TDC597 Linuxbased Network Management Lecture Three Perl

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TDC597Linux-based Network ManagementLecture
Three - Perl

• James T. Yu, Ph.D.
• jyu_at_cs.depaul.edu
• School of CTI
• DePaul University

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What is PERL

• Practical Extraction and Report Language
• Similar to shell script but a lot more powerful
• Is it more powerful than Aw?
• Easily available
• Lots of documentation and examples on the web

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Basic Concepts

• Perl files extension .pl
• Convention (not a requirement)
• Create self executing scripts
• Use system (Linux) commands
• Comment entry
• Print information on the screen

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First Perl Program
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Perl Basics

• Perl is a script language
• No compiler, no object file
• All perl commands end in "" .
• Perl can call Linux commands
• system(linux command")
• The pound sign "" is the symbol for comment.
  Same as Shell and Awk.
• The "print command" is used to write outputs on
  the screen.
• Note put \n at the end
• If you want to use formats for printing you can
  use printf which is similar to C.

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Scalar Variables ()

• Scalar variables are always preceded with the
  symbol.
• No variable declaration is needed before its use.
• It is a good practice to declare it my
  variable
• No data types, such as character or numeric.
• Scalar variable means that it can store only one
  value.
• A scalar variable can store a character, a
  string, or a number.
• Example, name "betty"
• The value betty is stored in the scalar variable
  name.
• Use single quote for literal string. price
  99.99
• Question what is wrong with price 99.99
• Default values for all variables is undef which
  is equivalent to null.

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List Variables (_at_)

• List variables are like arrays. It can be
  considered as a group of scalar variables.
• They are always preceded by the _at_ symbol.
• Example, _at_names ("betty","veronica","tom")
• Like in C the index starts at 0.
• To reference the second item, use names1
• Note the symbol here because each element is a
  scalar variable.

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List Variable
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Operations on list variablesstack operation

• Operators push, pop, shift, unshift, reverse
• Push and pop treat the list variable as a stack
  and operate on it. They act on the top (last)
  item of the stack.
• Example push(_at_names,"lily") will yield
• _at_names with content of ("betty","veronica","tom",
  "lily").
• Example pop(_at_names) returns "lily" which is the
  last value. And _at_names will contain
  ("betty","veronica","tom").

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Operations on list variables (cont.)queue
operation

• shift and unshift act on the bottom (first) of
  the stack
• Example, unshift(_at_names,"lily"), and _at_names
  contains ("lily","betty","veronica","tom").
• Example, shift(_at_names) returns "lily" and _at_names
  contains ("betty","veronica","tom").
• reverse reverses the list.

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Operations on List Variables
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Hash Variables ()

• Hash variables are like arrays but instead of
  having numbers as their index they can have any
  scalars as index.
• The same associate array in Awk.
• Hashes are preceded by the symbol.
• Example, hashArray ("A", 1, "B", 2, "C", 3)
• To get the hashArray, reference hashArrayA".
  This will return the value of hashArray of A.
• A is the key and 1 is its value.
• keys() returns a list of all the keys of the
  given hash.
• values() returns the list of all the values in a
  given hash.

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Hash Variables
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Hash Variable (sorting)
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Read / Write to Files

• To read from and write to files, we need to
  create handles which refer to the files. This is
  the same as the file descriptor in C.
• Return true if the file is opened successfully
  otherwise, return false.
• To create the handles we use the open command.
• Example open(FILEREAD, lt filename") create
  the handle called FILEREAD for the file
  "filename".
• This handle will be used for reading.
• The lt" symbol before the filename is to indicate
  the file is opened for reading.
• Example open(FILEWRITE, "gtfilename") create the
  handle called FILEWRITE for the file "filename".
• This handle will be used for writing.
• The "gt" symbol before the filename is to indicate
  the file is opened for writing.

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Read / Write to Files

• Once the file handles are created. the reading
  and writing to files is pretty simple.
• Example, linevalue ltFILEREADgt
• This will result in a line being read from the
  file pointed by FILEREAD and the that line is
  stored in the scalar variable linevalue.
• When the end of file is reached the ltFILEREADgt
  returns an undef.
• Example, print FILEWRITE "linevalue \n
• This will result in a line with the value as in
  linevalue being written to the file pointed by
  the FILEWRITE .
• To close a file for reading/writing, use
• close FILEHANDLE

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File Read/Write
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Control Structures

• If / unless statements
• while / until statements
• for statements
• foreach statements
• last , next , redo statements
• And as control structures

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if / unless

• The if command is similar to the if in C.
• unless the reverse of if
• Example of unless.
• unless(condition) .
• In an if command, in the case that you want to
  leave the then part empty and have just an else
  part, use unless.

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Numerical Comparison
Note same as C
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if unless
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while / until / for

• The while command is similar to the while of C.
• The for command is also similar to C
  implementation
• until.
• until( ltexpressiongt ) ltstatementsgt .
• The statements are executed till the condition is
  met.

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Loop while, for, until
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foreach statement

• This statement takes a list of values and assigns
  them one at a time to a scalar variable,
  executing a block of code with each successive
  assignment.
• Example foreach var (list) .

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foreach
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last / next / redo

• The last command is similar to the break
  statement of C.
• When you want to quit from a loop, use last.
• Do not use break (syntactically correct, but )
• To skip the current loop use the next statement.
• It immediately jumps to the next iteration of the
  loop.
• This is similar to the continue statement in C.
• Do not use continue
• The redo statement repeats the same iteration
  again.

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last, next, redo
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Functions

• Function declaration
• Calling a function
• Local variables
• Returning values

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Function Declaration

• The keyword sub describes the function.
• So the function should start with the keyword
  sub.
• Example, sub addnum . .
• It should be preferably either in the end or in
  the beginning of the main program to improve
  readability and also ease in debugging.
• Always have a prolog at the beginning of the
  function (a requirement for TDC597)

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Function Calls - Invocation

• name getname( )
• The symbol should precede the function name
  in any function call.

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Parameters of Functions

• Parameter are passed to the function as a list.
• The parameter is denoted by _at__ inside the
  function.
• So if you pass only one parameter, the size of _at__
  list will be one. If you pass two parameters then
  the _at__ size will be two. These two parameters are
  accessed by _0 and _1.

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More About Functions

• The variables declared in the main program are by
  default global so they are available for
  reference in the function.
• Local variables are declared by putting 'my'
  while declaring the variable.
• my x x is a local variable in the function

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More About Functions

• The result of the last operation is usually the
  value that is returned unless there is an
  explicit return statement returning a particular
  value.
• Recommendation always use return to return a
  value.
• Clarity is more important than saving one line of
  code.
• There are no pointers in Perl but we can
  manipulate and even create complicated data
  structures.

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Function (sub)
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String Operation

• split and join
• String Comparison
• Using different delimiter
• matching replacing
• Index and substring
• Others

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The split function

• split is used to form a list from a scalar data
  depending on the delimiter.
• The default delimiter is the space.
• It is usually used to get the independent fields
  from a record.  
• Example
• linevalue "R101 tom 89"
• _ linevalue.
• _at_data split()
• data0 is R101, data1 is tom , data2 is
  89.
• split by default acts on the _ variable.
• If split has to perform on some other scalar
  variable, than the syntax is.
• split (/ /, linevalue)
• To use other delimiter for split, the syntax is
• split(/ltdelimitergt/, linevalue)
• Example _at_arr split(/ /, linebuf)
• one or more space characters

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The join function

• join does the exact opposite job as that of the
  split.
• It takes a list and joins up all its values into
  a single scalar variable using the delimiter
  provided.
• Example, newlinevalue join(_at_data)

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split and join
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String Operations
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String Operations
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Matching and Replacing

• Suppose you need to look for a pattern and
  replace it with another one you can do the same
  thing as what you do in Linux. The command in
  perl is
• s/ltpatterngt/ltnew patterngt/
• This by default acts on the _ variable. If it
  has to act on a different variable (eg newval)
  then use.
• Example, _at_newvals/ltpatterngt/ltnew patterngt /.

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Pattern Match and Substitution
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index and substr

• index(s, c, i)
• Return the position of c in s, start searching
  from i
• substr(s, n1, n2)
• Create a subsring from s, starting at n1 with
  lengthn2

s ..12345.. my idx1
index(s, "", 0) my idx2 index(s, "",
(idx11)) my pid substr(s, (idx11),
(idx2-idx1-1))
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many, many more Perl features
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Hw03 (due 08/02)

• Objective analyze the Linux message file
• The message file is located in /var/log/messages
  (readable by root only)
• A sample copy of message file is saved on
• /home/tdc597/log/messages (hard code in your
  program)
• It has a lot of system log information
• User login and logout time
• Failed login attempt
• More
• Hw03 show a summary report of account usage
• Extra credit analysis of potential system
  intrusion
• Template /home/jyu/tdc597/lec03/hw03template.pl
• Your program HOME/hw03/hw03script.pl
• Your directory and files must be protected from
  other students.

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Input Data
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Extra Credit intrusion analysis
Failed login attempt to root (uid0) from an
unknown remote host
Report number of intrusion attempt from each
rhost