1. Introduction to Unix – CS 21
Lecture 15

2. Lecture Overview Perl Programming Why would you want to use it?
How does it compare to awk, sed, and bash?
Syntax
Semantics

3. Why Perl? Perl tries to be a little bit of everything
There’s More Than One Way To Do It
TMTOWTDI
Designed to perform string manipulation and regular expression matching
Capability to perform all the same tasks that awk and sed perform with little effort

4. Perl Mentality
Perl was designed to try to make common activities simple
Also designed to make not-so-common activities not that complicated
If you get good at perl, you can pack a lot of information into small programs
Perception: You’re not a hacker unless you know perl

5. Running Perl One liners Perl scripts
perl –e ‘print “Hello world!\n” ;’
Perl scripts
#!/usr/bin/perl –w
Adds warnings, which is VERY important

6. Major Differences From Bash and Awk programs
Every line must end with a ;
In bash and awk, simply hitting return ended a command
Every use of a variable must have the $
In bash: myVar=0
In perl: $myVar=0 ;

7. Printing In Perl The print statement
print $myVar ;
print $myVar, “ and “, $myOtherVar ;
Doesn’t go on to a new line like echo does in bash
Need to add an explicit marker for the end of the line: “\n”
print $myVar, “\n” ;

8. Variables In Perl Declared bash style (no previous declaration)
Scalars
A single value. (5, “Hello”, 4.3)
$variableName
Arrays
A group of values
@arrayName
Others
Automatic Interpretation

9. What Is An Array? “hi” $variable1 “there” $variable2 5.3 $variable3
4.1
$variable4
“hi”
“there”
5.3
4.1
@array 1 2 3

10. Accessing Array Elements In Perl
All elements are numbered starting from zero
Accessing the array as a whole requires
Accessing each individual element requires the $
@myArray = (5, “Hello”, 4.3”) ;
print $myArray[1], “\n” ;

11. File Input And Output
In order to read or write to an external file, we need a file handle
Special variable that refers to an external file
Should be in all caps to avoid confusion
Reading a line from a file:
<FILE>
Writing to a file:
print FILE “hello file!\n” ;

12. Declaring File Handles: The Open Command
open(HANDLE, “filename”) ;
Open for reading
open(HANDLE, “> filename”) ;
Open for writing
open(HANDLE, “>> filename”) ;
Open for writing by appending

13. STDIN, STDOUT, And STDERR
As usual, three files/streams are already ready to go whenever you run a perl program
STDIN, STDOUT, and STDERR
Read lines one at a time from STDIN
$line = <STDIN>

14. Example

15. Cutting Off The Newline: chomp
When reading in a line at a time, perl keeps the newline at the end of the line
Bash doesn’t, and awk doesn’t
Use the chomp command to get rid of it
chomp($line = <STDIN>) ;

16. Chomp Example

17. Special Cases Made Easy
Focusing on the idea that common cases should be made easy, there are a lot of shortcuts available in perl
$line = <> ;
Reads a line at a time from all of the files listed on the command line or STDIN if no files were specified
Acts just like other Unix programs

18. Example Of <>

19. The Default Variable: $_
To make shortcuts even easier, if you don’t assign a value, the results are automatically stored in a default variable named $_
<STDIN> ;
Stores the first line read into the default variable
Other commands will use this default variable if no variable is supplied, making a lot of work go on “behind the scenes”
Is this useful?

20. Example: Cat In Perl

21. Conditional Statements
Just like awk and bash, perl has a set of statements that control the flow of execution if
unless
Conditions are based upon comparisons or tests

22. Comparison Operators Numbers Strings Description == eq Equals != ne
Not equal
> gt
Greater than
< lt
Less than
<= le
Less than or equal
>= ge
Greater than or equal
<=>
cmp
Compare (0=, 1>, -1<)

23. File Operations -e -d -f -T File exists File is a directory
File is a normal file -T
File is a text file

24. If statements
if (CONDITION) {
STATEMENTS ; }

25. Examples Of if

26. If..elsif..else statements
if (CONDITION) { }
elsif (CONDITION)
else

27. Unless Statement
unless (CONDITION)
{ STATEMENTS ; }

28. Unless Flowchart
unless
Is Condition True? no
Statements
yes

29. Unless Example

30. Another Form Perl also has a form that is more like English
For single statements, you can place the if or unless and condition after the statement
if (5>3) { $myVar = 4 ; }
$myVar = 4 if (5 > 3) ;

31. Looping Statements
To allow repetition, several looping statements are allowed just like bash and awk
while
for
foreach

32. While Statements
while (CONDITION)
{ STATEMENTS ; }

33. For Statements
for (INIT ; CONDITION; INCREMENT)
{ STATEMENTS ; }

34. Example Of for

35. Foreach Statements
foreach $var )
{ STATEMENTS ; }

36. Example Of foreach

37. Regular Expressions
Syntax is mostly that of egrep with a couple of differences
Works on the default variable
Default behavior is greedy
Will match the largest string possible
? Restricts the match to the smallest possible

38. Example Jason Villarreal:11342:Midterm1:59:100 /:(.*):/ /:(.*?):/

39. Substitution In Perl Just like in vi s/old/new/
Works on the default variable
In order to work with other variables, another operator is needed

40. Sed Functionality In Perl
A special comparison operator: =~
Checks to see if a pattern appears in a variable
Example: $line =~ /Jason/ ;

41. Example: Deleting All Quiz #3’s
sed ‘/Quiz 3/d` database
Perl:
open(DATABASE, “database”) ;
while ($line = <DATABASE>) {
print $line if ! ($line =~ /Quiz 3/) ; }

42. Backreferences In R.E.
Every time you use parenthesis in a regular expression, the pattern matched becomes marked and can be accessed later
Marked by a number
\1 stands for the first ()
\2 stands for the second ()
Etc…

43. Example Of Back References
Matching HTML tags
<LI> … </LI>
<H1> … <H2> </H2> </H1>

44. Example

45. Awk Functionality In Perl: split
Breaking up a line or string based on a delimiter is done with a call to split
Usage: split( Delimiter, Record ) ;
Delimiter can be a regular expression
Examples:
@fields = split(“:”, $record) ;
($field1, $field2) = split(“:”, $record) ;

46. Example: Printing Out The Third Field
Awk:
{ print $3 }
Perl:
while ($line = <FILE>)
= split(“:”, $line) ;
print $fields[2] ; }

47. In Lab Today Practice with tar and perl
Writing very small perl programs
Rewriting some previous programs in perl

48. Next Week
LaTeX
Make