4
Recursive functions

• "recursion" is a way of defining a function in
  terms of itself and a base case
• for example, the fibonacci series is defined as
• the first fibonacci number is 0
• the second fibonacci number is 1
• every other fibonacci number is the sum of the
  previous two
• thus the fibonacci series begins
• 0, 1, 1, 2, 3, 5, 8, 13, 21, ...

5
Recursive functions continued

• exponentiation (xy) can be defined recursively
• return x y
• sub power
• my x shift
• my y shift
• if (y 1)
• return x
• else
• return x power (x, y - 1)

6
Variable scope and recursion

• Notice that each "call" of a recursive function
  (or any function) gets its own copy of variables
  declared with "my"
• It can be helpful to trace through the "call
  stack" to watch the variables as they change

7
Variable Scope

• my indicates that the variable has "local" scope
• without my, variable has "global" scope
• local scope means the variable is defined only
  within the enclosing braces
• global scope means the variable is defined
  everywhere, unless overridden by a local version
• this is useful for modularity

8
Variable Scope Example

• example
• sub mysub
• print "x x\n"
• print "y y\n"
• my x 7
• y 23
• print "x x\n"
• print "y y\n"
• try moving this before sub
• my x 3
• y 17
• print "x x\n"
• print "y y\n"
• mysub
• print "x x\n"
• print "y y\n"

9
Exercises

• Review session 2 exercises
• Review session 3 exercises

10
Next Up

• Arrays
• Hashes
• Regular Expressions
• see perl man pages for details
• man -M /usr/perl5/man perlintro