Introduction to Perl

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Software Tools

• Introduction to Perl

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Introduction to Perl

• Perl is a scripting language that makes
  manipulation of text, files, and processes easy.
• Perl is a cross between shell programming and the
  C programming language.

C (numbers)
Shell programming (text)
Smalltalk (objects)
C (numbers, objects)
Perl (text, numbers)
Java (objects)
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Introduction to Perl

• Perl provides a more concise and readable way to
  do many tasks compared to C and shell scripts.
• Perl has replaced shell programming as the most
  popular programming language for text processing
  and Unix system administration.
• Perl was originally designed under Unix, but now
  also runs under all operating systems (including
  Windows).
• Perl is also a popular language for CGI and GUI
  programming.

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Introduction to Perl

• The perl command on our system automatically
  invokes Perl 5.0.
• Some systems use Perl 4.0 as the default, but you
  also can run a perl 5.0 script explicitly with
  perl5
• cat simple
• perl5 simple
• You can run the script directly if you make the
  script executable, and the first line is of the
  following form ( !/usr/... must start from the
  first column)
• chmod x simple
• cat simple
• !/usr/local/bin/perl5 -w
• simple

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Basic Syntax

• The -w option tells Perl to produce extra warning
  messages about potential dangers. Use -w in all
  your Perl programs for now.
• !/usr/local/bin/perl5 -w
• Whitespace doesn't matter in Perl (like C),
  except for !/usr/local/bin/perl5 -w which must
  start from column 1 on line 1.

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Basic Syntax

• All perl statements end in a semicolon (like
  C)
• In Perl, comments begin with (like shell
  scripts)
• everything after the to the end of the line is
  ignored.
• need not be at the beginning of the line.
• there are no C-like multiline comments / /

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Perl Example 1

• Here is a hello world Perl program
• ls -l
• -rwxr-xr-x 1 horner cs 52 Mar 2 1550 hello
• cat hello
• !/usr/local/bin/perl5 -w
• comment lines start with the character
• print "Hello world\n"
• hello
• Hello world
• The print command sends the string to the screen,
  and \n adds a newline.

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Perl Example 1

• You can optionally use parenthesis around the
  argument in print
• print ("Hello world\n")
• or, if your prefer the C function style
• print("Hello world\n")

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Scalar Variables

• A scalar variable can hold a single number or
  string (like shell variables), including integers
  and floating-point numbers (unlike shell
  variables).
• Scalar variables begin with followed by a
  letter, and then possibly more letters, digits,
  or underscores. (e.g., n, n1, name,
  first_name).
• Scalar variables are case sensitive.

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Assigning Scalar Variables

• Scalars are assigned using
• scalar expression
• To assign a value to a scalar variable
• number 25
• name "Bill Gates"
• Unlike shell scripts, use the both when the
  variable is used and assigned
• cat test1
• !/usr/local/bin/perl5 -w
• number 25
• name "Bill Gates"
• print "number name\n"
• test1
• 25 Bill Gates

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Numerical Scalar Variables

• Internally, all numerical scalar values are
  stored as floats (so you dont have to worry
  about integer division in Perl like you do in
  C).
• Perl supports the usual C numerical operations
• a 25 a is now 25
• a 5 a is now 30
• a 3 a is now 90
• a a is now 91
• --a a is now 90
• result (a 2) 3.4 result is 312.8

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User Input

• Use ltSTDINgt to get input from the user
• cat test2
• !/usr/local/bin/perl5 -w
• print "Enter name "
• name ltSTDINgt
• chomp (name)
• print "How many girlfriends do you have? "
• number ltSTDINgt
• chomp(number)
• print "name has number girlfriends!\n"
• test2
• Enter name Bill Gates
• How many girlfriends do you have? more than you
• Bill Gates has more than you girlfriends!

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User Input

• ltSTDINgt grabs one line of input, including the
  newline character. So, after
• name ltSTDINgt
• if the user typed Bill GatesENTER, name
  will contain Bill Gates\n.
• To delete the newline, the chomp() function takes
  a scalar variable, and removes the trailing
  newline if present. (If there is no newline at
  the end, it does nothing.)
• A shortcut to do both operations in one line is
• chomp(name ltSTDINgt)

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• As with the shell scripts, use a backslash before
  if you really want to print the dollar sign
• cat test4
• !/usr/local/bin/perl5 -w
• print "Enter amount "
• cost ltSTDINgt
• print "The total is \cost"
• test4
• Enter amount 18.50
• The total is 18.50
• No need to use chomp() if the newline on cost
  can be used when it is printed.

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Numerical Example

• cat test6
• !/usr/local/bin/perl5 -w
• print "Enter height of rectangle "
• height ltSTDINgt
• print "Enter width of rectangle "
• width ltSTDINgt
• area height width
• print "The area of the rectangle is area\n"
• test6
• Enter height of rectangle 10
• Enter width of rectangle 5
• The area of the ractangle is 50
• test6
• Enter height of rectangle 10.1
• Enter width of rectangle 5.1
• The area of the rectangle is 51.51

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BackquotesCommand Substitution

• You can use command substitution in Perl like in
  shell scripts
• whoami
• gates
• cat test7
• !/usr/local/bin/perl5 -w
• user whoami
• chomp(user)
• num who wc -l
• chomp(num)
• print "Hi user! There are num users logged
  on.\n"
• test7
• Hi gates! There are 6 users logged on.
• Command substitution will usually include a
  newline, so use chomp().