Lecture Overview Perl Programming
Why would you want to use it? How does it compare to awk, sed, and
bash? Syntax Semantics
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
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
Running Perl One liners
perl –e ‘print “Hello world!\n” ;’ Perl scripts
#!/usr/bin/perl –w Adds warnings, which is VERY
important
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 ;
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” ;
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
What Is An Array?
$variable1
$variable2
$variable3
$variable4
“hi”
“there”
5.3
4.1
“hi” “there” 5.3 4.1@array
0 1 2 3
Accessing Array Elements In Perl All elements are numbered
starting from zero Accessing the array as a whole
requires the @ Accessing each individual element
requires the $ @myArray = (5, “Hello”, 4.3”) ; print $myArray[1], “\n” ;
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” ;
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
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 $line = <STDIN>
Cutting Off The Newline: chomp When reading in a line at a time,
perl keeps the newline at the end of the line
Bash doesn’t, and awk doesn’t Use the chomp command to get rid
of it chomp($line = <STDIN>) ;
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
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?
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
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<)
File Operations -e
File exists -d
File is a directory -f
File is a normal file -T
File is a text file
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) ;
Looping Statements To allow repetition, several looping
statements are allowed just like bash and awk while for foreach
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
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
Sed Functionality In Perl A special comparison operator:
=~ Checks to see if a pattern appears in
a variable Example: $line =~ /Jason/ ;
Example: Deleting All Quiz #3’s sed ‘/Quiz 3/d` database Perl:open(DATABASE, “database”) ;while ($line = <DATABASE>){ print $line if ! ($line =~ /Quiz 3/) ;}
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…
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) ;
Example: Printing Out The Third Field Awk: { print $3 } Perl: while ($line = <FILE>) { @fields = split(“:”, $line) ;
print $fields[2] ; }
In Lab Today Practice with tar and perl
Writing very small perl programs Rewriting some previous programs in
perl
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